Learning Objective

By the end of this section, the learner will be able to describe the concept of subthreshold stimuli, explain how membrane depolarization relates to stimulus strength, understand summation of subthreshold depolarizations, and predict whether a stimulus will generate an action potential.

Subthreshold Stimuli

A subthreshold stimulus is any depolarizing input that does not reach the threshold potential required to trigger an action potential.

Magnitude-dependent depolarization:

• The size of the membrane potential change (Em) is proportional to the stimulus strength.
• Small stimuli produce small depolarizations; larger but still subthreshold stimuli produce larger depolarizations.

Membrane repolarization:

• After the stimulus ends, the membrane returns to resting potential.
• No action potential is generated because the threshold was not reached.

Summation of depolarizations:

• If a second subthreshold stimulus is applied before repolarization is complete, the two depolarizations add together.
• This temporal summation can sometimes bring Em to threshold and trigger an action potential.

Clinical note:

Understanding subthreshold stimuli and summation is critical for interpreting graded potentials in dendrites and the initiation of neuronal firing.

Activity