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The body's ability to control bowel movements is made possible by several parts of the end of the digestive tract, including the anal canal, rectum, sphincters (circular muscles), and hemorrhoids. These parts are embedded in the pelvic floor muscles. The inner and outer sphincter muscles at the end of the anal canal close off the opening of the bowel (anus). The inner sphincter is ring-like and made up of smooth muscle, and the outer sphincter supports the inner sphincter and can be controlled voluntarily. Conditions such as anismus (dyssynergic defecation) can cause dysfunction in the pelvic floor muscles, making it difficult to pass stool and leading to chronic constipation.
| Characteristics | Values |
|---|---|
| Muscle that controls poop | External anal sphincter |
| Involuntary muscle that controls poop | Internal anal sphincter |
| Muscle that forms a flap-like valve to control stool passing | Puborectalis |
| Type of muscle that forms the internal anal sphincter | Smooth muscle |
| Type of muscle that forms the external anal sphincter | Striated muscle |
| Nerves that are vital to the functioning of the bowel | Pudendal nerve and pelvic splanchnic nerve |
| Muscles involved in constipation | Pelvic floor and lower colon |
| Muscles involved in defecation | Pelvic floor, expiratory chest, diaphragm, abdominal wall, pelvic diaphragm |
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What You'll Learn
- The pelvic floor is a group of muscles that form a hammock at the bottom of the pelvis
- The anal canal is closed off by sphincter muscles and hemorrhoids
- The inner sphincter is ring-shaped and made of smooth muscle
- The outer sphincter supports the inner sphincter and can be controlled voluntarily
- Anismus, or dyssynergic defecation, is a condition that makes it hard to relax pelvic muscles
The pelvic floor is a group of muscles that form a hammock at the bottom of the pelvis
The pelvic floor is a group of muscles that form a hammock-like structure at the bottom of the pelvis. It physically supports the pelvic organs and closes off the lower part of the pelvic cavity. The pelvic floor is made up of muscles, ligaments, and connective tissue. It plays a crucial role in bowel control and is essential for maintaining continence and facilitating complete bowel emptying.
An important muscle group within the pelvic floor is the levator ani, which includes the puborectalis muscle. The puborectalis forms a supportive strap around the junction between the rectum and the anal canal. This muscle helps to control bowel movements by forming a flap-like valve, preventing stool from passing too easily into the anal canal. The levator ani and puborectalis muscles work in coordination with the internal and external anal sphincters.
The internal anal sphincter (IAS) is composed of smooth muscle and is not under voluntary control. It works automatically to keep the anus closed until a bowel movement is initiated. On the other hand, the external anal sphincter (EAS) is made of striated muscle, similar to the pelvic floor muscles, and can be voluntarily controlled. This allows individuals to hold their bowel movements when necessary.
The coordination between the pelvic floor muscles, sphincters, and the brain is crucial for proper defecation. When the rectum fills up, stretch receptors in the anal canal sense the pressure and send signals to the brain. The brain then sends electrical signals back through the nerves to the pelvic floor muscles and sphincters, initiating the relaxation of the internal sphincter and the external sphincter voluntarily, allowing for the expulsion of stool.
Pelvic floor dysfunction can occur when these muscles and nerves do not coordinate correctly. This can lead to difficulties in relaxing the pelvic floor muscles during bowel movements, resulting in constipation or fecal impaction. It is important to seek medical advice if experiencing any issues related to bowel control or pelvic floor dysfunction, as treatments such as biofeedback therapy, constipation medications, and lifestyle changes may be recommended.
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The anal canal is closed off by sphincter muscles and hemorrhoids
The anal canal is a crucial component of the digestive system, facilitating the passage of food through the intestines. It is controlled by a complex set of muscles, including the internal and external anal sphincters, which work in tandem to regulate the release of feces. These sphincter muscles form two concentric rings that run along the length of the anal canal, ensuring the controlled release of waste.
The internal anal sphincter (IAS), composed of smooth muscle, operates involuntarily to maintain the anus in a closed position until a bowel movement is initiated. This automatic function is essential for preventing accidental defecation. On the other hand, the external anal sphincter (EAS) is composed of striated muscle, similar to the pelvic floor muscles, and is under voluntary control. This voluntary control allows individuals to hold their bowel movements when necessary, such as in cases of wind or diarrhoea.
In addition to the sphincter muscles, hemorrhoids play a role in closing off the anal canal. Hemorrhoidal tissue, composed of blood vessels, connective tissue, and a small amount of muscle, is present in all individuals as a normal part of their anatomy. However, in a small minority of people, hemorrhoids can become enlarged or symptomatic. Hemorrhoids fill with blood to aid in closing the anal canal and preventing leakage. When hemorrhoids become significantly enlarged, they can protrude into the anal canal or even appear externally, leading to potential discomfort and requiring treatment.
The proper functioning of the anal sphincters and hemorrhoids is critical for maintaining bowel control. Dysfunction in these muscles and tissues can lead to conditions such as anismus (dyssynergic defecation), where the muscles and nerves involved in defecation fail to coordinate correctly. This can result in the inability to relax or tighten the muscles appropriately during bowel movements, impacting the passage of stool. Additionally, conditions like anal fissures, which are small tears in the skin lining the anus, can be influenced by sphincter muscle tone. Patients with tight anal sphincter muscles are more susceptible to developing anal fissures, which can cause severe pain and bleeding during bowel movements.
