The absolute refractory period of an action potential: a. prevents summation of action potentials
b. allows a neuron to ignore a second signal sent that closely follows the first
c. ensures one-way travel down an axon
d. A and B
e. A, B, C

e. A, B, C

Explanation:

An action potential is a short event during which the membrane potential of an excitable cell increases and then falls rapidly. Membrane potentials correspond to potential differences across the membrane of excitable cells.

The absolute refractory period (P-RA) during which any stimulation is ignored, regardless of its intensity corresponds to the activation of fast sodium channels (quick response fibers) or calcium-sodium channels (slow response fibers). It begins in phase 0 of the action potential and ends in the repolarization phase (phase 3 for a fast fiber) when the membrane potential is sufficiently electronegative to restore the reactivation of these channels. It corresponds on the ECG to the interval between the onset of QRS and the top of the T wave.

The relative refractory period (P-RR) during which the cell is hypoexcitable. It corresponds to the start of the reactivation of the ion channels. Reactivation is incomplete and the action potential that results from stimulation during this period has unusual properties that expose conduction problems (block in phase 3) and may cause cells to desynchronize. Thus, an extra-stimulus which falls on the relative refractory period (R / T phenomenon) could cause ventricular tachycardia or ventricular fibrillation.

Thus; the absolute refractory period of an action potential prevents summation of action potentials, allows a neuron to ignore a second signal sent that closely follows the first and  ensures one-way travel down an axon .

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