Thursday, August 20, 2009

Cardiac Conduction System

The cardiac conduction system is the electrical conduction which makes the heart contract rhythmically to pump the blood out into the rest of the body. It is composed of sinoatrial node, the Bachmann's bundle, the atrioventricular node, the bundle of His, and the cardiac excitation wave pathway. The normal electrical conduction in the heart allows the impulse that is generated by the sinoatrial node of the heart to be spread to the myocardium, which contracts after stimulation. The ordered stimulation of the myocardium allows efficient contraction of the heart, pumping the blood throughout the body.

Substantial atrial to ventricular delay allows the atria to completely empty their contents into the ventricles; simultaneous contraction would cause inefficient filling and backflow. After contracting, the heart must relax to fill up again. Sustained contraction of the heart without relaxation would be fatal, and this is prevented by a temporary inactivation of certain ion channels.

The miocardium cells has some similarities both to neurons and skeletal muscle cells, and also have important unique properties. Like a neuron (nerve cell), a given myocardial cell has a negative membrane potential when at rest. Stimulation above a threshold value induces the opening of voltage-gated ion channels and a flood of cations into the cell. The positively charged ions entering the cell cause the depolarization characteristic of an action potential. Like skeletal muscle, depolarization causes the opening of voltage-gated calcium channels and release of Ca2+ from the t-tubules. This influx of calcium causes calcium-induced calcium release from the sarcoplasmic reticulum, and free Ca2+ causes muscle contraction. After a delay (the absolute refractory period), potassium channels reopen and the resulting flow of K+ out of the cell causes repolarization to the resting state.