Biology, Eighth Edition (Raven)

Chapter 44: The Nervous System

Action Potential Propagation in an Unmyelinated Axon

Why does an action potential travel down an axon? The basic unit of nerve transmission is the action potential. An action potential is a brief electrical signal that travels along an axon at great speed. The action potential is a rapid change in the relative ion concentrations on either side of the cellular membrane and a rapid return to the original concentrations. The change in ion concentration is called a depolarization of the membrane. The transmission of the action potential is caused by three properties of the system. The depolarization spreads to adjacent areas, the depolarization is self-correcting, and during the recovery period depolarization cannot re-occur. Therefore when an action potential starts it triggers further depolarization but only in the direction along the axon in which depolarization has not already occurred.

View the animation below, then complete the quiz to test your knowledge of the concept.



1.

An action potential
A)prevents the neuron cell membrane from altering its charge.
B)causes the inside of the neuron cell membrane to become positive and the outside negative.
C)causes the inside of the neuron cell membrane to become positive and the outside neutral.
D)causes the outside of the neuron cell membrane to become positive and the inside negative.
E)causes the outside of the neuron cell membrane to become positive and the inside neutral.
2.

An action potential generates local currents that tend to ________ the membrane immediately adjacent to the action potential.
A)depolarize
B)repolarize
C)hyperpolarize
D)stabilize
E)neutralize
3.

The absolute refractory period is the period of time during which
A)a second action potential cannot be generated.
B)a second action potential is generated.
C)the action potential ceases.
D)the action potential decreases in magnitude.
E)the action potential changes direction.
4.

Action potential propagation occurs in both directions along the axon.
A)True
B)False
5.

Threshold is the minimum current required for the cell membrane to generate an action potential.
A)True
B)False
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