Science, Natural Phenomena & Medicine

Primary Hyposecretion

2012-02-17T06:46:00.002-08:00

An endocrine gland may be secreting too little hormone because the gland cannot function normally. This is termed primary hyposecretion. Examples of primary hyposecretion include (1) destruction of the adrenal cortex, leading to decreased cortisol secretion, and (2) dietary deficiency of iodine leading to decreased secretion of thyroid hormones. There are many other causes— infections, toxic chemicals, and so on—all having the common denominator of damaging the endocrine gland.

In contrast to primary hyposecretion, a gland may be secreting too little hormone because there is not enough of its tropic hormone. This is termed secondary hyposecretion. There may also occur (though rarely) a situation called tertiary hyposecretion. In such a case, there is a deficiency of hormone production from a gland and also a deficiency of the gland’s tropic hormone. However, in tertiary hyposecretion the tropic hormone–producing gland is functional; instead, a third hormone (from the hypothalamus) that normally "starts the ball rolling" by stimulating the secretion of the tropic hormone is missing. Thus, hyposecretion can occur at any of three levels in situations where the secretion of a hormone is linked to the secretion of two other hormones in turn.

Hypersecretion

2012-02-16T07:25:00.002-08:00

Hypersecretion is an excessive secretion of a hormone by a gland. A hormone can undergo either primary hypersecretion (the gland is secreting too much of the hormone on its own) or secondary hypersecretion (there is excessive stimulation of the gland by its tropic hormone). One of the most common causes of primary or secondary hypersecretion is the presence of a hormone-secreting endocrine-cell tumor. These tumors tend to produce their hormones continually at a high rate, even in the absence of stimulation.

For the diagnosis of primary versus secondary hypersecretion, the concentrations of the hormone and, if relevant, its tropic hormone are measured. If both concentrations are elevated, then the hypersecretion is secondary. If the hypersecretion is primary, there will be a decreased concentration of the tropic hormone because of negative feedback by the high concentration of the hormone being hypersecreted. As with hyposecretion, tertiary hypersecretion of the peripheral endocrine gland can occur when there exists a three-step sequence of hormones. When an endocrine tumor is the cause of hypersecretion, it can often be removed surgically or destroyed with radiation if the tumor is confined to a small area. In many cases, hypersecretion can also be blocked by drugs that inhibit the hormone’s synthesis. Alternatively, the situation can be treated with drugs that do not alter the hormone’s secretion but instead block the hormone’s actions on its target cells.

Hydrogen Bonds

2012-02-15T06:39:00.001-08:00

The electrical attraction between the hydrogen atom in a polar bond in one molecule and an oxygen or nitrogen atom in a polar bond of another molecule forms a hydrogen bond. This type of bond is very weak, having only about 4 percent of the strength of the polar bonds linking the hydrogen and oxygen within a single water molecule (H2O). Hydrogen bonds are represented in diagrams by dashed or dotted lines to distinguish them from covalent bonds. Hydrogen bonds between and within molecules play an important role in molecular interactions and in determining the shape of large molecules.

Free Radicals

2012-02-14T09:25:00.005-08:00

The electrons that revolve around the nucleus of an atom occupy regions known as orbitals, each of which can be occupied by one or more pairs of electrons, depending on the distance of the orbital from the nucleus. An atom is most stable when each orbital is occupied by its full complement of electrons. An atom containing a single (unpaired) electron in its outermost orbital is known as a free radical, as are molecules containing such atoms. Free radicals can react with other atoms, thereby filling the unpaired orbital. Free radicals are diagramed with a dot next to the atomic symbol. Examples of biologically important free radicals are superoxide anion. A free radical configuration can occur in either an ionized (charged) or an un-ionized atom. A number of free radicals play important roles in the normal and abnormal functioning of the body.

Chordate Characteristics

2012-02-13T06:28:00.002-08:00

The chordates are animals which belong to the phylum Chordata. It includes the vertebrates (animals with spinal column) and animals that have notochord, such as tunicates and lancelets. This phylum gets its name from one of its three distinctive features or characteristics. The first one is the notochord; all chordates at some time during their lifetimes possess a flexible supporting rod along the back, called the notocord, which stiffens the animal's body. In most chordates, the notochord is lost in the adults, and is replaced by a series of jointed vertebrae, called the spinal column or backbone, which develops around the notochord. Thus, animals with backbones are called vertebrates; they comprise one of the three subphyla of the phylum Chordata.

The second important characteristic unique to chordates is the single tubelike nerve cord that develops along the back of the embryo. During development this nerve cord becomes the central nervous system, and in most members of the group, the front end enlarges greatly to form the brain. A third characteristic chordate feature is a series of openings in the wall of the pharynx called gill slits. In some aquatic forms they serve as strainer for filter feeding and respiration. In other forms they function mainly as respiration and provide the passageways through which water passes from the gills. In chordates adapted to land life, the gill slits and the part of the skeleton which supports them may appear only in the embryo and may later be altered to serve some purpose other than respiration. In reptiles, birds, and mammals, for instance, the embryonic gill slits disappear and some of their supporting tissues become part of the jaws, tongue, the larynx, facial muscles, and inner ear as well as the thyroid and parathyroid glands.

Effects of Peptide hormones

2012-02-12T08:55:00.000-08:00

Peptide hormones and catecholamine effects are very important as they influence ion channels, enzyme activity, and the cytoplasmic JAK kinases activity. The receptors for peptide hormones and the catecholamine hormones are located on the outer surface of the target cell's plasma membrane. This location is important, since these hormones are too large and hydrophilic to diffuse through the plasma membrane. When activated by hormone binding, the receptors trigger one or more of the signal transduction pathways. That is, the activated receptors directly influence: (1) ion channels that are part of the receptors; (2) enzyme activity that is part of the receptor; (3) activity of cytoplasmic JAK kinases associated with the receptor; or (4) G proteins coupled in the plasma membrane to effector proteins—ion channels and enzymes—that generate second messengers.

The opening or closing of ion channels changes the electrical potential across the membrane. When a calcium channel is involved, the cytosolic concentration of this important ionic second messenger is changed. The changes in enzyme activity are usually very rapid (for example, due to phosphorylation) and produce changes in the conformation and hence the activity of various cellular proteins. In some cases the signal transduction pathways also lead to activation (or inhibition) of particular genes, causing a change in the rate of synthesis of the proteins coded for by these genes. Thus, peptide hormones and catecholamines may exert both rapid and delayed (gene transcription) actions on the same target cell.

Hormone Metabolism and Excretion

2012-02-11T07:41:00.000-08:00

Once a hormone has acted on the target tissue, the concentration of the hormone in the blood must be restored to normal. This is necessary to prevent excessive, possibly harmful actions of prolonged exposure of target cells to hormones. A hormone’s concentration in the plasma depends upon (1) its rate of secretion by the endocrine gland, and (2) its rate of removal from the blood. Removal, or "clearance," of the hormone occurs either by excretion or by metabolic transformation. The liver and the kidneys are the major organs that excrete or metabolize hormones. The liver and kidneys, however, are not the only routes for eliminating hormones. Sometimes the hormone is metabolized by the cells upon which it acts. Very importantly, in the case of peptide hormones, endocytosis of hormone-receptor complexes on plasma membranes enables cells to remove the hormones rapidly from their surfaces and catabolize them intracellularly. The receptors are then often recycled to the plasma membrane.

In addition, catecholamine and peptide hormones are excreted rapidly or attacked by enzymes in the blood and tissues. These hormones therefore tend to remain in the bloodstream for only brief periods —minutes to an hour. In contrast, because protein-bound hormones are less vulnerable to excretion or metabolism by enzymes, removal of the circulating steroid and thyroid hormones generally takes longer, often several hours (with thyroid hormone remaining in the plasma for days). In some cases, metabolism of the hormone after its secretion activates the hormone rather than inactivates it. In other words, the secreted hormone may be relatively or completely unable to act upon a target cell until metabolism transforms it into a substance that can act. One example is provided by testosterone, which is converted either to estradiol or dihydrotestosterone in certain of its target cells. These molecules, rather than testosterone itself, then bind to receptors inside the target cell and elicit the cell’s response.

There is another kind of "activation" that applies to a few hormones. Instead of the hormone itself being activated after secretion, it acts enzymatically on a completely different plasma protein to split off a peptide that functions as the active hormone. The best known example of this is the renin-angiotensin system; renin is not technically a hormone but an enzyme that participates in the generation of the hormone angiotensin.

Hormone Receptors

2012-02-10T17:25:00.000-08:00

Because they are transported in the blood, hormones can reach virtually all tissues. Yet the response to a hormone is highly specific, involving only the target cells for that hormone. The ability to respond depends upon the presence on (or in) the target cells of specific receptors for those hormones. The receptors for peptide hormones and catecholamines are proteins located in the plasma membranes of the target cells. In contrast, the receptors for steroid hormones and the thyroid hormones are proteins located mainly inside the target cells. The response of a target cell to a chemical messenger is the final event in a sequence that begins when the messenger binds to specific cell receptors.

Hormones can influence the ability of target cells to respond by regulating hormone receptors. In the context of hormones, up-regulation is an increase in the number of a hormone’s receptors, often resulting from a prolonged exposure to a low concentration of the hormone. This has the effect of increasing target cell responsiveness to the hormone. Down-regulation is a decrease in receptor number, often from exposure to high concentrations of the hormone. This decreases target cell responsiveness to the hormone, thus preventing overstimulation. Hormones can down-regulate or up-regulate not only their own receptors but the receptors for other hormones as well. If one hormone induces a loss of a second hormone’s receptors, the result will be a reduction of the second hormone’s effectiveness. On the other hand, a hormone may induce an increase in the number of receptors for a second hormone. In this case the effectiveness of the second hormone is increased.

This latter phenomenon, in some cases, underlies the important hormone-hormone interaction known as permissiveness. In general terms, permissiveness means that hormone A must be present for the full strength of hormone B’s effect. A low concentration of hormone A is usually all that is needed for this permissive effect, which is due to A’s ability to upregulate B’s receptors. For example, epinephrine causes a large release of fatty acids from adipose tissue, but only in the presence of permissive amounts of thyroid hormone. The major reason is that thyroid hormone stimulates the synthesis of receptors for epinephrine in adipose tissue; thus, the tissue becomes much more sensitive to epinephrine. It should be noted, however, that receptor up-regulation does not explain all cases of permissiveness; often the explanation is not known.

Peptide Hormones

2012-02-09T10:37:00.000-08:00

Most hormones are either peptides or proteins. They range in size from small peptides having only three amino acids to small proteins (some of which are glycoproteins). For convenience, it is sometimes referred to all these hormones as peptide hormones. They are water-soluble hormones which consist of a few amino acids that introduce a series of chemical reactions to change the cell's metabolism. Examples of peptide hormones include hormones of the pituitary gland and parathyroid glands.

In many cases, peptides hormones are initially synthesized on the ribosomes of the endocrine cells as larger proteins known as preprohormones, which are then cleaved to prohormones by proteolytic enzymes in the rough endoplasmic reticulum. The prohormone is then packaged into secretory vesicles by the Golgi apparatus. In this process, the prohormone is cleaved to yield the active hormone and other peptide chains found in the prohormone. Therefore, when the cell is stimulated to release the contents of the secretory vesicles by exocytosis, the other peptides are co-secreted with the hormone. In certain cases they, too, may exert hormonal effects. In other words, instead of just one peptide hormone, the cell may be secreting multiple peptide hormones that differ in their effects on target cells.

Many peptides serve as both neurotransmitters (or neuromodulators) and as hormones. For example, most of the hormones secreted by the endocrine glands in the gastrointestinal tract (for example, cholecystokinin) are also produced by neurons in the brain, where they function as neurotransmitters.

