Describe the mechanisms involved by which the parasympathetic and sympathetic nervous systems affect cardiac output. Include ACh, NE, regulated K+ channels, If channels and T-type Ca2+ channels, vagus nerve, cardiac nerves, contractility, stroke volume, and heart rate in your answer
What will be an ideal response?
The parasympathetic nervous system stimulates the heart with ACh released from the vagus nerve. The ACh binds to receptors that ultimately cause the opening of ACh-regulated K+ channels. As K+ ions diffuse out of the cell, it takes longer for the pacemaker cells to reach threshold; therefore, the heart rate slows. The ACh also lessens the contractility of the heart by depressing the inward movement of Ca2+ through If and T-type Ca2+ channels; thus, the stroke volume and cardiac output decreases. The sympathetic nervous system stimulates the heart with NE, which causes the If and T-type Ca2+ channels to open more readily and allow more Ca2+ to enter the cell. This causes the pacemaker cells to reach threshold more quickly (faster heart rate) and it causes the contractile cells to increase their contractility (greater stroke volume). The result is a greater cardiac output.
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