Classifications
Sphincters can 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 can also be voluntarily or involuntarily controlled:
- Voluntary sphincters are supplied by somatic nerves.
- Involuntary sphincters are stimulated by autonomic nerves.
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