Lymph Flow Mechanism

2012-02-08T06:53:00.001-08:00

The lymphatic vessels beyond the lymphatic capillaries propel the lymph within them by their own contractions. The smooth muscle in the wall of the lymphatics exerts a pump-like action by inherent rhythmical contractions. Since the lymphatic vessels have valves similar to those in veins, these contractions produce a one-way flow toward the points at which the lymphatics enter the circulatory system. The lymphatic vessel smooth muscle is responsive to stretch, so when there is no accumulation of interstitial fluid, and therefore no entry of lymph into the lymphatics, the smooth muscle is inactive. As lymph formation increases, however, say as a result of increased fluid filtration out of blood vessel capillaries, the increased fluid entering the lymphatics stretches the walls and triggers rhythmical contractions of the smooth muscle. This constitutes a negative feedback mechanism for adjusting the rate of lymph flow to the rate of lymph formation and thereby preventing edema. In addition, the smooth muscle of the lymphatic vessels is innervated by sympathetic neurons, and excitation of these neurons in various physiological states such as exercise may contribute to increased lymph flow. Lymph flow is also enhanced by forces external to the lymphatic vessels. These include the same external forces we described for veins—the skeletal muscle pump and respiratory pump.

Electroencephalogram (EEG)

2012-02-07T07:48:00.000-08:00

Electroencephalogram (EEG) is the records of the brain electrical activity. Neural activity is manifested by the electrical signals known as graded potentials and action potentials. The electrical activity in the brain’s neurons, particularly those in the cortex near the surface of the brain, can be recorded from the outside of the head. Electrodes, which are wires attached to the head by a salty paste that conducts electricity, pick up electrical signals generated in the brain and transmit them to a machine that records them as the electroencephalogram (EEG). The majority of the electrical signal recorded in the EEG originates in the pyramidal cells of the cortex. The processes of these large cells are oriented perpendicularly to the brain’s surface, and the EEG records postsynaptic potentials in their dendrites. While we often think of electrical activity in neurons in terms of action potentials, action potentials do not usually contribute directly to the EEG. Rather, EEG patterns are largely due to graded potentials, in this case summed postsynaptic potentials in the many hundreds of thousands of brain neurons that underlie the recording electrodes.

EEG patterns are waves, albeit complex ones, with large variations in both amplitude and frequency. The wave’s amplitude, measured in microvolts, indicates how much electrical activity of a similar type is going on beneath the recording electrodes at any given time. If the amplitude is high, this indicates that many neurons are being activated simultaneously. In other words, it indicates the degree of synchronous firing of whichever neurons are generating the synaptic activity. If, on the other hand, amplitude is low, this indicates that these neurons are firing asynchronously. The amplitude may range from 0.5 to 100 microvolts. Note that EEG amplitudes are about 1000 times smaller than the amplitude of an action potential. The wave’s frequency indicates how often the wave cycles from its maximal amplitude to its minimal amplitude and back. The frequency is measured in hertz (Hz, or cycles per second) and may vary from 1 to 40 Hz or higher. Four distinct frequency ranges are found in EEG patterns. In general, lower EEG frequencies indicate less responsive states, such as sleep, whereas higher frequencies indicate increased alertness. As we will see, one stage of sleep is an exception to this general relationship.

Amine Hormones

2012-02-06T09:33:00.000-08:00

The amine hormones are all derivatives of the single amino acid tyrosine. They include the thyroid hormones, the catecholamines epinephrine and norepinephrine (produced by the adrenal medulla), and dopamine (produced by the hypothalamus). The amine hormones are stored in granules of cell cytoplasms.

Hormone Transport in the Blood

2012-02-05T06:56:00.000-08:00

Most peptide and all catecholamine hormones are water-soluble. Therefore, with the exception of a few peptides, these hormones are transported simply dissolved in plasma. In contrast, the poorly soluble steroid hormones and thyroid hormones are transported in the blood largely bound to plasma proteins. Even though the steroid and thyroid hormones exist in plasma mainly bound to large proteins, small concentrations of these hormones do exist dissolved in the plasma. The dissolved, or free, hormone is in equilibrium with the bound hormone: Free hormone + Binding protein = Hormone-protein complex. The total hormone concentration in plasma is the sum of the free and bound hormone. It is important to realize, however, that only the free hormone can diffuse across capillary walls and encounter its target cells. Therefore, the concentration of the free hormone is what is physiologically important rather than the concentration of the total hormone, most of which is bound. The degree of protein binding also influences the rate of metabolism and the excretion of the hormone.

Diffusion Across the Capillary Wall

2012-02-04T07:54:00.000-08:00

There are three basic mechanisms by which substances move across the capillary walls in most organs and tissues to enter or leave the interstitial fluid: diffusion, vesicle transport, and bulk flow. Mediated transport constitutes a fourth mechanism in the capillaries of the brain. Diffusion and vesicle transport are described in this section, and bulk flow in the next. In all capillaries, excluding those in the brain, diffusion constitutes the only important means by which net movement of nutrients, oxygen, and metabolic end products occurs across the capillary walls. As described in the next section, there is some movement of these substances by bulk flow, but the amount is negligible. Lipid-soluble substances, including oxygen and carbon dioxide, easily diffuse through the plasma membranes of the capillary endothelial cells. In contrast, ions and polar molecules are poorly soluble in lipid and must pass through small water-filled channels in the endothelial lining.

The presence of water-filled channels in the capillary walls causes the permeability of ions and small polar molecules to be quite high, although still much lower than that of lipid-soluble molecules. One location of these channels is the intercellular clefts—that is, the narrow water-filled spaces between adjacent cells. Another set of water-filled channels is provided by the fusedvesicle channels that penetrate the endothelial cells. The water-filled channels allow only very small amounts of protein to diffuse through them. Very small amounts of protein may also cross the endothelial cells by vesicle transport—endocytosis of plasma at the luminal border and exocytosis of the endocytotic vesicle at the interstitial side. Variations in the size of the water-filled channels account for great differences in the "leakiness" of capillaries in different organs. At one extreme are the "tight" capillaries of the brain, which have no intercellular clefts, only tight junctions. Therefore, water-soluble substances, even those of low molecular weight, can gain access to or exit from the brain interstitial space only by carriermediated transport through the blood-brain barrier.

At the other end of the spectrum are liver capillaries, which have large intercellular clefts as well as large holes in the plasma membranes of the endothelial cells so that even protein molecules can readily pass across them. This is very important because two of the major functions of the liver are the synthesis of plasma proteins and the metabolism of substances bound to plasma proteins. The leakiness of capillaries in most organs and tissues lies between these extremes of brain and liver capillaries. What is the sequence of events involved in transfers of nutrients and metabolic end products between capillary blood and cells? Nutrients diffuse first from the plasma across the capillary wall into the interstitial fluid, from which they gain entry to cells. Conversely, metabolic end products from the tissues move across the cells’ plasma membranes into interstitial fluid, from which they diffuse across the capillary endothelium into the plasma.

Medullary Cardiovascular Center

2012-02-03T07:17:00.000-08:00

The primary integrating center for the baroreceptor reflexes is a diffuse network of highly interconnected neurons called the medullary cardiovascular center, located in the brainstem medulla oblongata. The neurons in this center receive input from the various baroreceptors. This input determines the action potential frequency from the center along neural pathways that terminate upon the cell bodies and dendrites of the vagus (parasympathetic) neurons to the heart and the sympathetic neurons to the heart, arterioles, and veins. When the arterial baroreceptors increase their rate of discharge, the result is a decrease in sympathetic outflow to the heart, arterioles, and veins, and an increase in parasympathetic outflow to the heart. A decrease in baroreceptor firing rate results in just the opposite pattern.

As parts of the baroreceptor reflexes, angiotensin II generation and vasopressin secretion are also altered so as to help restore blood pressure. Thus, decreased arterial pressure elicits increased plasma concentrations of both these hormones, which raise arterial pressure by constricting arterioles. For simplicity, however, we focus in the rest of this chapter mainly on the sympathetic nervous system when discussing reflex control of arterioles. The roles of angiotensin II and vasopressin will be described further in Chapter 14 in the context of their effects on salt and water balance via the kidneys.

Active Hyperemia

2012-02-02T09:39:00.001-08:00

Most organs and tissues manifest an increased blood flow (hyperemia) when their metabolic activity is increased; this is termed active hyperemia. For example, the blood flow to exercising skeletal muscle increases in direct proportion to the increased activity of the muscle. Active hyperemia is the direct result of arteriolar dilation in the more active organ or tissue. The factors acting upon arteriolar smooth muscle in active hyperemia to cause it to relax are local chemical changes in the extracellular fluid surrounding the arterioles. These result from the increased metabolic activity in the cells near the arterioles. The relative contributions of the different factors implicated vary, depending upon the organs involved and on the duration of the increased activity.

The most obvious change that occurs when tissues become more active is a decrease in the local concentration of oxygen, which is used in the production of ATP by oxidative phosphorylation. On the other hand, a number of other chemical factors increase when metabolism exceeds blood flow, such as carbon dioxide (an end product of oxidative metabolism), hydrogen ions (for example, from lactic acid), and potassium ions (accumulated from repeated action
potential repolarization). Local changes in these chemical factors have been shown to cause arteriolar dilation under controlled experimental conditions, and they all probably contribute to the active hyperemia response in one or more organs.

Functions of Endothelial Cells

2012-02-01T07:05:00.000-08:00

Functions of endothelial cells are:
1- Serve as a physical lining of heart and blood vessels to which blood cells do not normally adhere.
2- Serve as a permeability barrier for the exchange of nutrients, metabolic end products, and fluid between plasma and interstitial fluid; regulate transport of macromolecules and other substances.
3- Secrete paracrine agents that act on adjacent vascular smooth muscle cells; these include vasodilators, such as prostacyclin and nitric oxide (endothelium-derived relaxing factor, EDRF), and vasoconstrictors—notably endothelin-1.
4- Mediate angiogenesis (new capillary growth).
5- Play a central role in vascular remodeling by detecting signals and releasing paracrine agents that act on adjacent cells in the blood vessel wall.
6- Contribute to the formation and maintenance of extracellular matrix.
7- Produce growth factors in response to damage.
8- Secrete substances that regulate platelet clumping, clotting, and anticlotting.
9- Synthesize active hormones from inactive precursors.
10- Extract or degrade hormones and other mediators.
11- Secrete cytokines during immune responses.
12- Influence vascular smooth-muscle proliferation in the disease atherosclerosis.

Arterial Baroreceptors

2012-01-31T07:37:00.000-08:00

The arterial baroreceptors are nerve ending receptors (located in the carotid sinuses and the aortic arch) which react to changes in blood pressure. The reflexes that homeostatically regulate arterial pressure originate primarily with this arterial receptors that respond to changes in pressure. High in the neck, each of the two major vessels supplying the head, the common carotid arteries, divides into two smaller arteries. At this division, the wall of the artery is thinner than usual and contains a large number of branching, vinelike nerve endings. This portion of the artery is called the carotid sinus (the term "sinus" denotes a recess, space, or dilated channel). Its nerve endings are highly sensitive to stretch or distortion. Since the degree of wall stretching is directly related to the pressure within the artery, the carotid sinuses serve as pressure receptors, or baroreceptors. An area functionally similar to the carotid sinuses is found in the arch of the aorta and is termed the aortic arch baroreceptor. The two carotid sinuses and the aortic arch baroreceptor constitute the arterial baroreceptors. Afferent neurons from them travel to the brainstem and provide input to the neurons of cardiovascular control centers there.

Velocity of Capillary Blood Flow

2012-01-30T14:49:00.000-08:00

When a continuous stream moves through consecutive sets of tubes, the velocity of flow decreases as the sum of the cross-sectional areas of the tubes increases. This is precisely the case in the cardiovascular system. The blood velocity is very great in the aorta, slows progressively in the arteries and arterioles, and then slows markedly as the blood passes through the huge cross-sectional area of the capillaries. Slow forward flow through the capillaries maximizes the time available for substances to exchange between the blood and interstitial fluid. The velocity of flow then progressively increases in the venules and veins because the cross-sectional area decreases. To reemphasize, flow velocity is not dependent on proximity to the heart but rather on total cross-sectional area of the vessel type. The cross-sectional area of the capillaries accounts for another important feature of capillaries: Because each capillary is very narrow, it offers considerable resistance to flow, but the huge total number of capillaries provides such a large cross-sectional area that the total resistance of all the capillaries is much less than that of the arterioles.

