Tonic in physiology refers to a physiological response which is slow and may be graded. 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
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.
[Kardong, K. 2008. Vertebrates: Comparative Anatomy, Function, Evolution. 5th edition. McGraw-Hill Science/Engineering/Math. ] These muscles are rarely found in mammals (only in the muscles moving the eye and in the middle ear), but are common in reptiles and amphibians.
Tonic sensory receptors
Tonic sensory input adapts slowly to a stimulus
[http://caspar.bgsu.edu/~courses/Glossary.htm] and continues to produce
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
s over the duration of the stimulus.
[Dawson, M., Schell, A., & Filion, D. (2007). The Electrodermal System. In J. Cacioppo, L. Tassinary, & G. Berntson (Eds.), ''Handbook of Psychophysiology'' (pp. 159-181). Cambridge: Cambridge University Press.] In this way it conveys information about the duration of the stimulus. In contrast, phasic receptors adapt rapidly to a stimulus. The response of the cell diminishes very quickly and then stops.
It does not provide information on the duration of the stimulus;
instead some of them convey information on rapid changes in stimulus intensity and rate.
Examples of tonic receptors are
pain receptors, the
joint capsule
In anatomy, a joint capsule or articular capsule is an envelope surrounding a synovial joint.muscle spindle
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...
,
and the
Ruffini corpuscle
The Bulbous corpuscle or Ruffini ending or Ruffini corpuscle is a slowly adapting mechanoreceptor located in the cutaneous tissue between the dermal papillae and the hypodermis. It is named after Angelo Ruffini
Angelo Ruffini (Pretare of Arquata ...
.
See also
*
Tonic-clonic seizure
References
Physiology
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