The internal anal sphincter, IAS, or sphincter ani internus is a ring of

smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...

that surrounds about 2.5–4.0 cm of the

anal canal The anal canal is the part that connects the rectum to the anus, located below the level of the pelvic diaphragm. It is located within the anal triangle of the perineum, between the right and left ischioanal fossa. As the final functional s ...

. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum. The internal anal sphincter aids the sphincter ani externus to occlude the anal aperture and aids in the expulsion of the

feces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...

. Its action is entirely involuntary. It is normally in a state of continuous maximal contraction to prevent leakage of faeces or gases. Sympathetic stimulation stimulates and maintains the sphincter's contraction, and

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulat ...

stimulation inhibits it. It becomes relaxed in response to distention of the rectal ampulla, requiring voluntary contraction of the puborectalis and external anal sphincter to maintain continence, and also contracts during the bulbospongiosus reflex.

Structure

The internal anal sphincter is the specialised thickened terminal portion of the inner circular layer of smooth muscle of the large intestine. It extends from the pectinate line (anorectal junction) proximally to just proximal to the anal orifice distally (the distal termination is palpable). Its muscle fibres are arranged in a spiral (rather than a circular) manner. At its distal extremity, it is in contact with but separate from the external anal sphincter.

Innervation

The sphincter receives extrinsic autonomic innervation via the

inferior hypogastric plexus Inferior may refer to: * Inferiority complex * An anatomical term of location * Inferior angle of the scapula, in the human skeleton * ''Inferior'' (book), by Angela Saini * '' The Inferior'', a 2007 novel by Peadar Ó Guilín * Inferior good: ...

, with sympathetic innervation derived from spinal levels L1-L2, and parasympathetic innervation derived from S2-S4. The internal anal sphincter is ''not'' innervated by the

pudendal nerve The pudendal nerve is the main nerve of the perineum. It is a Mixed nerve, mixed (motor and sensory) nerve and also conveys Sympathetic nervous system, sympathetic Autonomic nervous system, autonomic fibers. It carries sensation from the exter ...

(which provides motor and sensory innervation to the external anal sphincter).

Function

The sphincter is contracted in its resting state, but reflexively relaxes in certain contexts (most notably during

defecation Defecation (or defaecation) follows digestion and is the necessary biological process by which organisms eliminate a solid, semisolid, or liquid metabolic waste, waste material known as feces (or faeces) from the digestive tract via the anus o ...

). Transient relaxation of its proximal portion occurs with rectal distension and post-prandial rectal contraction (the recto-anal inhibitory reflex and sampling reflex, respectively) while the distal portion of the sphincter remains contracted and the external anal sphincter becomes contracted to maintain continence; this transient relaxation allows passage of stool into the proximal anal canal - this filling is sensed.

Continence

The IAS contributes 55% of the resting pressure of the anal canal. It is very important for bowel continence, especially for liquid and gas. When the rectum fills beyond a certain capacity, the rectal walls are distended, triggering the defecation cycle. This begins with the rectoanal inhibitory reflex (RAIR), where the IAS relaxes. This is thought to allow a small amount of rectal contents to descend into the anal canal where specialized mucosa samples whether it is gas, liquid or solid. Problems with the IAS often present as degrees of

fecal incontinence Fecal incontinence (FI), or in some forms, encopresis, is a lack of control over defecation, leading to involuntary loss of bowel contents—including flatus (gas), liquid stool elements and mucus, or solid feces. FI is a sign or a symptom ...

(especially partial incontinence to liquid) or mucous rectal discharge.

Physiology

Neurophysiology

Sympathetic stimulation is mediated by alpha-2 adrenergic receptors and results in contraction of the sphincter. Parasympathetic stimulation is mediated by muscarinic acetylcholine receptors and results in relaxation of the sphincter. Nitrergic stimulation also produces relaxation which has pharmacological significance.

Clinical significance

Clinical pharmacology

Nitrergic pharmaceutical agents produce relaxation of the muscular tone of the sphincter and are applicable in pathological contexts where this tone is abnormally increased.

Regenerative medicine

In 2011, it was announced by the Wake Forest School of Medicine that the first bioengineered, functional anal sphincters had been constructed in a laboratory made from muscle and nerve cells, providing a proposed solution for anal incontinence."Anal Sphincters"
, Wake Forest School of Medicine. December 12, 2015. Retrieved 3 feb 2017

Additional images

File:illu_intestine.jpg, Intestines File:Human anus-en.svg, Anatomy of the human anus

References

External links

* () {{Authority control Perineum Anus