Heart Sounds (Murmurs)

2012-01-29T08:54:00.000-08:00

Two heart sounds resulting from cardiac contraction are normally heard through a stethoscope placed on the chest wall. The first sound, a soft low-pitched lub, is associated with closure of the atrioventricular (AV) valves; the second sound, a louder dup, is associated with closure of the pulmonary and aortic valves. The lub marks the onset of systole while the dup occurs at the onset of diastole. These sounds, which result from vibrations caused by the closing valves, are perfectly normal, but other sounds, known as heart murmurs, can be a sign of heart disease.

Murmurs can be produced by blood flowing rapidly in the usual direction through an abnormally narrowed valve (stenosis), by blood flowing backward through a damaged, leaky valve (insufficiency), or by blood flowing between the two atria or two ventricles through a small hole in the wall separating them (called a septal defect). The exact timing and location of the murmur provide the physician with a powerful diagnostic clue. For example, a murmur heard throughout systole suggests a stenotic pulmonary or aortic valve, an insufficient AV valve, or a hole in the interventricular septum. In contrast, a murmur heard during diastole suggests a stenotic AV valve or an insufficient pulmonary or aortic valve.

Sympathetic and Parasympathetic Influence on Heart Rate

2012-01-28T10:05:00.000-08:00

Rhythmical beating of the heart at a rate of approximately 100 beats/min will occur in the complete absence of any nervous or hormonal influences on the SA node. This is the inherent autonomous discharge rate of the SA node. The heart rate may be much lower or higher than this, however, since the SA node is normally under the constant influence of nerves and hormones. A large number of parasympathetic and sympathetic postganglionic fibers end on the SA node. Activity in the parasympathetic (vagus) nerves causes the heart rate to decrease, whereas activity in the sympathetic nerves increases the heart rate. In the resting state, there is considerably more parasympathetic activity to the heart than sympathetic, and so the normal resting heart rate of about 70 beats/min is well below the inherent rate of 100 beats/min.

Sympathetic stimulation increases the slope of the pacemaker potential by increasing the If (sodium) and T-type calcium currents. This causes the SA node cells to reach threshold more rapidly and the heart rate to increase. Stimulation of the parasympathetics has the opposite effect—the slope of the pacemaker potential decreases due to a reduction in the inward currents. Threshold is thus reached more slowly, and heart rate decreases. Parasympathetic stimulation also hyperpolarizes the plasma membrane of SA node cells by increasing the permeability to potassium. The pacemaker potential thus starts from a more negative value (closer to the potassium equilibrium potential). Factors other than the cardiac nerves can also alter heart rate. Epinephrine, the main hormone liberated from the adrenal medulla, speeds the heart by acting on the same beta-adrenergic receptors in the SA node as norepinephrine released from neurons.

The heart rate is also sensitive to changes in body temperature, plasma electrolyte concentrations, hormones other than epinephrine, and a metabolite—adenosine— produced by myocardial cells. These factors are normally of lesser importance, however, than the cardiac nerves. sympathetic and parasympathetic neurons innervate not only the SA node but other parts of the conducting system as well. Sympathetic stimulation also increases conduction velocity through the AV node and other cells of the conducting system, whereas parasympathetic stimulation decreases the rate of spread of excitation through the AV node.

Cardiac Stroke Volume

2012-01-27T09:14:00.000-08:00

Cardiac stroke volume is the volume of blood ejected by each ventricle during each contraction, equal to the difference between the end-diastolic volume and the end-systolic volume. So, stroke volume determines cardiac output. Let us remember that the ventricles do not completely empty themselves of blood during contraction. Therefore, a more forceful contraction can produce an increase in stroke volume by causing greater emptying. Changes in the force of contraction can be produced by a variety of factors, but three are dominant under most physiological and pathophysiological conditions: 1) changes in the end-diastolic volume (the volume of blood in the ventricles just before contraction, sometimes referred to as the pre-load); 2) changes in the magnitude of sympathetic nervous system input to the ventricles; and 3) afterload, that is, the arterial pressures against which the ventricles pump.

The mechanical properties of cardiac muscle are the basis for an inherent mechanism for altering stroke volume: The ventricle contracts more forcefully during systole when it has been filled to a greater degree during diastole. In other words, all other factors being equal, the stroke volume increases as the end-diastolic volume increases. This is illustrated graphically as a ventricular function curve. This relationship between stroke volume and end-diastolic volume is known as the Frank-Starling mechanism (also called Starling’s law of the heart) in recognition of the two physiologists who identified it. What accounts for the Frank-Starling mechanism? Basically it is simply a length-tension relationship, as described for skeletal muscle in Chapter 9, in that enddiastolic volume is a major determinant of how stretched the ventricular sarcomeres are just before contraction. Thus, the greater the end-diastolic volume, the greater the stretch, and the more forceful the contraction.

However, there is an important difference between the length-tension relationship in skeletal and cardiac muscle. The normal point for cardiac muscle in a resting individual is not at its optimal length for contraction, as it is for most resting skeletal muscles, but is on the rising phase of the curve. For this reason, additional stretching of the cardiac muscle fibers by greater filling causes increased force of contraction. The significance of the Frank-Starling mechanism is as follows: At any given heart rate, an increase in the venous return—the flow of blood from the veins into the heart—automatically forces an increase in cardiac output by increasing end-diastolic volume and thus stroke volume. One important function of this relationship is maintaining the equality of right and left cardiac outputs. Should the right heart, for example, suddenly begin to pump more blood than the left, the increased blood flow to the left ventricle would automatically produce an increase in left ventricular output. This ensures that blood will not accumulate in the lungs.

Cardiac Output

2012-01-26T07:29:00.000-08:00

The volume of blood pumped by each ventricle per minute is called the cardiac output (CO), usually expressed in liters per minute. It is also the volume of blood flowing through either the systemic or the pulmonary circuit per minute. The cardiac output is determined by multiplying the heart rate (HR)—the number of beats per minute— and the stroke volume (SV)—the blood volume ejected by each ventricle with each beat: CO = HR x SV. Thus, if each ventricle has a rate of 72 beats/min and ejects 70 ml of blood with each beat, the cardiac output is: CO = 72 beats/min x 0.07 L/beat = 5.0 L/min

These values are within the normal range for a resting average-sized adult. Coincidentally, total blood volume is also approximately 5 L, and so essentially all the blood is pumped around the circuit once each minute. During periods of strenuous exercise in well-trained athletes, the cardiac output may reach 35 L/min; that is, the entire blood volume is pumped around the circuit seven times a minute! Even sedentary, untrained individuals can reach cardiac outputs of 20–25 L/min during exercise. The following description of the factors that alter the two determinants of cardiac output—heart rate and stroke volume—applies in all respects to both the right and left sides of the heart since stroke volume and heart rate are the same for both under steady-state conditions. It must also be emphasized that heart rate and stroke volume do not always change in the same direction. For example, stroke volume decreases following blood loss while heart rate increases. These changes produce opposing effects on cardiac output.

Cardiac Excitation Sequence

2012-01-25T09:27:00.000-08:00

The SA node is the normal pacemaker for the entire heart. Its depolarization normally initiates the cardiac excitation sequence, generating the action potential that leads to depolarization of all other cardiac muscle cells, and so its discharge rate determines the heart rate, the number of times the heart contracts per minute. The action potential initiated in the SA node spreads throughout the myocardium, passing from cell to cell by way of gap junctions. The spread throughout the right atrium and from the right atrium to the left atrium does not depend on fibers of the conducting system. The conduction through atrial muscle cells is rapid enough that the two atria are depolarized and contract at essentially the same time. However, the spread of the action potential to the ventricles is more complicated and involves the rest of the conducting system. The link between atrial depolarization and ventricular depolarization is a portion of the conducting system called the atrioventricular (AV) node, which is located at the base of the right atrium. The action potential spreading through the muscle cells of the right atrium causes depolarization of the AV node. This node has a particularly important characteristic: the propagation of action potentials through the AV node is relatively slow (requiring approximately 0.1 s). This results in a delay that allows atrial contraction to be completed before ventricular excitation occurs.

After leaving the AV node, the impulse enters the wall—the interventricular septum—between the two ventricles. This pathway has conducting-system fibers termed the bundle of His (or atrioventricular bundle) after its discoverer (pronounced Hiss). It should be emphasized that the AV node and the bundle of His constitute the only electrical link between the atria and the ventricles. Except for this pathway, the atria are completely separated from the ventricles by a layer of nonconducting connective tissue. Within the interventricular septum the bundle of His divides into right and left bundle branches, which eventually leave the septum to enter the walls of both ventricles. These fibers in turn make contact with Purkinje fibers, large conducting cells that rapidly distribute the impulse throughout much of the ventricles. Finally, the Purkinje fibers make contact with ventricular myocardial cells, by which the impulse spreads through the rest of the ventricles. The rapid conduction along the Purkinje fibers and the diffuse distribution of these fibers cause depolarization of all right and left ventricular cells more or less simultaneously and ensure a single coordinated contraction. Actually, though, depolarization and contraction begin slightly earlier in the bottom (apex) of the ventricles and spread upward. The result is a more efficient contraction, like squeezing a tube of toothpaste from the bottom up.

Molecular Shape

2012-01-24T09:14:00.000-08:00

When atoms are linked together, molecules with various shapes can be formed. Although we draw diagrammatic structures of molecules on flat sheets of paper, molecules are actually three-dimensional. When more than one covalent bond is formed with a given atom, the bonds are distributed around the atom in a pattern that may or may not be symmetrical. Molecules are not rigid, inflexible structures. Within certain limits, the shape of a molecule can be changed without breaking the covalent bonds linking its atoms together. A covalent bond is like an axle around which the joined atoms can rotate. A sequence of six carbon atoms can assume a number of shapes as a result of rotations around various covalent bonds. As we shall see, the three-dimensional, flexible shape of molecules is one of the major factors governing molecular interactions.

Lateral Superior Genicular Artery

2012-01-23T08:01:00.000-08:00

The lateral superior genicular artery is a blood vessel arising from the descending branch of lateral femoral circumflex artery. It runs around the exterior border of the femur bone lower end (lateral candyle), towards the front of the knee, just above the patella. The lateral superior genicular artery supplies the vastus lateralis muscle and the lower part of femur, forming an anastomotic arch across the front of this bone with the highest genicular and the medial inferior genicular arteries.

Inferior Thyroid Artery

2012-01-22T11:24:00.000-08:00

The inferior thyroid artery is a blood vessel which springs from the thyrocervical trunk. It runs up diagonally towards the center of the neck and then goes into the tracheo-oesophageal region. The inferior thyroid gland supplies the thyroid gland with oxygenated blood.

Lateral Sacral Artery

2012-01-21T06:53:00.000-08:00

The lateral sacral arteries are oxygen-rich blood vessels that originate from the second branch of the posterior division of internal iliac artery, giving off branches which run into the anterior sacral foramina to supply related bone and soft tissues, structures in the vertebral canal, and skin and muscle posterior to the sacrum.

As a matter of fact, the lateral sacral arteries (or artery) show great irregularity in development. According to a study, from 100 subjects: 48 had two lateral sacral arteries, 30 had a single lateral sacral artery, and 22 had three lateral sacral arteries. In some cases, one lateral sacral artery came off directly from the internal iliac artery.

Paracrine and Autocrine Agents

2012-01-20T07:47:00.000-08:00

Paracrine agents are chemical messengers that participate in local communications between cells. Paracrine agents are synthesized by cells and released, once given the appropriate stimulus, into the extracellular fluid. They then diffuse to neighboring cells, some of which are their target cells. Given this broad definition, neurotransmitters theoretically could be classified as a subgroup of paracrine agents, but by convention they are not. Paracrine agents are generally inactivated rapidly by locally existing enzymes so that they do not enter the bloodstream in large quantities. There is one category of local chemical messengers that are not intercellular messengers—that is, they do not communicate between cells. Rather, the chemical is secreted by a cell into the extracellular fluid and then acts upon the very cell that secreted it. Such messengers are termed autocrine agents. Frequently a messenger may serve both paracrine and autocrine functions simultaneously—that is, molecules of the messenger released by a cell may act locally on adjacent cells as well as on the same cell that released the messenger.

