Question

Which of the following is TRUE of the absolute refractory period?

a) It establishes the maximum frequency with which a nerve cell can conduct action potentials

b) It does not end until the Na+/K+ pump has restored the ions to their original locations

c) It establishes the maximum number of graded potentials that can summate

d) Two of the above are correct

d) a) b) and c) are correct

Please provide an explanation. Thank you.

Answer 1

Option A - It establishes the maximum frequency with which a nerve cell can conduct action potentials

opening of the Na+ channels, spontaneously and rapidly leads to their inactivation. At the peak of the action potential, all Na+ channels become inactivated. When Na+ channels are inactivated, they cannot be immediately opened again (see figure on Na+ channel inactivation). Recovery from inactivation is a time- and voltage-dependent process, and full recovery usually takes about 3-4 ms. Therefore, it takes about 3-4 ms for all Na+ channels to come out of inactivation in order to be ready for activation (opening) again. The period from the initiation of the action potential to immediately after the peak is referred to as the absolute refractory period (ARP) .This is the time during which another stimulus given to the neuron (no matter how strong) will not lead to a second action potential. Thus, because Na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials. The absolute refractory period takes about 1-2 ms.

relative refractory period (RRP)

After the absolute refractory period, Na+ channels begin to recover from inactivation and if strong enough stimuli are given to the neuron, it may respond again by generating action potentials. However, during this time, the stimuli given must be stronger than was originally needed when the neuron was at rest. This situation will continue until all Na+ channels have come out of inactivation. The period during which a stronger than normal stimulus is needed in order to elicit an action potential is referred to as the relative refractory period (RRP). During the relative refractory period, since pK remains above its resting value , continued K+ flow out of the cell would tend to oppose any depolarization caused by opening of Na+ channels that have recovered from inactivation.