Monday, October 17, 2011

Ultrastructure of the Nerve Cell

The ultrastructure of the nerve cell (neuron) can be observed through electron micrographs, which show the nerve cell nucleus to be enclosed by a double-layered membrane, containing the nuclear pores that probably open only temporarily. The karyoplasm of the nucleus contains finely dispersed chromatin granules, which consist of DNA and proteins. The nucleolus, a spongiform area of the nucleus made up of a dense granular component and a loose filamentous component, consists of RNA and proteins. In the cytoplasm, the Nissl bodies appear as rough endoplasmic reticulum, a lamellar system of membranes that enclose flattened, intercommunicating cisternae. Attached to the cytoplasmic side of the membranes are the protein-synthesizing ribosomes.

To maintain the long axon, it is essential that the cell has an extremely high rate of protein synthesis (structural metabolism). Ribosomefree membranes form the agranular or smooth endoplasmic reticulum. The rough endoplasmic reticulum communicates with the perinuclear space and with the marginal cisternae below the cell surface. Marginal cisternae are often found at sites where boutons or glial cell processes are attached. The cytoplasm is crossed by neurofilaments and neurotubules that are arranged into long parallel bundles inside the axon. The neurotubules correspond to the microtubules of other cells. The transport of substances takes place along neurofilaments and neurotubules. Neurofibrils are the light-microscopic equivalent of densely packed neurotubules.

The neuron contains a large number of mitochondria. These are enclosed in a double membrane; the inner membrane shows projections (cristae) into the inner space (matrix). The mitochondria are of various shapes (short and plump in the perikaryon, long and slender in the dendrites and the axon) and move constantly along fixed cytoplasmic paths between the Nissl bodies. The mitochondria are the site of cellular respiration and, hence, of energy generation. Numerous enzymes are localized in the inner membrane and in the matrix, among others the enzymes of the citric acid cycle and respiratory-chain oxidative phosphorylation.

The Golgi complex consists of a number of dictyosomes, which are stacks of flattened, noncommunicating cisternae. The dictyosome has a forming side (cis face) and a maturing side (trans face). The forming side receives transport vesicles from the endoplasmic reticulum. At the margins of the maturing side, Golgi vesicles are formed by budding. The Golgi complex is mainly involved in the modification (e.g., glycosylation, phosphorylation) of proteins from the endoplasmic reticulum. The numerous lysosomes contain various enzymes (e.g., esterases, proteases) and are mainly involved in intracellular digestion.