One of the most exciting developments in physiology today is the identification of a growing number of paracrine /autocrine agents and the extremely diverse effects they exert. Their structures span the gamut from a simple gas (nitric oxide) to fatty acid derivatives to peptides and amino acid derivatives. They tend to be secreted by multiple cell types in many tissues and organs. According to their structures and functions, they can be gathered into families; for example, one such family constitutes the "growth factors," encompassing more than 50 distinct molecules, each of which is highly effective in stimulating certain cells to divide and/or differentiate.

Hibernation

2012-01-19T13:44:00.000-08:00

Hibernation is a period of dormancy (sleep) characterized by slow metabolism, inactivity, and in some cases low body temperature. Hibernation ends when the organism is exposed to favorable environmental conditions. Bears, bats, squirrels, and some species of lemurs are known to fall into this slow metabolic state in winter time in order save energy.

Intercellular Chemical Messengers

2012-01-19T08:05:00.000-08:00

Essential to reflexes and local homeostatic responses, and therefore to homeostasis, is the ability of cells to communicate with one another. In the majority of cases, this communication between cells—intercellular communication— is performed by chemical messengers. There are three categories of such messengers: hormones, neurotransmitters, and paracrine agents. A hormone functions as a chemical messenger that enables the hormone-secreting cell to communicate with cells acted upon by the hormone—its target cells—with the blood acting as the delivery system. Most nerve cells (neurons) communicate with each other or with effector cells by means of chemical messengers called neurotransmitters. Thus, one nerve cell alters the activity of another by releasing from its ending a neurotransmitter that diffuses through the extracellular fluid separating the two nerve cells and acts upon the second cell. Similarly, neurotransmitters released from nerve cells into the extracellular fluid in the immediate vicinity of effector cells constitute the controlling input to the effector cells. Chemical messengers participate not only in reflexes but also in local responses. Chemical messengers involved in local communication between cells are known as paracrine agents.

Pathway of the Protopathic Sensibility

2012-01-18T06:41:00.000-08:00

The pathway of the protopathic sensibility originates from small nerve cells in the spinal ganglia, called protopathic neurons. They send thin, poorly myelinated or unmyelinated nerve fibers for the senses of pain and temperature. Their centripetal axons enter the spinal cord through the lateral part of the posterior root. They bifurcate in Lissauer’s tract and terminate in the dorsal border region of the substantia gelatinosa and in the posterior horn of the grey substance of spinal cord. The secondary fibers cross to the opposite side and ascend in the anterolateral funiculus as lateral spinothalamic tract (2nd neuron). The tract does not form a discrete fiber bundle but consists of loosely arranged fibers that are mixed with fibers of other systems. The fibers entering at various root levels join ventromedially. Thus, the sacral fibers lie at the surface, and the cervical fibers that joined last lie in the inner part of the anterolateral funiculus.

In the medulla oblongata, the lateral spinothalamic tract (spinal lemniscus) is located at its lateral margin above the olive and gives off numerous collaterals to the reticular formation. Here, too, a considerable portion of the fibers (spinoreticular tract) terminate. The reticular formation is part of the ascending activation system, the stimulation of which puts the organism into a state of alertness. Hence, the impulses transmitted via the pain pathway not only cause a conscious sensation but also increase the attention via the reticular formation. By contrast, the pathway of the epicritic sensibility runs through the brain stem without giving off any collaterals.

Septal Area

2012-01-17T09:51:00.000-08:00

The septal area, or septal nuclei, are structures in the brain hemispheres in the form of thin sheets of brain tissue. They form the medial wall of the lateral ventricle frontal horn. From the septal area, there are strong connections to the hippocampus, the central structure of the limbic system. Cholinergic and GABAergic neurons of the medial septal nucleus project to the hippocampus and the dentate gyrus; collaterals of the pyramidal cells project back to the lateral septal nucleus. As with stimulation of the amygdaloid body, electrical stimulation of the septal area triggers oral reactions (licking, chewing, retching), excretory reactions (defecation, urination), and sexual reactions (erection). The septal area, especially the diagonal band of Broca, is also the preferred localization for self-stimulation experiments in the rat.

Geotropism

2012-01-16T15:18:00.000-08:00

Geotropism is a growth response which is oriented by gravitational stimulus. For example, regardles of the orientation in which the plant is placed the stem generally grows up and the roots down.

Saphenous Nerve

2012-01-16T09:36:00.000-08:00

The saphenous nerve is an exclusively sensory nerve that arises from the femoral nerve as its longest branch. It runs to the adductor canal and enters into it as it penetrates the vastoadductor membrane, extending along the medial side of the knee joint and the lower leg together with the great saphenous vein down to the medial ankle. Below the knee joint, the saphenous nerve gives off the infrapatellar branch which supplies the skin below the patella. The remaining branches, the medial crural cutaneous branches, supply the skin of the anterior and medial aspects of the lower leg. The supplied area extends on the anterior side over the edge of the tibia and may reach to the great toe along the medial aspect of the foot.

Inferior Gluteal Nerve

2012-01-15T09:13:00.000-08:00

The inferior gluteal nerve is a nerve which arises from the sacral plexus (L5-S2). It leaves the pelvis through the infrapiriform foramen and gives off several branches to the gluteus maximus muscle. Paralysis of the nerve weakens extension of the hip joint, for example, when standing up or climbing stairs.

Lateral Cutaneous Nerve of Thigh

2012-01-14T07:25:00.000-08:00

The lateral cutaneous nerve of thigh originates from the lumbar plexus (L2-L3) and runs over the iliac muscle to below the superior anterior iliac spine. It then extends underneath the inguinal ligament through the lateral part of the muscular lacuna to the outer aspect of the thigh and passes through the fascia lata to the skin. The lateral cutaneous nerve of thigh is exclusively sensory and supplies the skin of the lateral aspect of the thigh down to the level of the knee.

Obturator Nerve

2012-01-13T09:37:00.000-08:00

The obturator nerve is a nerve which arises from the lumbar plexus (L2–L4) and provides motor innervation to the adductor muscles of the thigh. Medial to the greater psoas muscle, the obturator nerve extends along the lateral wall of the small pelvis down to the obturator canal through which it passes to reach the thigh. It gives off a muscular branch to the external obturator muscle and then divides into a superficial branch and a deep branch. The superficial branch runs between the long adductor muscle and short adductor muscle and innervates both. The nerve also gives off branches to the pectineal muscle and the gracilis muscle and finally terminates in a cutaneous branch to the distal region of the medial aspect of the thigh. The deep branch runs along the external obturator muscle and then down to the great adductor muscle. Paralysis of the obturator nerve, for example, as a result of pelvic fracture, causes loss of adductor muscle function. This restricts standing and walking, and the affected leg can no longer be crossed over the other leg.

Cross Sections of the Spinal Cord

2012-01-12T06:53:00.000-08:00

Cross sections of the spinal cord at different levels vary considerably. In the regions of cervical enlargement and lumbar enlargement, the cross sectional area is larger than in the rest of the spinal cord; it is largest at the C4–C5 and L4–L5 levels. In both swellings, the numerous nerves that supply the extremities cause an increase in gray matter. The white matter is most extensive in the cervical region and diminishes gradually in caudal direction; the ascending sensory tracts increase in number from the sacral to the cervical region as more fibers are added, while the descending motor tracts decrease from the cervical to the sacral regions as fibers terminate at various levels.

The butterfly configuration of the gray matter changes in shape at the various levels, and so does the posterolateral tract. The posterior horn is narrowing the cervical spinal cord; its tip ends in the cap-shaped marginal zone (nucleus posteromarginalis). The lateral angle between the posterior and anterior horn is occupied by the reticular formation. The gelatinous substance (Rolando’s substance) contains small, mostly peptidergic neurons where posterior root fibers of various calibers terminate; it also contains descending fibers from the brain stem. Unmyelinated processes of neurons ascend or descend for one to four root levels within the posterolateral tract (Lissauer’s tract) and then reenter into the gelatinous substance. Some of the processes run within the lateral spinothalamic tract to the thalamus. The fibers of proprioceptive sensibility in the muscles (muscle spindles) terminate in the posterior thoracic nucleus (dorsal nucleus of Clarke) where the tracts to the cerebellum begin. The reduced gray matter of the thoracic spinal cord has a slender posterior horn with a prominent dorsal nucleus. In the plump posterior horn of the lumbar and sacral spinal cords, the gelatinous substance is much enlarged and borders dorsally on the narrow band of the marginal zone.

The lateral horn forms in the thoracic spinal cord the lateral intermediate substance. It contains sympathetic nerve fibers mainly for the vasomotor system, the efferent fibers of which emerge via the anterior root. Sympathetic neurons also lie medially in the intermediomedial nucleus. In the sacral spinal cord, parasympathetic neurons form the intermediolateral nucleus und intermediomedial nucleus. The anterior horn expands in the cervical spinal cord and contains several nuclei with large motor neurons, all of which are cholinergic.

Fatty Acids and Eicosanoids

2012-01-11T07:12:00.000-08:00

A fatty acid consists of a chain of carbon and hydrogen atoms with a carboxyl group at one end. Thus, fatty acids contain two oxygen atoms in addition to their complement of carbons and hydrogen. Because fatty acids are synthesized in the body by the linking together of two-carbon fragments, most fatty acids have an even number of carbon atoms (with 16- and 18-carbon). When all the carbons in a fatty acid are linked by single covalent bonds, the fatty acid is said to be a saturated fatty acid (because all the carbons are saturated with covalently bound H). Some fatty acids contain one or more double bonds, and these are known as unsaturated fatty acids. If one double bond is present, the acid is said to be monounsaturated, and if there is more than one double bond, polyunsaturated. Some fatty acids can be altered to produce a special class of molecules that regulate a number of cell functions. These modified fatty acids, collectively termed eicosanoids, are derived from the 20-carbon, polyunsaturated fatty acid arachidonic acid.

Corticostriate Fibers

2012-01-10T07:15:00.000-08:00

Corticostriate fibers are fibers that connect the cerebral cortex with the striatum, extending from all areas of the neocortex to the striatum. They are made up of medium-sized axons and small pyramidal cells of the fifth layer. However, there are no fiber connections extending from the striatum to the cortex. The corticostriate projection reveals a topical organization: the frontal lobe projects to the head of the caudate nucleus and is followed by the parietal lobe, the occipital lobe, and the temporal lobe. The projection of the precentral motor area in the putamen reveals a somatotopic organization: head, arm, and leg. A somatotopic projection of the postcentral sensory area to the dorsolateral region of the caudate nucleus has been demonstrated. The fibers from areas adjoining the central sulcus are the only ones that partly cross via the corpus callosum to the contralateral neostriatum.

Internal Mammary Artery

2012-01-09T07:58:00.000-08:00

The internal mammary artery, also known as the internal thoracic artery, is one of a pair of blood vessels that originates from the subclavian artery. It runs down along the sternum, giving off the sternal branches and perforating branches as it goes, to finally divides into two smaller arteries: the musculophrenic artery and the superior epigastric artery. The internal mammary artery supplies the anterior chest wall and the breast with oxygenated wall.

Lateral Thoracic Artery

2012-01-08T07:29:00.000-08:00

The lateral thoracic artery is an oxygen-rich blood vessels that arises from the axillary artery and runs along the lower border of the pectoralis minor muscle, giving off branches to the axillary limph nodes and subscapularis muscle. These branches anastomoses with the subscapular, intercostal arteries, and internal thoracic artery. The lateral thoracic artery supplies the lateral structures of the thorax and breast.

