Tonic (physiology)

This term is typically used in opposition to a fast response.

For instance, tonic muscles are contrasted by the more typical and much faster twitch muscles, while tonic sensory nerve endings are contrasted to the much faster phasic sensory nerve endings.

Tonic muscles are much slower than twitch fibers in terms of time from stimulus to full activation, time to full relaxation upon cessation of stimuli, and maximal shortening velocity.

In contrast, phasic receptors adapt rapidly to a stimulus.

The response of the cell diminishes very quickly and then stops.