The action potentials typical of cardiac pacemaker cells differ from those measu

Question

The action potentials typical of cardiac pacemaker cells differ from those measured in cardiac myocardium in which of the following ways (Check all that apply)

Question 3 options:

Long plateau in depolarized state due to slow leak of calcium

Rapid depolarization due to sodium influx into the cell

enable cells to be 'refractory' or resistant to multiple sequential stimuli

Exhibit autorhythmicity

Long plateau in depolarized state due to slow leak of calcium

Rapid depolarization due to sodium influx into the cell

enable cells to be 'refractory' or resistant to multiple sequential stimuli

Exhibit autorhythmicity

Explanation / Answer

Rapid depolarization due to sodium influx into the cell

Phase 0 is the rapid depolarization phase. Action potentials are unidirectional, all-or-none signals, because once they are initiated they only fire/move in one direction and happen fully at constant strength or not at all.The slope of phase 0 represents the maximum rate of potential change and is known as dV/dtmax. Its behavior is different in contractile and pacemaker heart cells. In heart muscle cells, this slope is directly proportional to the net ionic current.[16] This phase is due to the opening of the fast Na+ channels causing a rapid increase in the membrane conductance to Na+ (gNa)[nb 2] and thus a rapid influx of Na+ ions (INa) into the cell; a Na+ current. The membrane potential is reversed from negative to positive and peaks at about +25mV inside the cell.The ability of the cell to open the fast Na+ channels during phase 0 is related to the membrane potential at the moment of excitation

enable cells to be 'refractory' or resistant to multiple sequential stimuli

From the beginning of phase 0 until part way through phase 3 when the membrane potential reaches -60mV, each cell is in an absolute refractory period, also known as the effective refractory period, during which it is impossible to evoke another action potential. This is immediately followed until the end of phase 3 by a relative refractory period, during which a stronger-than-usual stimulus is required

Exhibit autorhythmicity

This differentiation of the action potentials allows the different electrical characteristics of the different portions of the heart. For instance, the specialized excitatory system of the heart has the special property of spontaneous depolarization. This means the heart depolarizes without any external influence via a slow, positive increase in voltage known as the pacemaker potential across the cell's membrane (the membrane potential) that occurs between the end of one action potential and the beginning of the next action potential. This increase in membrane potential (depolarization) typically allows it to reach the threshold potential at which the next pacemaker potential is fired. Thus, it is the pacemaker potential that drives the self-generated rhythmic firing, known as cardiac muscle automaticity or autorhythmicity.

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