Phospholipids

2012-01-07T06:04:00.000-08:00

Phospholipids are similar in overall structure to triglycerides, with one important difference. The third hydroxyl group of glycerol, rather than being attached to a fatty acid, is linked to phosphate. In addition, a small polar or ionized nitrogen-containing molecule is usually attached to this phosphate. These groups constitute a polar (hydrophilic) region at one end of the phospholipid, whereas the fatty acid chains provide a nonpolar (hydrophobic) region at the opposite end. Therefore, phospholipids are amphipathic. In water, they become organized into clusters, with their polar ends attracted to the water molecules.

Triglycerides

2012-01-06T09:06:00.000-08:00

Triglycerides, also known as triacylglycerols, constitute the majority of the lipids in the body, and it is these molecules that are generally referred to simply as "fat." Triglycerides are formed by the linking together of glycerol, a three-carbon alcohol, with three fatty acids. Each of the three hydroxyl groups in glycerol is linked to the carboxyl group of a fatty acid by the removal of a molecule of water. The three fatty acids in a molecule of triglyceride need not be identical. Therefore, a variety of fats can be formed with fatty acids of different chain lengths and degrees of saturation. Animal fats generally contain a high proportion of saturated fatty acids, whereas vegetable fats contain more unsaturated fatty acids. Saturated fats tend to be solid at low temperatures.

Unsaturated fats, on the other hand, have a very low melting point, and thus they are liquids (oil) even at very low temperatures. Thus, heating a hamburger on the stove melts the saturated animal fats, resulting in grease appearing in the frying pan. When allowed to cool, however, the oily grease returns to its solid form. Hydrolysis of triglycerides releases the fatty acids from glycerol, and these products can then be metabolized to provide energy for cell functions. Thus, the storage of energy in the form of triglycerides and polysaccharides requires dehydration reactions (removal of a molecule of water). Similarly, both are broken down to usable forms of fuel by hydrolysis.

Superior Thyroid Artery

2012-01-05T09:35:00.000-08:00

The superior thyroid artery is an oxygen-rich blood vessel that originates from the external carotid artery. From its point of origin, it descends to the anterior surface of the thyroid gland, giving off the superior laryngeal artery and other smaller branches to the hyoid bone and sometimes to the externocleidomastoid muscle. The superior thyroid artery supplies the thyroid gland, the larynx, hyoid bone, and muscles of the neck.

Thyrocervical Trunk

2012-01-04T07:39:00.000-08:00

The thyrocervical trunk is a short and thick artery which arises from subclavian artery, close to the medial border of the scalenus anterior muscle. It divides into the inferior thyroid, suprascapular, and transverse cervical arteries. The thyrocervical trunk and its branches supplies oxygen-rich blood to the numerous muscles and bones in the head, neck, and back, as well as the inferior portion of the thyroid gland.

Suprascapular Artery

2012-01-03T09:41:00.000-08:00

The suprascapular artery arises from thyrocervical trunk, which in turn comes off the subclavian artery. It runs roughly parallel to the clavicle through the lateral cervical region and a cross the tansverse ligament of the scapula to the supra- and infraspinatus muscles. The suprascapular artery supplies the clavicular, deltoid and scapular regions.

Superior Gluteal Artery

2012-01-02T07:32:00.000-08:00

The superior gluteal artery originates from the internal iliac artery as its largest branch. Running backward between the lumbosacral trunk and the first sacral nerve, the superior gluteal artery leaves the pelvis through the suprapiriform part of the greater sciatic foramen to supply the gluteal muscles. It divides into several branches which anastomoses with inferior gluteal artery.

Circumflex Scapular Artery

2012-01-01T10:25:00.000-08:00

The circumflex scapular artery is an oxygen-rich blood vessel that arises from the subscapular artery. Going around the axillary border of the scapula, it runs through between the Teres minor and Teres major muscles as it gives off two branches, one of which anastomoses with the suprascapular artery. The circumflex scapular supplies the muscles of the shoulder and shoulder blade.

Highest Thoracic Artery

2011-12-31T06:52:00.000-08:00

The highest thoracic artery is an oxygen-rich blood vessel which normally arises from the axillary artery. Also known as superior thoracic artery, it travels forward along the upper border of the pectoralis minor; then it runs between it and the pectoralis major. The point of origin of the highest thoracic artery varies according to individuals. Dividing in two smaller branches, it supplies the muscles of the chest wall.

Plantar Metatarsal Arteries

2011-12-30T15:33:00.000-08:00

The plantar metatarsal arteries arise from the plantar arch and gives off the plantar digital arteries, which supply the sides of the toes, and the perforating branches to the dorsal metatarsal branches.

Membrane Junctions (Desmosomes)

2011-12-29T08:57:00.000-08:00

In addition to providing a barrier to the movements of molecules between the intracellular and extracellular fluids, plasma membranes are involved in interactions between cells to form tissues. Many cells are physically joined at discrete locations along their membranes by specialized types of junctions known as desmosomes, tight junctions, and gap junctions. Desmosomes consist of a region between two adjacent cells where the apposed plasma membranes are separated by about 20 nm and have a dense accumulation of protein at the cytoplasmic surface of each membrane and in the space between the two membranes. Protein fibers extend from the cytoplasmic surface of desmosomes into the cell and are linked to other desmosomes on the opposite side of the cell. Desmosomes hold adjacent cells firmly together in areas that are subject to considerable stretching, such as in the skin. The specialized area of the membrane in the region of a desmosome is usually disk-shaped, and these membrane junctions could be likened to rivets or spot-welds.

Asecond type of membrane junction, the tight junction, is formed when the extracellular surfaces of two adjacent plasma membranes are joined together so that there is no extracellular space between them. Unlike the desmosome, which is limited to a diskshaped area of the membrane, the tight junction occurs in a band around the entire circumference of the cell. Most epithelial cells are joined by tight junctions. For example, epithelial cells cover the inner surface of the intestinal tract, where they come in contact with the digestion products in the cavity (lumen) of the tract. During absorption, the products of digestion move across the epithelium and enter the blood. This transfer could take place theoretically by movement either through the extracellular space between the epithelial cells or through the epithelial cells themselves.

Cell Membrane Structure

2011-12-28T09:21:00.000-08:00

The cell membrane structure consist of a double layer of lipid molecules in which proteins are embedded. The major membrane lipids are phospholipids. These are amphipathic molecules: one end has a charged region, and the remainder of the molecule, which consists of two long fatty acid chains, is nonpolar. The phospholipids in cell membranes are organized into a bimolecular layer with the nonpolar fatty acid chains in the middle. The polar regions of the phospholipids are oriented toward the surfaces of the membrane as a result of their attraction to the polar water molecules in the extracellular fluid and cytosol. No chemical bonds link the phospholipids to each other or to the membrane proteins, and therefore, each molecule is free to move independently of the others. This results in considerable random lateral movement of both membrane lipids and proteins parallel to the surfaces of the bilayer. In addition, the long fatty acid chains can bend and wiggle back and forth. Thus, the lipid bilayer has the characteristics of a fluid, much like a thin layer of oil on a water surface, and this makes the membrane quite flexible. This flexibility, along with the fact that cells are filled with fluid, allows cells to undergo considerable changes in shape without disruption of their structural integrity. Like a piece of cloth, membranes can be bent and folded but cannot be stretched without being torn.

The plasma membrane also contains cholesterol (about one molecule of cholesterol for each molecule of phospholipid), whereas intracellular membranes contain very little cholesterol. Cholesterol, a steroid, is slightly amphipathic because of a single polar hydroxyl group on its nonpolar ring structure. Therefore, cholesterol, like the phospholipids, is inserted into the lipid bilayer with its polar region at a bilayer surface and its nonpolar rings in the interior in association with the fatty acid chains. Cholesterol associates with certain classes of plasma membrane phospholipids and proteins, forming organized clusters that function in the pinching off of portions of the plasma membrane to form vesicles that deliver their contents to various intracellular organelles. There are two classes of membrane proteins: integral and peripheral. Integral membrane proteins are closely associated with the membrane lipids and cannot be extracted from the membrane without disrupting the lipid bilayer. Like the phospholipids, the integral proteins are amphipathic, having polar amino acid side chains in one region of the molecule and nonpolar side chains clustered together in a separate region. Because they are amphipathic, integral proteins are arranged in the membrane with the same orientation as amphipathic lipids—the polar regions are at the surfaces in association with polar water molecules, and the nonpolar regions are in the interior in association with nonpolar fatty acid chains. Like the membrane lipids, many of the integral proteins can move laterally in the plane of the membrane, but others are immobilized because they are linked to a network of peripheral proteins located primarily at the cytosolic surface of the membrane.

Most integral proteins span the entire membrane and are referred to as transmembrane proteins. Most of these transmembrane proteins cross the lipid bilayer several times. These proteins have polar regions connected by nonpolar segments that associate with the nonpolar regions of the lipids in the membrane interior. The polar regions of transmembrane proteins may extend far beyond the surfaces of the lipid bilayer. Some transmembrane proteins form channels through which ions or water can cross the membrane, whereas others are associated with the transmission of chemical signals across the membrane or the anchoring of extracellular and intracellular protein filaments to the plasma membrane. Peripheral membrane proteins are not amphipathic and do not associate with the nonpolar regions of the lipids in the interior of the membrane. They are located at the membrane surface where they are bound to the polar regions of the integral membrane proteins. Most of the peripheral proteins are on the cytosolic surface of the plasma membrane where they are associated with cytoskeletal elements that influence cell shape and motility.

Polypeptides

2011-12-27T11:38:00.000-08:00

A polypeptide is a sequence of amino acids linked by peptide bonds, which are formed between the amino and carboxyl group. Thus, the peptide bonds form the backbone of polypeptides, and the side chain of each amino acid sticks out from the side of the chain. By convention, if the number of amino acids in a polypeptide is 50 or less, the molecule is known as a peptide; if the sequence is more than 50 amino acid units, it is known as a protein. The number 50 is somewhat arbitrary but is useful in distinguishing among large and small polypeptides. Small peptides have certain chemical properties that differ from proteins, e.g., peptides are generally soluble in acid, while proteins generally are not.

Atom

2011-12-27T06:56:00.000-08:00

The units of matter that form all chemical substances are called atoms. The smallest atom, hydrogen, is approximately 2.7 billionths of an inch in diameter. Each type of atom, such as carbon, hydrogen, oxygen, and so on, is called a chemical element. A one- or two-letter symbol is used as a shorthand identification for each element. Although slightly more than 100 elements exist in the universe, only 24 are known to be essential for the structure and function of the human body. The chemical properties of atoms can be described in terms of three subatomic particles—protons, neutrons, and electrons. The protons and neutrons are confined to a very small volume at the center of an atom, the atomic nucleus, whereas the electrons revolve in orbits at various distances from the nucleus. This miniature solar-system model of an atom is an oversimplification, but it is sufficient to provide a conceptual framework for understanding the chemical and physical interactions of atoms.

Each of the subatomic particles has a different electric charge: Protons have one unit of positive charge, electrons have one unit of negative charge, and neutrons are electrically neutral. Since the protons are located in the atomic nucleus, the nucleus has a net positive charge equal to the number of protons it contains. The entire atom has no net electric charge, however, because the number of negatively charged electrons orbiting the nucleus is equal to the number of positively charged protons in the nucleus.

Atomic Weight

2011-12-26T12:12:00.000-08:00

Atoms have very little mass. The atomic weight is the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12, which is known as the unified atomic mass unit. Thus, the atomic weight scale indicates an atom’s mass relative to the mass of other atoms. This scale is based upon assigning the carbon atom a mass of 12. On this scale, a hydrogen atom has an atomic weight of approximately 1, indicating that it has one-twelfth the mass of a carbon atom; a magnesium atom, with an atomic weight of 24, has twice the mass of a carbon atom.

Atomic Number

2011-12-26T07:23:00.000-08:00

Each chemical element (atom) contains a specific number of protons, which are located in its nucleus, and it is this number that distinguishes one type of atom from another. This number is known as the atomic number. For example, hydrogen, the simplest atom, has an atomic number of 1, corresponding to its single proton; calcium has an atomic number of 20, corresponding to its 20 protons. Since an atom is electrically neutral, the atomic number is also equal to the number of electrons in the atom.

