Question

Why do you need to stimulate the nerve for longer at a lower stimulus intensity?

Answer 1

Progressive stronger stimulation causes activation of multiple individual nerve fibers. These nerve fibers yield individual action potential due to dephosphorylation of their membrane. The individual action potential combines to yield a combined action potential or CAP. Hence, a stronger stimulation causes more fibers to reach threshold.

Depolarization of an excitable nerve membrane is the total charge transferred across the membrane. For a short duration stimulus that generates a stead transmembrane current, the charge (Q) = Current (I) * time (T).

Q= I*T

If the duration of stimulation is D and the current is It, then charge required (Qt) to activate the membrane is a product of It X D

Qt= It*D

It = Qt / D

A graph of threshold stimulus strength (I) vs stimulus duration (D) will decline when the duration of stimulus is increased. As the stimulus duration decreases, the strength of stimulus required to reach threshold increases. If the strength of stimulus decreases, then the duration of stimulus increases. Hence, at lower stimulus intensity, more time (duration) is required for the membrane potential to reach the threshold. Thus, lower stimulus intensity requires longer duration to stimulate a nerve.