Question

If a stimulating electrode is placed in the middle of a resting axon and an above-threshold voltage is applied to the electrode action potentials

A. will not occur.

B. will start at that point and proceed only toward the axon terminal.

C. will start at that point and proceed only toward the cell body.

D. will start at that point and travel in both directions in the axon.

Answer 1

An action potential is an all-or-nothing event that travels down the axon as voltage-gated ion channels are opened by depolarization. In myelinated axons, propagation is faster due to saltatory conduction. If the threshold is not reached, no action potential occurs.

Step-by-step explanation:

An action potential is an all-or-nothing event; it either happens or it does not. If the threshold is not reached, then no action potential occurs. If depolarization reaches -55 mV, then the action potential continues and runs all the way to +30 mV, at which K+ causes repolarization, including the hyperpolarizing overshoot.

The action potential travels down the axon as voltage-gated ion channels are opened by the spreading depolarization. In unmyelinated axons, this happens in a continuous fashion because there are voltage-gated channels throughout the membrane. In myelinated axons, propagation is described as saltatory because voltage-gated channels are only found at the nodes of Ranvier and the electrical events seem to 'jump' from one node to the next. Saltatory conduction is faster than continuous conduction, meaning that myelinated axons propagate their signals faster.

Because of the threshold, the action potential can be likened to a digital event-it either happens or it does not. If the threshold is not reached, then no action potential occurs. If depolarization reaches -55 mV, then the action potential continues and runs all the way to +30 mV, at which K+ causes repolarization, including the hyperpolarizing overshoot.