Lateral Plantar Artery

2011-12-25T12:07:00.000-08:00

The lateral plantar artery originates from the posterior tibial artery and then runs obliquely outward and forward, across the quadratus plantae muscle, to the base of the fifth metatarsal bone, descending deep to the lateral border of the foot where it curves medialward to the space between the bases of the first and second metatarsal bone, giving off the plantar metatarsal arteries. The lateral plantar artery supplies the sole with oxygenated blood.

Thoracoacromial Artery

2011-12-24T11:42:00.000-08:00

The thoracoacromial artery is a short oxygen-rich blood vessel which arises from the axillary artery and supplies the pectoral muscles and the shoulder (deltoid). The thoracoacromial artery gives off four branches: the pectoral, deltoid, clavicular, and acromial.

Medial Plantar Artery

2011-12-23T10:15:00.000-08:00

The medial plantar artery arises from the posterior tibial artery. Running forward along the medial side of the foot, the medial plantar gives off a superficial branch to the great toe and a deep plantar artery to the plantar arch of the foot. It supplies the sole.

Heartbeat Coordination and Generation

2011-12-22T07:50:00.000-08:00

The heartbeat coordination is the rhythmic and harmonious contractions of the heart chambers. The heart is a dual pump in that the left and right sides of the heart pump blood separately, but simultaneously, into the systemic and pulmonary circuits. Efficient pumping of blood requires that the atria contract first, followed almost immediately by the ventricles. Contraction of cardiac muscle, like that of skeletal muscle and many smooth muscles, is triggered or generated by depolarization of the plasma membrane. The gap junctions that connect myocardial cells allow action potentials to spread from one cell to another. Thus, the initial excitation of one cardiac cell eventually results in the excitation of all cardiac cells. This initial depolarization normally arises in a small group of conducting-system cells, the sinoatrial (SA) node, located in the right atrium near the entrance of the superior vena cava. The action potential then spreads from the SA node throughout the atria and then into and throughout the ventricles. The action potential initiated in the SA node spreads throughout the myocardium, passing from cell to cell by way of gap junctions. The spread throughout the right atrium and from the right atrium to the left atrium does not depend on fibers of the conducting system. The conduction through atrial muscle cells is rapid enough that the two atria are depolarized and contract at essentially the same time.

The spread of the action potential to the ventricles is more complicated and involves the rest of the conducting system. The link between atrial depolarization and ventricular depolarization is a portion of the conducting system called the atrioventricular (AV) node, which is located at the base of the right atrium. The action potential spreading through the muscle cells of the right atrium causes depolarization of the AV node. This node has a particularly important characteristic: the propagation of action potentials through the AV node is relatively slow (requiring approximately 0.1 s). This results in a delay that allows atrial contraction to be completed before ventricular excitation occurs. After leaving the AV node, the impulse enters the wall—the interventricular septum—between the two ventricles. This pathway has conducting-system fibers termed the bundle of His (or atrioventricular bundle) after its discoverer (pronounced Hiss). It should be emphasized that the AV node and the bundle of His constitute the only electrical link between the atria and the ventricles. Except for this pathway, the atria are completely separated from the ventricles by a layer of nonconducting connective tissue. Within the interventricular septum the bundle of His divides into right and left bundle branches, which eventually leave the septum to enter the walls of both ventricles. These fibers in turn make contact with Purkinje fibers, large conducting cells that rapidly distribute the impulse throughout much of the ventricles. Finally, the Purkinje fibers make contact with ventricular myocardial cells, by which the impulse spreads through the rest of the ventricles. The rapid conduction along the Purkinje fibers and the diffuse distribution of these fibers cause depolarization of all right and left ventricular cells more or less simultaneously and ensure a single coordinated contraction. Actually, though, depolarization and contraction begin slightly earlier in the bottom (apex) of the ventricles and spread upward. The result is a more efficient contraction, like squeezing a tube of toothpaste from the bottom up.

Vertical Columns (Cerebral Cortex)

2011-12-21T08:46:00.000-08:00

The basic functional units of the neocortex are vertical nerve cell columns that reach through all six layers of the cerebral cortex and have a diameter between 200 and 300 µm (micrometer). Electrophysiological studies have shown that, in the cortical projection areas, each cell column is connected to a defined peripheral group of sensory cells. Stimulation of the peripheral field always yields a response from the entire column. Fiber tracts connect the cortical columns with each other (D): the fibers of a column (D20) run either to columns of the ipsilateral hemisphere (association fibers, see p. 260) or via the corpus callosum to mostly symmetrically localized columns of the contralateral hemisphere (commissural fibers, see p. 260). Branches of individual fibers terminate in different columns (D21). It is estimated that the neocortex is made up of 4 million columns.

Tonsils

2011-12-20T10:29:00.000-08:00

The tonsils are lumps of lymphoepithelial tissue surrounding the exits of the oral and nasal spaces into the pharynx (throat). They form a circle and have a protective function by early activation of specific defense mechanisms. The tonsils form the pharyngeal circle of Waldeyer, which consists of the unpaired pharyngeal tonsil in the roof of the pharynx, the paired palatine tonsils on each side of the palatine arch, and the lingual tonsil in the retrolingual region. There is additional lymphatic tissue in the lateral wall of the pharynx which, at the entrance to the Eustachian tube, is condensed into the tonsilla tubaria.

The lymphoid tissue in the tonsils consists of aggregated lymph follicles lying directly beneath the covering epithelium, the surface of which is deeply fissured by crypts. The lymphocytes (type of leukocytes) immigrate into the intercellular gaps between the epithelial cells. In the crypts there are plugs of detritus from shed epithelial cells and leukocytes. The tonsil is demarcated from its surroundings by a tout fibrous capsule which allows to be surgically removed. The tonsil is covered by stratified epithelium.

Peroneal Artery

2011-12-19T08:55:00.000-08:00

Also called the fibular artery, the peroneal artery arises from the posterior tibial artery and runs down along the fibula to the lateral malleolus. The peroneal artery supplies the soleus muscle and the peroneus muscles, giving off the perforating, communicating, calcaneal, and lateral and medial malleolar branches, and feeding the calcaneal retia.

Posterior Tibial Artery

2011-12-18T09:11:00.000-08:00

Arising from the popliteal artery, the posterior tibial artery is the larger of the two tibial arteries. It passes between the tendinous arch of the soleous muscle, between the superficial and deep flexor muscles. Then, the posterior tibial artery runs behind the malleolus together with the tibial nerve, and, finally, covered by the abductor hallucis, it reaches the sole of the foot. Supplying the muscles and bones of the leg and foot with oxygenated blood, the posterior tibial artery gives off three branches: the peroneal (or fibular) artery, the medial plantar artery, and the lateral plantar artery.

Anterior Tibial Artery

2011-12-17T07:18:00.000-08:00

The anterior tibial artery is one of the two arteries into which the popliteal artery divides, the other being the posterior tibial artery. It arises from behind the upper leg (tibia), near the back of the knee, and perforates the interosseous membrane at the lower border of the popliteus muscle and emerges in the front of the leg; then the anterior tibial artery runs with the deep peroneal nerve, embedded in the extensor muscles to the dorsom of the foot, where it continues as the dorsal artery of the foot. The anterior tibial artery supplies oxygen-rich blood to several muscles of the leg and foot as it gives off six branches: posterior tibial recurrent artery, anterior tibial recurrent artery, anterior medial malleolar artery, anterior artery malleolar artery, dorsalis pedis artery, circumflex fibular artery.

Popliteal Artery

2011-12-16T07:32:00.000-08:00

The popliteal artery arises from the femoral artery. It runs through the adductor canal to enter the popliteal fossa at the flexor side of the knee. It is called the popliteal artery until it divides into the anterior and posterior tibial arteries. During its course, the popliteal artery gives off the lateral superior genicular, the medial superior genicular, and the sural branches. It supplies the knee joint and the calf muscles.

Artery Wall Structure

2011-12-15T09:30:00.000-08:00

The artery wall structure of the aorta and other large arteries near the heart are composed of three well-defined layers: 1) the tunica adventitia, the external layer, which consists of connective tissue; 2) tunica media, the middle and thickest layer, composed of elastic fibers arranged in circles in the form of fenestrated (with tiny gaps) elastic membranes, polisacharide substances and connective tissue; 3) the tunica intima, the inner and thinnest layer, which is a single layer made up of simple squamous endothelial cells.

Capillary Wall

2011-12-14T08:37:00.000-08:00

The capillary wall consists of an endothelial cell layer and a basal membrane, visible under the electron microscope. In some organs, such as the brain, there are also additional contractile cells, called pericytes, attached to the outer wall. The endothelial cells of the capillary wall, which are between 25 and 50 micrometer long, as a rule adjoin each other tightly, leaving or not gaps in between, and form and endothelial tube, which measures between 5 and 10 micrometer in diameter. Transport of materials occurs from the capillaries into the surrounding tissue and vice versa, and in both cases through the tube of endothelial cells. The capillary makes it possible the exchange of oxygen, carbondioxide, water, food, and waste materials between surrounding tissue and blood.

There are three types of capillary: 1) continuous capillary, which provide uninterrupted lining, allowing only small molecules, such as water molecules and ions to diffuse through it; 2) fenestrated capillary, which has pores or gaps, through which limited amount of protein diffuse; 3) sinusoidal, which are similar to fenestrated capillaries, but with larger pores in between the endothelial cells to allow red and white blood cells, as well as serum protein, to go through them.

Capillaries and End Artery Occlusion

2011-12-13T08:00:00.000-08:00

The capillaries are a network of tiny, oxygen-rich blood vessels, supplying oxygenated blood from the arteries to the different body tissues. The exchange of gases and substances between the blood and the tissues is facilitated by the large cross-sectional area of all the capillaries and by slowing down of the blood flow (to about 0.3mm/s as compared to the 320mm/s in the aorta). All organs have capillaries, except the stratified squamous epithelia, the cornea and lens of the eye, and cartilage. Capillaries are usually 1mm long and 5-15 microms in diameter. Capillaries form three dimensional networks that are supplied by several arteries. Occlusion of one artery may, therefore, be of no consequence to the organ in question. Nevertheless, if a particular capillary area depends on a single artery, such as an end artery without adequate cross communications with other vessels, occlusion to that artery results in necrosis (death) of the affected tissue, which is known as infarct. The branches of the arteries to the liver, kidneys, spleen, brain, retina, and the heart coronary vessels are end arteries.

Lymphatic System

2011-12-12T06:01:00.000-08:00

The lymphatic system is a system of drainage vessels which return fluids (lymph) from between the cells to the heart via the veins. The lymphatic system also slowly delivers fatty substances absorbed by the intestine into the blood. This allows the blood to accommodate lipids which might block the flow of blood if they were taken directly into the blood from the digestive system. Another main function of the lymphatic system is fighting microorganism in the immune response. The smallest vessels of the lymphatic system are closed at one end. These microscopic vessels are the lymphatic capillaries. The lympth capillaries unite to form lymphatic vessels, which contain many valve to assure that the lymph in them flows in one direction. Lymphatic vessels flow into lymph nodes, which are situated mainly in the lumbar region, axillae, neck, and in the serosa of the abdomen. The tonsils are a ring of lymph nodes located in the pharynx. Vessels flowing into a node are called afferent lymphatic vessels; on the other hand, lymphatic vessels that exit lymph nodes (from a depression called the hilus) are called efferent lymphatic vessels.

Lymphatic vessels from the left side of the thorax and head, left arm, and everything below the diaphragm on both side of the body, empty into the largest lymphatic vessel, the thoracic duct, which in turn empties into the left subclavian vein. Lymphatic vessels from the right arm, the right side of the thorax, head, and neck empty into the right lymphatic duct, which empties into the right subclavian vein.

