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a ring of muscle that contracts to close an opening (同)anatomical sphincter, sphincter_muscle

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/04/10 11:25:59」(JST)

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A sphincter is a cylindrical muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals; there are over 60 types in the human body, some microscopically small, in particular the millions of precapillary sphincters.^[1] Sphincters relax at death, possibly releasing fluids.^[2]

Functions

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

Many sphincters are used every day in the normal course of digestion and vision. For example, the lower esophageal sphincter (aka: cardiac sphincter), which resides at the top of the stomach, keeps stomach acids and other stomach contents from pushing up and into the esophagus. During flexion of sphincter/circular muscles, the lumen associated with the sphincter dilates (opens). This dilation is caused from the "shortening" of the sphincter muscle. While relaxation of a sphincter muscle causes the lumen to close, which is caused by the lengthening of the associated sphincter muscle.

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 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.

Examples

Diagram of Ileocaecal valve and sphincter

The sphincter pupillae, or pupillary sphincter, belonging to the iris in the eye.
The orbicularis oculi muscle, a muscle around the eye.
The upper and lower oesophageal sphincters.
The cardia (lower esophageal 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 (ileum) 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 (see internal anal sphincter and external anal sphincter). 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.^[1]

References

^ ^a ^b Vander, Arthur; Sherman, James; Luciano, Dorothy (1994). Human Physiology: The Mechanism of Body Function (Sixth Edition, International Edition). McGraw Hill, Inc. pp. 437–440. ISBN 0-07-113761-0.
^ http://www.medscape.com/viewarticle/716463_5

English Journal

The use of a bulldog clamp to control the dorsal vein complex during robot-assisted radical prostatectomy.

Tüfek I, Atuğ F, Argun B, Keskin S, Obek C, Coşkuner E, Kural AR.SourceDepartment of Urology, Istanbul Acibadem University, Istanbul, Turkey. iltertuf@gmail.com
Journal of endourology / Endourological Society.J Endourol.2012 Dec;26(12):1605-8. doi: 10.1089/end.2012.0153. Epub 2012 Aug 27.
Optimal control of the dorsal venous complex (DVC) is a critical step in robot-assisted radical prostatectomy (RARP). If DVC is not controlled properly, bleeding may occur during the apical dissection. On the other hand, if it is controlled well, a bloodless field is attained and, thus, a precise ap
PMID 22691123

[Anterior thoracic transdural spinal cord herniation: a sporadic, treatable cause of progressive myelopathy].

Vergeer RA, Groen RJ.SourceUniversitair Medisch Centrum Groningen, afd. Neurochirurgie, Groningen, the Netherlands. ra.vergeer@umcg.nl
Nederlands tijdschrift voor geneeskunde.Ned Tijdschr Geneeskd.2012;156(28):A4824.
Anterior thoracic transdural spinal cord herniation (ATTSCH) is a rare cause of slowly progressing thoracic myelopathy. The myelopathy is caused by herniation and subsequent trapping of the thoracic spinal cord through a defect in the anterior dural sac. We describe the case of a man, 38 years of ag
PMID 22805793

[Characteristics of motility function of the esophagus in patients with aphagia].

Pinchuk TP, Abakumov MM, Kvardakova OV, Avdiunina IA.AbstractIn article is shown the results of investigation of motility function of esophagus in 18 patients with pharyngoesophageal obstruction. 16 patients were operated. The essence of method was in simultaneous registration of UES intracavitary pressure and impedance. We discovered that all patients have both not opening of for the swallow and disturbances of sphincteral function of LES and contractive activity of thoracic esophagus. Dynamic of this parameters in 7 operated patients was monitored. Obtained data were analyzed according to clinical results of surgery.
Patologicheskaia fiziologiia i èksperimental'naia terapiia.Patol Fiziol Eksp Ter.2010 Apr-Jun;(2):38-42.
In article is shown the results of investigation of motility function of esophagus in 18 patients with pharyngoesophageal obstruction. 16 patients were operated. The essence of method was in simultaneous registration of UES intracavitary pressure and impedance. We discovered that all patients have b
PMID 20804073