Chapter 16 Endocrine and Neuroendocrine Physiology

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The Action Potential

Action potential is when neurons communicate over long distances by generating and sending an electrical signal (otherwise known as nerve impulse)

The Action Potential: An Overview

The action potential is a large change in membrane potential from a resting value of about -70 millivolts (mV) to a peak of about +30 mV, and back to -70 mV again

The action potential results from a rapid change in the permeability of the neuronal membrane to sodium and potassium. The permeability changes as voltage-gated ion channels open and close.

The Action Potential Begins at the Axon Hillock

The action potential is generated at the axon hillock, where the density of voltage-gated sodium channels is greatest

The action potential begins when signals from the dendrites and cell body reach the axon hillock and cause the membrane potential there to become more positive, a process called depolarization.

These local signals travel for only a short distance and are very different from action potentials.

During Depolarization Sodium Moves into the Neuron

As the axon hillock depolarizes, voltage-gated channels for sodium open rapidly, increasing membrane permeability to sodium.

Sodium moves down its electrochemical gradient into the cell

Threshold

If the stimulus to the axon hillock is great enough, the neuron depolarizes about 15 mV and reaches a trigger point called threshold.

At threshold, an action potential is generated. Weak stimuli that do not reach threshold do not produce an action potential. Thus we say that the action potential is an all-or-nothing event.

Action potentials always have the same amplitude and the same duration.

At -55 mV the membrane is depolarized to threshold, and an action potential is generated.

Threshold is a special membrane potential where the process of depolarization becomes regenerative, that is, where a positive feedback loop is generated.

An Action Potential is Generated When a Positive Feedback Loop is Established

• When, and only when, a neuron reaches threshold, a positive feedback loop is established.

• At threshold, depolarization opens more voltage-gated sodium channels.

• This causes more sodium to flow into the cell, which in turn causes the cell to depolarize further andopens still more voltage-gated sodium channels.

• This positive feedback loop produces the rising phase of the action potential.

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