Epicritic Sensibility Pathway

2011-12-11T07:35:00.000-08:00

The epicritic sensibility pathway, whose nerve fibers transmit impulses for the senses of touch, vibration, and joints, arises from neurons of the spinal ganglia, while the fibers for face and sinuses stem from neurons of the trigeminal ganglion (Gasser’s ganglion, semilunar ganglion) (1st neuron). Touch stimuli are transmitted by two types of fibers; thick, well-myelinated nerve fibers terminate at the corpuscular end organs, while thin nerve fibers terminate at hair follicles. The centripetal axons of the neurons enter the spinal cord via the posterior root, with the thick myelinated fibers running through the medial portion of the root. They merge with the posterior funiculi in such a way that the newly entering fibers border laterally; as a result, the sacral and lumbar fibers lie medially and the thoracic and cervical fibers laterally. The sacral, lumbar, and thoracic bundles form the gracile fasciculus (Goll’s tract), while the cervical fibers form the cuneate fasciculus (Burdach’s tract).

The primary fibers (gracile funiculus and cuneate funiculus) terminate in a corresponding arrangement on the neurons of the dorsal column nuclei (2nd neuron), gracile nucleus, and cuneate nucleus, which therefore exhibit the same somatotopic arrangement as the posterior funiculi. Each neuron in the dorsal column nuclei receives its impulses from a specific type of receptor. The cutaneous supply area of a neuron is small in the distal segments of the limbs (hand, finger) but larger in the proximal segments. As electrophysiological studies have demonstrated, the neurons receiving impulses from specific receptors show a somatotopic arrangement as well; close to the surface of the nuclei lie the neurons for the hair follicle receptors, in the middle those for the touch organs, and still deeper those for the vibration receptors.

Corticofugal fibers from the central region (precentral gyrus and postcentral gyrus) run via the corticospinal (pyramidal) tract to the dorsal column nuclei; fibers from the lower limb area of the central region terminate in the gracile nucleus, while fibers from the upper limb area terminate in the cuneate nucleus. The corticofugal fibers have a postsynaptic or presynaptic inhibitory effect on neurons of the dorsal column nuclei and therefore attenuate the incoming afferent impulses. Thus, the cortex is able to regulate in these relay nuclei the input of impulses coming from the periphery. The secondary fibers ascending from the dorsal column nuclei (2nd neuron) form the medial lemniscus. In the decussation of the medial lemnisci the fibers cross to the opposite sides, with the fibers from the gracile nucleus lying ventrally and those from the cuneate nucleus lying dorsally. Later, the gracile fibers take a lateral position and the cuneate fibers take a medial position. The secondary fibers from the trigeminal nuclei, trigeminal lemniscus, join the medial lemniscus at the level of the pons and become located dorsomedially to it in the midbrain.

The medial lemniscus extends to the lateral part of the ventral posterior nucleus of thalamus; the fibers of the gracile nucleus terminate laterally, while those of the cuneate nucleus terminate medially. The trigeminal fibers terminate in the medial part of the ventral posterior nucleus. This results in a somatotopic organization of the nucleus. The arrangement of fibers is preserved in the projections of the thalamocortical fibers (3rd neuron) to the cortex of the postcentral gyrus and forms the basis for the somatotopic organization of the postcentral area. Hence, the pathway of the epicritic sensibility consists of three neurons relayed in tandem, with a demonstrated somatotopic organization in each relay station and at the terminal station.

Solution

2011-12-11T04:53:00.000-08:00

Solution is a mixture in which the molecules of a substance, such as sugar (called solute), are are dissolved and dispersed among the molecules of a liquid, such water (called solvent).

Bulbothalamic Tract

2011-12-10T04:58:00.000-08:00

The bulbothalamic tract is a bundle of fibers which represents the extension of the posterior funiculi of the spinal cord (epicritic sensibility). These fibers originate in the gracile nucleus and the cuneate nucleus, cross as arcuate fibers (decussation of lemnisci), and form the medial lemniscus in the narrower sense. The cuneate fibers initially lie dorsally to the gracile fibers, while they lie medially to them in the pons and midbrain. They terminate in the thalamus.

Femoral Nerve

2011-12-09T07:09:00.000-08:00

The femoral nerve is a branch of the lumbar plexus, composed by L1, L2, L3, and L4 spinal roots. The femoral nerve runs along the margin of the greater psoas muscle up to the inguinal ligament and underneath it through the muscular lacuna to the front of the thigh. The nerve trunk divides below the inguinal ligament into several branches, namely, a mostly sensory group, the anterior cutaneous branches, a lateral and medial group of motor branches for the extensor muscles of the thigh, and the saphenous nerve. The saphenous nerve extends to the adductor canal and enters into it. It penetrates the vastoadductor membrane and runs along the medial side of the knee joint and the lower leg together with the great saphenous vein down to the medial ankle. In the small pelvis, the femoral nerve gives off fine branches to the greater psoas muscle and to the iliac muscle. Below the inguinal ligament, a branch extends to the pectineal muscle. The anterior cutaneous branches originate slightly more distally, with the strongest one continuing along the middle of the thigh down to the knee. They supply sensory fibers to the skin of the anterior and medial aspects of the thigh.

Vena Cava

2011-12-08T12:21:00.000-08:00

The vena cava is a large blood vessel which leads directly into the right atrium of the heart, carrying deoxygenated blood from the rest of the body. It is composed of two branches: the superior and inferior vena cava. The superior vena cava is a short thick blood vessel that arises from the junction of the right and left innominate veins. The inferior vena cava is a long vein which originates at the junction of the left and right common iliac veins in the lower abdomen, and then runs up parallel to the vertebral column, on its right side, entering the right atrium on its lower right side.

Anterior Cerebral Vein

2011-12-08T07:53:00.000-08:00

The anterior cerebral vein is a blood vessel in the brain. Running parallel to the anterior cerebral artery, it receives deoxygenated blood from the anterior two-thirds of the corpus callosum and the adjacent convolutions. The anterior cerebral vein extends around the genu of the corpus callosum to the base of the frontal lobe.

Great Cerebral Vein

2011-12-07T08:48:00.000-08:00

The great cerebral vein is a short vascular trunk formed by the confluence of four veins, namely, the two internal cerebral veins and the two basal veins. It curves around the splenium of the corpus callosum and empties into the straight sinus. Veins from the surface of the cerebellum and from the occipital lobe may drain into it.

Deep Cerebral Veins

2011-12-06T06:58:00.000-08:00

The deep cerebral veins collect the blood from the diencephalon, the deep structures of the hemispheres, and the deep white matter. In addition, there are thin transcerebral veins running along the fibers of the corona radiata from the outer white matter and from the cortex. They connect the superficial drainage areas with the deep ones. The deep cranial veins empty their blood into the great cerebral vein, also called great vein of Galen. The drainage system of the deep veins is therefore also known as the system of the great cerebral vein. The deep cerebral veins comprise: the basal vein, which passes backward around the cerebral peduncle and ends in the great vein of Galen; the anterior cerebral vein; the deep middle cerebral brain, which receives tributaries from the insula and neighboring gyri and runs in the lower part of the lateral cerebral fissure; the thalamostriate vein; and the internal cerebral vein, which drains the deep parts of each hemisphere.

Sacral Plexus

2011-12-05T11:13:00.000-08:00

The sacral plexus is a network of nerves composed of lumbosacral trunk and the sacral nerve. The lumbosacral trunk (parts of L4 and L5) and the anterior branches of the sacral nerve S1 – S3 join on the anterior surface of the piriform muscle (on the back of the pelvis) to form the sacral plexus. Direct branches extend from the plexus to the muscles of the pelvic region, namely, to the piriform muscle, the gemellus muscles, the internal obturator muscle, and the quadrate muscle of thigh.

Lumbar Plexus

2011-12-04T10:00:00.000-08:00

The lumbar plexus is composed of the anterior branches of the lumbar nerves and fibers of the subcostal nerve. The lumbar plexus gives off direct short muscular branches to the hip muscles, namely, to the greater and lesser psoasmuscles (L1–L5), the lumbar quadrate muscle (T12–L3), and the lumbar intercostal muscles. The upper nerves of the plexus are still roughly organized in the same way as the intercostal nerves. Together with the subcostal nerve, they represent transitional nerves between the intercostal nerves and the lumbar nerves.

Axillary Nerve

2011-12-03T06:42:00.000-08:00

Originating from the posterior cord of the brachial plexus, the axillary nerve runs deep inside the axilla and across the capsule of the shoulder joint around the surgical neck on the back of the humerus. It passes through the lateral axillary gap and extends beneath the deltoid muscle to the anterior margin of the latter. Before the axillary nerve trunk passes through the lateral axillary gap, it gives off a motor branch to the teres minor muscle, which also passes through the lateral axillary gap. The axillary nerve supplies the teres minor and the deltoid muscles, as well as the shoulder skin.

Biome

2011-12-02T11:31:00.001-08:00

A Biome is a group of large natural communities characterized by distinctive climate and vegetation, such as the grassland biome or the tropical rain forest biome. All tropical rain forests collectively form the tropical rain forest biome. All Biomes are usually identified with particular patterns of ecological succession. An ecosystem has many biotopes and a biome is a major habitat type.

Lumbosacral Plexus

2011-12-02T08:08:00.000-08:00

The lumbosacral plexus is formed by the anterior branches of the lumbar and sacral spinal nerves. Its branches provide sensory and motor innervation to the lower limb. The branches of lumbar spinal nerves L1–L3 and part of L4 form the lumbar plexus, the roots of which lie within the psoas muscle. The obturator nerve and the femoral nerve originate from here, in addition to several short muscular branches. The remainder of the fourth lumbar nerve and the L5 nerve join to form the lumbosacral trunk, which then unites in the small pelvis with sacral branches 1 – 3 to form the sacral plexus. The sacral branches emerge from the anterior sacral foramina of the sacrum and form together with the lumbosacral trunk the sacral plexus; the main nerves originating from here are the sciatic nerve (common peroneal nerve and tibial nerve).

Thoracic Spinal Nerves

2011-12-01T07:14:00.000-08:00

There are twelve thoracic spinal nerves, each of them dividing into a posterior branch and an anterior branch. Each posterior branch divides into a medial and a lateral branch; both supply motor fibers to the deep autochthonous back muscles. Sensory innervation of the back comes mainly from the lateral branches of the posterior branches. The anterior branches of the spinal thoracic nerves run as intercostal nerves between the ribs, initially on the inner surface of the thorax and later within the internal intercostal muscles. An upper group and a lower group of intercostal nerves can be distinguished: the nerves of the upper group (T1–T6) run up to the sternum and supply the intercostal muscles, the superior and inferior posterior serrate muscles, and the transverse thoracic muscle, giving off sensory branches to the skin of the thorax; the nerves of the lower group (T7–T12), the intercostal segments of which no longer end at the sternum, extend across the costal cartilages up to the white line, taking an increasingly oblique downward path and supplying the muscles of the abdominal wall (abdominal transverse muscle, external and internal abdominal oblique muscles, rectus abdominis muscle and pyramidal muscle).

Brachial Plexus

2011-11-30T06:49:00.000-08:00

The brachial plexus is formed by the anterior branches of cervical spinal nerves from C5 to C8 and by a part of the thoracic spinal nerve (T1) nerve. The brachial plexus innervates the skin and muscles of the whole upper limb, except for the trapezius muscle and the skin region next to the axilla (armpit). The anterior branches pass through the scalene gap into the posterior cervical triangle, where they form three primary trunks above the clavicle: 1) the superior trunk (C5, C6); 2) the medial trunk (C7); 3) the inferior trunk (C8, T1).

Cervical Plexus

2011-11-29T11:50:00.000-08:00

The cervical plexus is a nerves network formed by the spinal nerve of the neck. It consists of the anterior branch of the first four spinal nerves (C1, C2, C3, C4). The following nerves originate here: the lesser occipital nerve, the greater auricular nerve, the transverse nerve of the neck, the supraclavicular nerves, the phrenic nerve, and also the roots of the deep cervical ansa. The cervical plexus innervates the deep neck muscles, such as the anterior and lateral rectus capitis muscles, the long muscle of the head and the long muscle of the neck.

