Question

You are performing an experiment on the axon of a neuron. You give the axon two suprathreshold stimuli less than 0.002 seconds apart. However, you only see one action potential occur on your monitor. You decide to try the experiment again using the same voltage on the first stimulus and a stronger stimulus for the second stimulus. The two stimuli are again given 0.002 seconds apart. This time you see two action potentials.

Which of the following concepts best explains why you saw only one action potential during the first experiment, but observed two action potentials during the second experiment?

All or none law

Absolute refractory period

Membrane potential

Relative refractory period

Answer 1

The phenomena observed are due to the effects of the absolute and relative refractory periods on neuron excitability. No action potentials can be generated during the absolute period, whereas during the relative period, a stronger-than-normal stimulus is required.

Step-by-step explanation:

The concept that best explains why only one action potential occurred during the first experiment but two action potentials were observed during the second experiment is related to the absolute refractory period and relative refractory period. During the absolute refractory period, a second action potential cannot be initiated, no matter the strength of the stimulus, due to the inactivation of the voltage-gated Na+ channels. Once the neuron enters the relative refractory period, it is possible to initiate a new action potential, but it requires a stronger stimulus to overcome the outward flow of K+.

A suprathreshold stimulus is one that is strong enough to depolarize the membrane to the action potential threshold, leading to an action potential. However, during the relative refractory period, because of the K+ outflow, an even stronger stimulus than usual is necessary to reach the action potential threshold and initiate a second action potential.