Sphincter

Functions

Classifications

• 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 through muscle contraction around or within the structure.

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

  Examples

• The sphincter pupillae, or pupillary sphincter, belonging to the iris in the eye.
• The orbicularis oculi muscle, a muscle around the eye.
• The upper oesophageal sphincters
• The lower sphincter, or cardiac sphincter, at the upper portion of the stomach. This sphincter prevents the acidic contents of the stomach from moving upward into the esophagus.
• The pyloric sphincter, at the lower end of the stomach.
• The ileocecal sphincter at the junction of the small intestine and the large intestine, which functions to limit the reflux of colonic contents back into the ileum.
• The sphincter of Oddi, or Glisson's sphincter, controlling secretions from the liver, pancreas and gall bladder into the duodenum.
• The sphincter urethrae, or urethral sphincter, controlling the exit of urine from the body.
• At the anus, there are two sphincters which control the exit of feces from the body. The inner sphincter is involuntary and the outer is voluntary.
• The microscopic precapillary sphincters function to control the blood flow into each capillary in response to local metabolic activity.
• The preputial sphincter, belonging to the human foreskin.