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The inner sphincter is ring-shaped and made of smooth muscle
The human body has over 50 types of sphincters, which are circular muscles that open and close passages to regulate the flow of substances like bile, urine, and faeces. The anal sphincter, which is a part of the gastrointestinal (GI) tract, has both inner and outer muscles. The inner sphincter is ring-shaped and made of smooth muscle. It is involuntary and prevents stool from leaking out. The outer sphincter is predominantly voluntary and allows one to pass or hold stool on demand.
The inner sphincter is also known as the internal anal sphincter (IAS) and is composed of smooth muscle. It is a part of the anal sphincter complex, which is the most sophisticated sphincter mechanism in the human body. The anal sphincter complex includes the internal anal sphincter and the external anal sphincter muscle, both of which play important roles in maintaining faecal continence. The internal anal sphincter is a smooth muscle, measuring about 30mm in height and 3mm in thickness, and originates from extensions of the circular layers of the rectum into the anal canal.
The internal anal sphincter contracts to close the anal canal and prevent the passage of faeces, and it relaxes to propel faeces. It remains in a continuous state of contraction and only relaxes in response to inhibitory neural input. The inner sphincter is controlled by the autonomic nervous system and acts through the rectoanal inhibitory reflex (RAIR). This reflex involves the reflexive relaxation of the internal anal sphincter in response to rectal distension.
The inner sphincter is crucial for maintaining continence and ensuring that the rectum remains closed when one is sleeping, for example. It works in conjunction with the outer sphincter to control the passage of stool. The inner sphincter opens when the rectum fills up, and stretch receptors in the wall of the anus send signals to the brain. The brain then processes these signals and sends back signals to the lining of the anus, causing the inner sphincter to open and creating the urge to empty one's bowels.
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The outer sphincter supports the inner sphincter and can be controlled voluntarily
The human digestive system is a complex mechanism that pushes food through the intestines using muscular contractions (peristalsis). The food is then separated by bands of muscles, or sphincters, which act as valves. These sphincters, along with the rectum and anal canal, help control bowel movements.
The anal sphincter muscles are made up of an internal and external sphincter, which form two concentric rings that run along the length of the anal canal. The internal anal sphincter (IAS) is made of smooth muscle and is not under voluntary control. It works automatically to keep the anus closed until we are ready to have a bowel movement. The external anal sphincter (EAS), on the other hand, is made of striated muscle, similar to the pelvic floor muscles, and can be controlled voluntarily.
The ability to voluntarily control the outer sphincter is learned during toilet training in early childhood. However, it is important to note that this control can be lost due to various factors, such as physical injury, nerve damage, prior surgeries, constipation, or intense fright. In such cases, individuals may experience faecal incontinence.
In summary, the outer sphincter plays a crucial role in supporting the inner sphincter and can be voluntarily controlled to manage bowel movements. This voluntary control is a learned skill that allows us to manage our bodily functions in various social and private settings.
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Anismus, or dyssynergic defecation, is a condition that makes it hard to relax pelvic muscles
Anismus, or dyssynergic defecation, is a condition that makes it difficult to pass stools due to the failure of normal relaxation of the pelvic floor muscles during defecation. It is characterised by the inability to properly relax the pelvic floor muscles, which are responsible for bowel movements, during defecation. This can lead to chronic constipation, as hardened stool becomes impacted in the bowels, making it even harder to pass. The condition can affect both children and adults and occurs in both men and women, although it is more common in women.
The pelvic floor muscles, along with the nerves they work with, play a crucial role in bowel control. When functioning properly, these muscles contract and relax in coordination with the anal and rectal muscles to facilitate the passage of stool. However, in individuals with anismus, the pelvic floor muscles, specifically the puborectalis and the external anal sphincter muscles, fail to relax during defecation. This results in a functional blockage, making it challenging to pass stool and impacting daily life.
The symptoms of anismus can vary in intensity and may include persistent difficulty in starting or completing bowel movements, often requiring excessive pushing or straining. Individuals may also experience a frequent sensation of incomplete evacuation, leading to multiple bathroom visits throughout the day. In some cases, manual assistance or changing positions may be necessary to complete a bowel movement.
Biofeedback therapy is the most effective treatment for anismus, with success rates of up to 80%. This therapy involves a mind-body physiotherapy technique that uses painless electrodes placed on the skin to sense physiological parameters such as heart rate, muscle tension, and brain activity. During therapy, individuals are guided through various muscle-activating and tension-relieving strategies. Through practice, they can train their pelvic floor muscles to relax effectively, improving their ability to pass stool.
In addition to biofeedback therapy, other treatments for anismus may include laxatives or pharmaceutical interventions for constipation. While these medications may not directly address the muscle coordination issues, they can help prevent stool from becoming severely impacted and ease the passage of stool. Pelvic floor physical therapy is also recommended, focusing on retraining muscle coordination, strengthening pelvic floor muscles, and improving overall bowel function.
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Frequently asked questions
The muscles involved in controlling pooping are the anal sphincters (inner and outer), the pelvic floor muscles, and the rectal muscles.
The inner sphincter closes off the rectum and opens involuntarily when the rectum fills up, creating the urge to defecate. The pelvic floor relaxes and drops down, allowing the muscle in the rectum to push stool out. The individual can then voluntarily relax the outer sphincter to release the stool or tense the outer sphincter to hold it in.
Conditions such as anismus (dyssynergic defecation) and pelvic floor dyssynergia can cause dysfunction in these muscles, leading to chronic constipation. Treatments include biofeedback therapy, constipation medications, and lifestyle changes. Anorectal manometry testing can be used to assess muscle functioning and provide guidance for treatment.
