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The human bowel is part of the digestive system, working to digest food, absorb nutrients, and expel waste. This process involves several muscles, including the pelvic floor muscles, anal sphincters, and the levator ani muscle. The pelvic floor is a sheet of muscular tissue that supports the pelvic organs and separates the rectum from the anal canal. The anal sphincters, both internal and external, control the opening and closing of the anus, allowing for the passage of stool. The levator ani muscle, part of the pelvic floor, is crucial for bowel control. Together, these muscles and their coordinated contractions and relaxations enable humans to control their bowel movements.
| Characteristics | Values |
|---|---|
| Muscles controlling bowel movements | Pelvic floor muscles, anal sphincters (internal and external), rectal muscles, abdominal muscles |
| Involuntary muscles | Internal anal sphincter (made of smooth muscle) |
| Voluntary muscles | External anal sphincter (made of striated muscle) |
| Exercises to improve bowel control | Kegel exercises, biofeedback therapy |
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What You'll Learn
The pelvic floor and abdomen muscles
During a bowel movement, the pelvic floor muscles relax and descend slightly, allowing the muscles in the rectum to push stool out. Conversely, when we need to hold in stool, the pelvic floor muscles contract and squeeze, preventing its release. The brain communicates with the pelvic floor muscles through electrical signals transmitted via nerves in the sacral area of the back. Two sacral nerves critical to bowel function are the pudendal nerve and the pelvic splanchnic nerve.
The anal sphincters, which are circular muscles at the end of the anal canal, are also key to bowel control. There are two sets of anal sphincters: internal (or inner) and external (or outer). The internal sphincter, composed of smooth muscle, operates involuntarily to keep the anus closed until a bowel movement is desired. The external sphincter, composed of striated muscle similar to the pelvic floor muscles, can be controlled voluntarily to either hold in or release stool.
To enhance bowel control, individuals can perform Kegel exercises, which strengthen the pelvic and rectal muscles. These exercises are particularly beneficial for individuals with weak pelvic floor muscles or those experiencing fecal incontinence. Additionally, contracting the abdomen muscles while bearing down can increase abdominal pressure and facilitate bowel evacuation. Maintaining a healthy bowel routine involves regular bowel movements, a high-fibre diet, adequate fluid intake, and physical activity.
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The anal canal and rectum
The anal canal is surrounded by a mucous membrane lining, known as the anoderm, which is rich in nerves. These nerves send signals to the brain, allowing it to recognise the consistency of stool and the fullness of the bowel. This information is then processed by the cerebrum, which sends signals back to the anus, causing the inner sphincter to open and creating the urge to empty the bowels.
The inner anal sphincter is ring-shaped and made of smooth muscle. It is involuntary, keeping the anus closed until a bowel movement is required. The outer anal sphincter supports the inner sphincter and can be controlled voluntarily, allowing us to hold or pass stool when needed. However, the outer sphincter can also act involuntarily, working with the inner sphincter to control the passage of stool.
The pelvic floor muscles also play a crucial role in bowel control. These muscles form a supportive strap around the junction of the rectum and anal canal, helping to regulate stool passage. They relax and contract in coordination with the anal sphincters to facilitate bowel movements. Weakened pelvic floor muscles can lead to ineffective support for the pelvic openings, potentially resulting in fecal incontinence.
To improve bowel control and manage issues like fecal incontinence or constipation, individuals may undergo bowel retraining, which includes exercises to strengthen the pelvic and rectal muscles, such as Kegel exercises and biofeedback therapy.
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The internal anal sphincter
The IAS plays a crucial role in fecal continence, generating tone and providing resting anal pressure. It is responsible for more than 70% of this pressure, which prevents the unintentional release of stool. When the rectum fills beyond a certain capacity, the rectal walls distend, triggering the defecation cycle. During this process, the IAS relaxes, allowing rectal contents to descend into the anal canal, where specialised mucosa determines whether it is gas, liquid, or solid.
Impaired IAS function can lead to fecal incontinence, particularly partial incontinence to liquid, or mucous rectal discharge. Studies have shown that patients with fecal incontinence often exhibit extremely low resting pressures in the IAS. The IAS is a critical structure for maintaining bowel control, and its dysfunction can significantly impact an individual's quality of life.
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The external anal sphincter
The sphincter receives innervation from the bilaterally paired inferior anal nerve, a branch of the pudendal nerve, which is derived from the ventral rami of S2-S4. It may also receive additional motor innervation from the nerve to levator ani. The external anal sphincter is largely under voluntary control by the somatic nervous system, which allows it to stay in a contracted state. This voluntary control allows individuals to empty their bowels or hold back their stool when needed.
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The role of Kegel exercises
Bowel movements are controlled by several parts of the end of the digestive tract: the rectum and anal canal, the sphincters (circular muscles), and hemorrhoids. These parts are embedded in the pelvic floor muscles. The pelvic floor is made up of muscles, ligaments, and connective tissue. It supports the pelvic organs and closes off the lower part of the pelvic cavity.
Kegel exercises, also known as pelvic floor exercises, are a way to strengthen the pelvic floor muscles. These muscles support the organs in the pelvis, such as the bladder, bowel, and vagina. By tightening and releasing the pelvic floor muscles, Kegel exercises can improve bladder and bowel health, as well as sexual health.
The pelvic floor muscles can be weakened by age, pregnancy, childbirth, surgery, obesity, or other health conditions. Weak pelvic floor muscles can lead to urinary or fecal incontinence, as well as pelvic organ prolapse. By doing Kegel exercises, individuals can improve their control over bladder and bowel functions, reducing instances of leakage or accidental passing of gas.
To perform Kegel exercises correctly, one should focus on isolating the contraction of the pelvic floor muscles. While sitting or standing, the individual should squeeze the pelvic floor muscles without engaging the inner thighs, back, buttocks, or stomach. It is important to maintain normal breathing throughout the exercise. Over time, Kegel exercises can help improve bowel control and reduce related symptoms.
In summary, Kegel exercises play a crucial role in strengthening the pelvic floor muscles, which are essential for controlling bowel movements. By performing these exercises correctly and consistently, individuals can improve their bowel health and manage related conditions effectively.
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Frequently asked questions
The muscles that control bowel movements are the internal and external anal sphincters, the pelvic floor muscles, and the levator ani muscle.
The internal anal sphincter (IAS) is made of smooth muscle and we do not have voluntary control over this muscle. It works automatically to keep the anus closed until we are ready to pass stool.
The external anal sphincter (EAS) is made of striated muscle, similar to the pelvic floor muscles. We have voluntary control over the EAS, which allows us to hold on in cases of flatulence or diarrhoea.
To strengthen the muscles that control bowel movements, you can perform Kegel exercises, which increase pelvic and rectal muscle tone.
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