Local Response and Local Excitation

Characteristics of the Local Response

For local excitation to occur in a tissue, it must be stimulated by an insufficient intensity—namely, a subthreshold stimulus. The primary feature of this response lies in its name: it is strictly local.

The excitation appears exclusively at the point of stimulus application. Any attempt by this signal to spread to neighboring membrane regions fails because it decays very rapidly. Furthermore, the amplitude of the resulting response is not constant; it depends directly on the strength of the applied stimulus. Due to this direct dependency, the local response does not obey the classic all-or-none law.

A crucial property of local excitation is the ability to undergo summation. If subthreshold stimuli are applied repeatedly, their effects can accumulate. During this process, membrane excitability always increases in the area of the local response. If summation brings the excitation to a critical threshold, the local process develops into a full propagating action potential.

Features of Propagating Excitation

Unlike the local response, propagating excitation manifests as a full-fledged action potential (AP). Its generation requires threshold or suprathreshold stimuli, or it can result from the successful summation of a series of local excitations.

The action potential differs fundamentally in its physiological characteristics:

• Propagation without decrement. The signal can travel over vast distances (on a cellular scale) without any reduction in its initial amplitude.
• Absence of summation. Unlike local responses, action potentials cannot be summed together.
• The all-or-none law. This excitation strictly obeys the rule. The tissue either produces the maximum possible response for its current state (if the stimulus reaches threshold) or fails to respond at all (if the stimulus is too weak).

Brief Comparison