Refractory Period

For a very short time following passage of a nerve impulse, a threshold stimulus will not trigger another impulse on an axon. This brief period, called the refractory period, has two parts. During the absolute refractory period, which lasts about 1/2,500 of a second, the axon's membrane is changing in sodium permeability and cannot be stimulated. This is followed by a relative refractory period, during which the membrane is reestablishing its resting potential. While the membrane is in the rela tive refractory period, even though repolarization is incomplete, a threshold stimulus of high intensity may trigger an impulse.

As time passes, the intensity of stimulation required to trigger an impulse decreases until the axon's original excitability is restored. This return to the resting state usually takes from 10 to 30 milliseconds.

The refractory period limits how many action potentials may be generated in a neuron in a given amount of time. Remembering that the action potential itself takes about a millisecond, and adding the time of the absolute refractory period to this, the maximum theoretical frequency of impulses in a neuron is about 700 per second. In the body, this limit is rarely achieved—frequencies of about 100 impulses per second are common.

Events Leading to Nerve Impulse Conduction

1. Nerve cell membrane maintains resting potential by diffusion of Na+ and K+ down their concentration gradients as the cell pumps them up the gradients.

2. Neurons receive stimulation, causing local potentials, which may sum to reach threshold.

3. Sodium channels in a local region of the membrane open.

4. Sodium ions diffuse inward, depolarizing the membrane.

5. Potassium channels in the membrane open.

6. Potassium ions diffuse outward, repolarizing the membrane.

7. The resulting action potential causes an electric current that stimulates adjacent portions of the membrane.

8. Series of action potentials occurs sequentially along the length of the axon as a nerve impulse.

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Essentials of Human Physiology

Essentials of Human Physiology

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