pupillary sphincter

Sphincter

[sfingk-ter]
A sphincter is a structure, usually a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. There are over 40 different sphincters in the human body; some of these sphincters are microscopic in size.

Many sphincters are used every day in the normal course of digestion. For example, the epiglottis is used to seal off the windpipe when swallowing, so as to ensure that no food or liquid enters the lungs. The use of the epiglottis is a typical example of an involuntary action by the body.

Examples of sphincters

Sphincters prove effective in the mediation of the entrance or release of liquids and fluids; this is evident in the blowholes of numerous marine mammals, for example.

Sphincters can also be further classified into functional and anatomical sphincters:

  • Anatomical sphincters have a localised and often circular muscle thickening to facilitate their action as a sphincter.
  • Functional sphincters do not have this localised muscle thickening and achieve their sphincteric action indirectly through muscle contraction around (extrinsic) or within (intrinsic) the structure.

Sphincters also can be voluntarily or involuntarily controlled:

  • Voluntary sphincters are supplied by somatic nerves
  • Involuntary sphincters are stimulated by autonomic nerves

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