Osteoblast

2011-11-28T07:13:00.000-08:00

The osteoblast is a bone tissue cell responsible for the formation of bones. Located on the surface of bone, osteoblasts produce the extracellular matrix and enzymes needed for the calcification of the matrix (osteoid). After an osteoblast is encased by osteoid matrix produced by itself, it becomes an osteocyte. When the extracellular matrix becomes hard, it is located in small cavity called lacuna, and contacts its neighboring osteocytes through thin cellular extensions, lying in small bony ducts or canals, called canaliculi.

Osseous Tissue (Bone)

2011-11-27T15:19:00.000-08:00

The bone tissue, also known as osseous tissue, is the forming component of the skeletal system. It protects inner body cavities and provides movement, as a lever, by converting muscle contractions into movements; another important function of the bone tissue is blood cell formation and storing minerals like calcium. The bone tisue consists of cells, which are called osteoblasts, and an extracellular matrix in which fibers are embedded; it is unlike other connective tissues in that the extracellular matrix becomes calcified. There are two types of bone tissue: 1) spongy bone, which is composed of a network of branching bone spicules or trabeculae, with spaces between them containing bone marrow; 2) compact bone, which appears as a mass of bony tissue lacking spaces visible to the unaided eye.

Adipose Tissue

2011-11-27T06:12:00.000-08:00

Adipose tissue is a special type of connective tissue specialized in lipid (fat) storage. It is formed by large cells, called adypocytes, with small extracellular matrix. It can be divided into a yellow, found anywhere in adults body, and a brown,infant, adipose tissue. Yellow adipose tissue store lipids, provides thermic and mechanic isolation, and body shape. Brown adipose tissue generates heat by utilizing fatty acids.

Free Nerve Endings

2011-11-25T15:48:00.000-08:00

Free nerve endings are found in almost every tissue of the body. In the skin, they reach into the lower layers of the epidermis (stratum germinativum). At the end of their terminal branches, the axons send nodular or finger-like evaginations through gaps in the Schwann cell sheath. These evaginations are covered only by the basement membrane and represent the receptor segments of free nerve endings to which the sensations of pain and cold are attributed. Delicate nerve fibers encircle the hair follicles, and their terminal segments ascend or descend parallel to the hair shaft. The receptor terminals lose their myelin sheath and are enclosed between two Schwann cells (sandwich arrangement), which leave a cleft along the entire terminal segment. Through this cleft the axon terminal reaches the surface where it is covered only by the basement membrane. The nerve endings are radially arranged around the hair follicle in such a way that the sensory clefts face the follicle. Every movement of the hair causes mechanical stimulation of the nerve endings, which is perceived as touch.

Also associated with hair follicles are Merkel’s touch cells. These are large, clear epithelial cells that lie between the basal cells of the outer root sheath and send out finger-like processes into their surroundings. Deformation of these cells through movement of the hair results in stimulation of the associated nerve fiber. The nerve fiber loses its myelin sheath as it penetrates the basement membrane and sends branches to several tactile cells. The terminal segment widens into a tactile meniscus and forms synapse-like membrane contacts with the Merkelcell.

Connective Tissues

2011-11-24T05:26:00.000-08:00

Connective tissues consists of fiber proteins like collagen or elastin, glycoproteins, glycosamino-glycans, and tissue fluids. Connective tissues provide support, protection and stability, produce blood cells, and fill empty spaces. The cells are separated by extracellular matrix produced by themselves. There are five types of connective tissues: 1) Loose connective tissue, found around blood vessels, groups of muscle cells, etc., is well vascularized and flexible but not very resistant to stretch. 2) Dense connective tissue, which is found in ligaments and tendons and has many collagen fibers arranged in bundles; it is less flexible but much stronger than the first type. 3) Elastic connective tissue, found in ligaments and has more bundles of elastic fibers than collagen fibers. 4) Reticular connective tissue, which consists of reticular fibers produced by special cells, forming the supporting framework for bone marrow and lymphatic tissue. 5) carthilage, which is a connective tissue with a solid extracellular matrix, specialized to bear mechanical stress.

Common Terminal Motor Pathway

2011-11-23T13:37:00.000-08:00

The common terminal pathway of all centers involved in motor activity is the large anterior horn cell and its axon (alfa motor neuron), which innervates the voluntary skeletal muscles. Most of the tracts running to the anterior horn do not end directly on the anterior horn cells but terminate on interneurons. The latter influence the motor neurons either directly or act by inhibiting or activating the reflexes between muscle receptors and motor neurons. The anterior horn is therefore not simply a relay station as described earlier but a complex integration apparatus that regulates motor activity. The central regions that influence motor activity via descending pathways are interconnected in many ways.

The most important afferent pathways stem from the cerebellum, which receives the impulses of muscle receptors via the spinocerebellar tracts and the stimuli of the cortex via the corticopontine tracts. The cerebellar impulses are transmitted via the parvocellular part of the dentate nucleus and the ventral lateral nucleus of thalamus to the precentral cortex (area 4). The corticospinal (pyramidal) tract descends from area 4 to the anterior horn and gives off collaterals in the pons that return to the cerebellum. Additional cerebellar impulses are transmitted via the emboliform nucleus and the centromedian nucleus of the thalamus to the striatum and via the magnocellular part of the dentate nucleus to the red nucleus. From here fibers run in the central tegmental tract via the olive back to the cerebellum and in the rubroreticulospinal tract to the anterior horn. Fibers from the globose nucleus run to the interstitial nucleus of Cajal and from there in the interstitiospinal fasciculus to the anterior horn. Finally, cerebellofugal fibers are relayed in the vestibular nuclei and in the reticular formation to the vestibulospinal tract and the reticulospinal tract, respectively.

The descending pathways can be divided into two groups according to their effect on the muscles: one group stimulates the flexor muscles, and another group stimulates the extensor muscles. The corticospinal tract and the rubroreticulospinal tract activate mainly the neurons of the flexor muscles and inhibit the neurons of the extensor muscles. This corresponds to the functional importance of the corticospinal tract for delicate and precise movements, especially those of hand and finger muscles where flexor muscles play an important role. In contrast, the fibers of the vestibulospinal tract and the fibers from the pontine reticular formation inhibit the flexors and activate the extensors. They belong to a phylogenetically old motor system that is directed against the effect of gravity and, thus, is of special importance for body posture and balance.

Epithelial Tissues

2011-11-23T06:38:00.000-08:00

Epithelial tissues are composed of closely aggregated cells, which usually have little intercellular material between them. Epithelial tissues can line cavities, cover a body organ or structure, and form glands. Further functions of epithelial tissues are absorption of nutrients and secretion of substances. The cells may be joined to a single layer, for example, alveolar epithelia in lungs, which provide a little diffusion distance for the oxygen, or as a multiple layers, for example the skin epithelial.

The type of cells that compose the epithelial tissues may vary from cuboidal (for example in kidney tubules), to columnar (for example in the digestive tract), to flattened squamous cells (for example in blood vessels). Epithelial tissues can also form glands, which secret their products into ducts (exocrine glands), or into blood vessels (endocrine glands).

Submandibular Ganglion

2011-11-22T06:28:00.000-08:00

The submandibular ganglion is a group of parasympathetic nerve cell bodies which lies, together with several small secondary ganglia, in the floor of the mouth above the submandibular gland and below the lingual nerve, to which it connects via several ganglionic branches. The preganglionic parasympathetic fibers of the submandibular ganglion originate from the superior salivatory nucleus (located in the pontine tegmentum of the pons), run in the facial nerve (intermediate nerve), and leave the nerve together with the taste fibers in the chorda tympani. In the latter, the fibers reach the lingual nerve and extend in it to the floor of the mouth where they cross over into the ganglion. Postganglionic sympathetic fibers from the plexus of the external carotid artery reach the ganglion via the sympathetic branch given off by the plexus of the facial artery; they pass through the ganglion without synapsing. The postganglionic parasympathetic and sympathetic fibers pass partly in the glandular branches to the submandibular gland, partly in the lingual nerve to the sublingual gland and to the glands in the distal two-thirds of the tongue.

Parasympathetic Ganglia

2011-11-21T07:27:00.000-08:00

The parasympathetic ganglia are the clusters of cell bodies of cholinergic neurons of the parasympathetic nervous system. They are situated near the wall of the organs being innervated. Supplying all parasympathetic innervation to the head and neck, these paired ganglia are: 1) ciliary ganglion, which supply pupil sphincter and ciliary muscle; 2) otic ganglion (parotid gland); 3) submandibular ganglion (submandibular and sublingual glands); 4) pterygopalatine ganglion (lacrimal gland, glands of nasal cavity).

Each ganglion has three roots: a motor root; a sympathetic root, synapsing in the ganglia of the sympathetic chain; and a sensory root, the fibers of which pass the ganglion without interruption.

Neuroendocrine System

2011-11-20T07:46:00.000-08:00

The neuroendocrine system is the system which overlaps the nervous and endocrines systems, comprising the hypothalamus, and the anterior and posterior areas of the hypophysis (pituitary gland). In the neuroendocrine system, the hypophysis is under the control of the hyopothalamic centers. Unmyelinated fiber bundles from the hypothalamus run in the hypophysial stalk and in the hypophysial posterior lobe. Severing the hypophysial stalk leads to retrograde cellular changes, namely, to extensive loss of neurons in the nuclei of the tuber cinereum when transection is carried out at a high level.

Neurons in the hypothalamus produce substances that migrate within the axons into the hypophysis and enter there into the bloodstream. This endocrine function of neurons is called neurosecretion. The substances are produced in the perikarya and appear there as small secretory droplets. The cells, which are genuine neurons with dendrites and axons, represent a transitional stage between neurons and secretory cells. Both these cell types are of ectodermal origin and are closely related with respect to physiology and metabolism. Both of them produce a specific substance that they secrete in response to a nervous or humoral stimulus: the neurons release transmitter substances (neurotransmitters) and the secretory cells release their secretion. Transitional forms between the two cell types are the neurosecretory cells and the endocrine cells, both of which release their secretion into the bloodstream.

Corresponding to the structure of the hypophysis with an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis), there are two different fiber systems extending from the hypothalamus to the hypophysis, namely, the tuberoinfundibular tract and the hypothalamohypophysial tract. In both, the coupling of the neural system with the endocrine system is achieved by the sequential arrangement of nerve fibers and capillaries (neurovascular chain).

Insula (Brain)

2011-11-18T05:26:00.000-08:00

The insula is the region of the cerebral cortex located at the lateral aspect of the hemisphere that lags behind during development and becomes covered by a fold formed by the fissure of Sylvius (lateral cerebral sulcus), which are adjacent regions of the hemisphere that grow more rapidly during fetal development. The parts of the hemisphere overlapping the insula are called opercula. They are named according to the cerebral lobe they belong to: the frontal operculum, the parietal operculum, and the temporal operculum. The insula can be seen by removing the opercula apart to expose it. They normally leave only a cleft, the lateral cerebral sulcus (fissure of Sylvius), which widens over the insula into the lateral fossa. The insula has roughly the shape of a triangle and is bordered at its three sides by the circular sulcus of the insula. The central sulcus of the insula divides the insula into a rostral and a caudal part. At its lower pole, the limen of insula, the insular region merges into the olfactory area, the paleocortex.

The insular cortex represents a transitional region between paleocortex and neocortex. The lower pole of the insula is occupied by the prepiriform area which belongs to the paleocortex. The upper part of the insula is covered by the isocortex with the familiar six layers. Between both parts lies a transitional region, the mesocortex. Unlike the paleocortex, it has six layers; however, these are only poorly developed as compared to the neocortex.

Parieto-occipital Sulcus

2011-11-17T07:50:00.000-08:00

The parieto-occipital sulcus is a deep groove in the cerebral cortex, located near the posterior end of each hemisphere that separates the parietal lobes and the occipital lobes in both hemispheres. It begins on the external, lateral surface of the hemisphere and runs up for about 2 cm to the hemispheric border, and then travels down and forward on the medial (internal) surface, ending up under the posterior end of the corpus callosum. Along the way the parieto-occipital sulcus meets the calcarine fissure, forming the lingual gyrus below.

Parieto-occipital Sulcus on the internal (medial) surface of the brain hemisphere