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

smooth muscle Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...

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 segment ...

; its inferior border is in contact with, but quite separate from, the

external anal sphincter The external anal sphincter (or sphincter ani externus ) is a flat plane of skeletal muscle fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus. Anatomy The external anal sphincter measures about 8 ...

. It is myogenic in nature and playing phasic and tonic state for relaxation and contraction. This myogenic tone is based on Ca2+/Calmodulin/MLCK/RhoA/ROCK signaling cascade pathway. Recent studies suggested, BDNF is the member of the neurotrophin family; having next target for basal IAS and NANC relaxation. It is about 5 mm thick, and is formed by an aggregation of the involuntary circular fibers of the rectum. Its lower border is about 6 mm from the orifice of the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

Actions

Its action is entirely

involuntary Involuntary means ''unintended''. An involuntary action is one that is unintentional, i.e. without volition or will; see volition (psychology) and will (philosophy). ''Involuntary'' may also refer to: * ''Involuntary'' (film), a 2008 Swedish film ...

, and it is in a state of continuous maximal contraction. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of the

feces Feces ( or faeces), known colloquially and in slang as poo and poop, 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 large intestine. Feces contain a relati ...

. Sympathetic fibers from the superior rectal and hypogastric plexuses stimulate and maintain internal anal sphincter contraction. Its contraction is inhibited by

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 enteric nervous system is sometimes considered part of t ...

fiber stimulation. This sphincter is tonically contracted most of the time to prevent leakage of fluid or gas, but is relaxed upon distention of the rectal ampulla, requiring voluntary contraction of the puborectalis and external anal sphincter. The internal anal sphincter is not innervated by the

pudendal nerve The pudendal nerve is the main nerve of the perineum. It carries sensation from the external genitalia of both sexes and the skin around the anus and perineum, as well as the motor supply to various pelvic muscles, including the male or fem ...

, which carries somatic (motor and sensory) fibers that provide the innervation to the external anal sphincter.

Role in 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 The rectoanal inhibitory reflex (RAIR) (also known as the anal sampling mechanism, anal sampling reflex, rectosphincteric reflex, or anorectal sampling reflex) is a reflex characterized by a transient involuntary relaxation of the internal anal ...

(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, both liquid stool elements and mucus, or solid feces. When this loss includes flatus (gas), it is referred ...

(especially partial incontinence to liquid) or mucous

rectal discharge Rectal discharge is intermittent or continuous expression of liquid from the anus ( per rectum). Normal rectal mucus is needed for proper excretion of waste. Otherwise, this is closely related to types of fecal incontinence (e.g., fecal leakage) bu ...

Regenerative medicine

In 2011 it was announced by the

Wake Forest School of Medicine Wake Forest University School of Medicine is the medical school of Wake Forest University, with two campuses located in Winston-Salem, North Carolina and Charlotte, North Carolina, United States. It is affiliated with Atrium Health Wake Forest B ...

that the first

bioengineered Biological engineering or bioengineering is the application of principles of biology and the tools of engineering to create usable, tangible, economically-viable products. Biological engineering employs knowledge and expertise from a number o ...

, functional anal sphincters had been constructed in a laboratory made from muscle and nerve cells, providing a 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