This slowing of the action potential allows the ventricle more time to fill

This slowing of the action potential allows the

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atria to ventricle. This slowing of the action potential allows the ventricle more time to fill beforeits contractile cycle is initiated. Thus, the contraction of the atrium provides the last chance to fillthe ventricles before their contraction begins. After traversing the atrioventricular node, the action potential gains speed as it travels along the bundle of His and bifurcates into the left and right bundle branches. These branches move the action potential toward the apex of the heart before the conduction pathway further branches and ultimately ends at the Purkinje fibers. It is these fibers that transmit the action potential to the ventricular myocyte. The contraction of the ventricle originates at its apex, pushing blood up and out of the ventricle.Diff: 5 Page Ref: 37027
4) Contraction of the ventricular contractile cells originates from the Purkinje cells. Describe the electrical events responsible for the action potential of the ventricle.Answer: Contraction of the ventricular contractile cell is originated by the action potential that has traveled along the conduction pathway and ended within the Purkinje fibers. The current responsible for the action potential in the Purkinje cells passes to the ventricular contractile cell through gap junctions. That depolarizing current causes the Na+channels to open, thereby inducing the rapid upstroke of the action potential within the contractile cell (Phase 0). Those channels are rapidly closed by their inactivation gates, as was observed in neurons. However, membrane potential does not return immediately to resting values. This reduction in the rate of repolarization (Phase 1) is driven by two events: 1) the closure of the voltage-gated inwardly rectified K+channels and 2) the opening of the voltage-gated L-type Ca2+channels. These two events slow the rate of repolarization of the membrane maintaining it in a relatively depolarized state for a longer period of time, which is described as the plateau (Phase 2). After a short delay (approximately 200 ms), delayed rectifier K+channels begin to open, thereby speeding the rate of repolarization (Phase 3). This repolarization causes the Ca2+channels to close, further repolarizing the membrane. Thereafter, the membrane maintains a stable resting membrane potential (-80 mV) through the activity of K+channels (Phase 4).Diff: 7 Page Ref: 3735) Describe the changes in pressure and volume that occur within the left ventricle throughout the course of the cardiac cycle, starting at the beginning of diastole.Answer: At the beginning of diastole, intraventricular pressure is just below aortic pressure (the aortic semilunar valve has just closed). Ventricular pressure continues to decrease without any change in volume (isovolumetric relaxation), until pressure within the left ventricle is less than pressure within the left atrium. Since the movement of blood within the cardiovascular system is driven by pressure gradients, once pressure is greater in the atrium than in the ventricle, the
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