Internal anal sphincter muscle

The internal anal sphincter is a continuation and thickening of the inner circular layer of smooth muscle of the rectum. It is composed entirely of smooth muscle fibers and forms the inner circumferential muscle ring at the terminal part of the rectum. Unlike the external anal sphincter, which is made of skeletal muscle, the internal anal sphincter operates involuntarily and is regulated by the autonomic nervous system.

It is located beneath the mucosa of the anal canal and deep to the external anal sphincter, contributing to the maintenance of resting anal tone and playing a critical role in fecal continence.

Species-specific Details

In carnivores such as dogs and cats, the internal anal sphincter is well-developed and clearly distinguishable. It lies just internal to the external anal sphincter. It appears as a circular band of smooth muscle that is tonically contracted under resting conditions and relaxes reflexively during defecation. The external sphincter, being voluntary, works in coordination with the internal sphincter during this process.

In the horse, the internal anal sphincter is present as a thickened area of smooth muscle at the terminal rectum. It is somewhat less prominent than in carnivores but still serves the same physiological function. It contributes to anal closure and continence.

In ruminants, the internal anal sphincter is formed by the thickened smooth muscle of the inner circular layer of the rectal wall. It blends partially with surrounding connective tissue and the muscularis mucosae. It is evident at the anorectal junction and helps maintain continence.

In the pig, the internal anal sphincter is easily recognized as a concentrically thickened region of smooth muscle around the anal canal. It plays a significant role in controlling the passage of feces by maintaining tonic contraction and relaxing during defecation.

Nervous Control and Function

The internal anal sphincter is under involuntary control. Its activity is modulated by the following nervous pathways:

• Sympathetic fibers, primarily from the hypogastric nerves, maintain the tonic contraction of the muscle at rest.

• Parasympathetic fibers, mainly from the pelvic nerves, induce relaxation during defecation.

• The enteric nervous system also contributes through local reflexes and neural networks intrinsic to the intestinal wall.

The sphincter remains contracted most of the time to prevent passive leakage of feces. During defecation, it relaxes in coordination with rectal contraction and external sphincter relaxation.