The rectal sphincter is a group of muscles located at the end of the digestive tract. These muscles are essential for maintaining continence, the ability to control bowel movements. Their coordinated action allows individuals to hold stool until an appropriate time for defecation.
Anatomy of the Rectal Sphincter
The rectal sphincter complex consists of two distinct muscles: the internal anal sphincter (IAS) and the external anal sphincter (EAS). The internal anal sphincter is an involuntary muscle, meaning it operates without conscious thought or control. It is composed of smooth muscle tissue and represents a thickened continuation of the circular muscle layer of the rectum. This inner muscle maintains a constant state of mild contraction, providing approximately 70-85% of the resting pressure within the anal canal.
Encircling the internal anal sphincter is the external anal sphincter, a voluntary muscle. Unlike the internal sphincter, it is made of skeletal muscle, allowing conscious control. This outer muscle can be consciously contracted to increase pressure in the anal canal, preventing the leakage of stool or gas and allowing an individual to delay defecation.
The Mechanism of Bowel Control
Bowel control involves an interplay between the internal and external anal sphincters and sensory input from the anal canal. When stool enters the rectum, its walls stretch, triggering the rectoanal inhibitory reflex (RAIR). This reflex causes the internal anal sphincter to momentarily relax, allowing rectal content to descend into the anal canal.
The anal canal contains specialized sensory receptors that distinguish between solid, liquid, or gas, informing the brain about the contents. If it is an inappropriate time for defecation, the external anal sphincter is consciously contracted, closing off the anal canal and preventing leakage. This voluntary contraction can be sustained until a suitable time for a bowel movement is found.
During defecation, both the internal and external anal sphincters relax in coordination with abdominal muscle contractions. The internal anal sphincter relaxes automatically due to the rectoanal inhibitory reflex, while the external anal sphincter relaxes voluntarily. This coordinated relaxation allows stool to pass through the anal canal and out of the body.
Common Sphincter Dysfunctions
Rectal sphincter dysfunction can lead to issues like fecal incontinence, the involuntary leakage of stool or gas. This often results from weakness or damage to the anal sphincter muscles or their controlling nerves. Childbirth, particularly vaginal delivery, is a common cause of external anal sphincter injury, directly impairing muscle contraction.
Nerve damage from conditions like diabetes, spinal cord injuries, or prolonged straining can also weaken the sphincter muscles. Other issues include anal fissures, small tears in the anal canal lining often caused by hard stools or straining, leading to pain and bleeding.
Sphincter spasms, characterized by involuntary tightening of the anal muscles, can cause discomfort. These spasms may contribute to conditions like proctalgia fugax, a sudden, severe rectal pain lasting a short duration.
Management and Treatment Options
Addressing rectal sphincter dysfunctions often begins with conservative approaches. Dietary adjustments, like increasing fiber and fluid intake, promote softer stools and reduce straining. Pelvic floor muscle training, such as Kegel exercises, strengthens the external anal sphincter and supporting pelvic floor muscles, improving voluntary control.
If conservative methods are insufficient, advanced treatments may be considered. Biofeedback therapy teaches individuals to better control their anal sphincter muscles using real-time feedback. Medications, such as anti-diarrheal agents, can firm stools and reduce bowel movement frequency, aiding continence.
For severe cases, surgical interventions may be necessary. Sphincteroplasty repairs damaged sphincter muscles, often after obstetric injuries, by rejoining muscle ends. Sacral nerve stimulation involves implanting a device that sends electrical pulses to nerves controlling bowel function, improving sphincter control and reducing incontinence.