Applied anatomy of Anal canal 2 - Anal sphincter complex & continence mechanism - Edusurg Clinics
The anal sphincters are muscular structures located around the anal canal, which is the terminal part of the large intestine that opens to the exterior of the body. The anal sphincters play a crucial role in regulating bowel movements and maintaining continence (control over defecation). There are two main types of anal sphincters: the internal anal sphincter and the external anal sphincter.
Internal Anal Sphincter (IAS):
The internal anal sphincter is an involuntary, smooth muscle ring that surrounds the upper part of the anal canal. It is controlled by the autonomic nervous system and is composed of thick circular muscle fibers. The IAS remains contracted most of the time, providing basal tone to the anal canal and helping to maintain continence by preventing involuntary passage of feces or gas. The contraction of the internal sphincter can be overridden during defecation.
External Anal Sphincter (EAS):
The external anal sphincter is a voluntary muscle that surrounds the distal part of the internal sphincter. It consists of skeletal muscle fibers and is under conscious control. The external sphincter can be contracted voluntarily to enhance continence or prevent defecation when necessary. During defecation, this sphincter is consciously relaxed to allow feces to pass through the anal canal.
In addition to these two main sphincters, there are also submucosal glands and folds in the anal canal that secrete mucus to aid in the passage of stool and reduce friction during bowel movements.
The coordination between the involuntary internal anal sphincter and the voluntary external anal sphincter, along with the sensory feedback from the rectum and anus, helps to regulate the process of defecation and maintain fecal continence. Dysfunction of these sphincters or damage to the nerves controlling them can lead to various bowel control issues, such as fecal incontinence or constipation.