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The process of expelling faeces from the body, known as defecation, involves the coordination of multiple muscle groups. This complex bodily function is regulated by the gastrointestinal, nervous, and musculoskeletal systems. The pelvic floor muscles, in particular, play a crucial role in controlling the passage of stool. These muscles contract to prevent the release of stool and relax to allow defecation. Additionally, the anal sphincters, both internal and external, work in tandem to control the exit of faeces from the rectum. While the internal sphincter operates involuntarily, the external sphincter can be consciously controlled, enabling individuals to voluntarily hold or release stool.
| Characteristics | Values |
|---|---|
| Process | Defecation is the term for the act of expelling feces from the digestive tract via the anus |
| Involuntary Process | Defecation is involuntary from the colon to the internal anal sphincter, where smooth muscles push feces out |
| Voluntary Process | Defecation is under conscious or subconscious control at the level of the external anal sphincter, which is lined by striated muscles |
| Fecal Movement | Fecal movement from the colon to the rectum induces the urge to defecate |
| Muscle Relaxation | The external anal sphincter and puborectalis muscle relax to let feces pass out of the rectum |
| Muscle Contraction | The levator ani and puborectalis muscles work together to allow complete fecal expulsion |
| Muscle Coordination | Rectal afferent nerves are responsible for the sensation of rectal fullness and the urge to defecate |
| Muscle Control | People can prevent defecation by voluntarily tightening the external anal sphincter |
| Pelvic Floor Muscles | Pelvic floor muscles help stabilize your core while assisting with essential bodily functions, like pooping, peeing, and having sex |
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What You'll Learn
The role of the pelvic floor muscles
The pelvic floor is made up of muscles, ligaments, and connective tissue. It physically supports the pelvic organs, including the bladder, bowel (large intestine), and internal reproductive organs, and closes off the lower part of the pelvic cavity. Openings in this strong sheet of muscular tissue allow the vagina, the last part of the bowel (rectum), and the urethra to pass through.
The pelvic floor muscles help stabilize your core while assisting with essential bodily functions, such as defecation, urination, and sexual intercourse. They can weaken over time due to injury, trauma, and the normal aging process, leading to conditions like incontinence or pelvic organ prolapse. Pelvic floor muscles can become stressed during pregnancy or from overuse, such as repeated heavy lifting, chronic coughing, or constipation. They may also grow weaker due to hormone changes during menopause and lose strength as a natural part of aging. Conditions like diabetes may also contribute to weakening pelvic floor muscles.
The pelvic floor muscles play a crucial role in controlling defecation. When the rectum fills up, stretch receptors in the wall of the anus are activated. Signals are sent along nerve pathways to the cerebrum, where they are processed, and signals are sent back to the lining of the anus. The nerve impulse causes the inner sphincter to open, creating the urge to empty the bowels. The pelvic floor then relaxes and drops down slightly, allowing the muscles in the rectum to push the stool out.
To prevent defecation, individuals can voluntarily tighten their external anal sphincter muscle, along with the muscles in their pelvic floor and abdomen. This allows them to hold back stool for some time. However, if defecation is delayed for an extended period, the rectal wall relaxes, and the urge to defecate subsides until another mass movement occurs and stimulates the reflex again.
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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 derived from the ventral rami of S2-S4. It may also receive additional motor innervation from the nerve to levator ani. The sphincter consists mostly of slow-twitch fibres that allow extended continuous contraction.
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The internal anal sphincter
The IAS works in conjunction with the external anal sphincter to produce a bowel movement. The internal anal sphincter is located inside the rectum, while the external anal sphincter is outside, surrounding the end of the anal canal. The external anal sphincter is a cylindrical sheet of striated, voluntary muscle, whereas the internal sphincter is made of smooth muscle. The external sphincter supports the internal sphincter and helps it to perform its function.
Problems with the IAS can lead to fecal incontinence, especially partial incontinence to liquid, or mucous rectal discharge. Impaired internal sphincter function can be caused by various factors, including anal dilatation, perineal descent, and age. Anal fissures, tears in the lining of the anus, can also extend into the internal anal sphincter and cause pain during bowel movements.
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The levator ani and puborectalis muscles
The levator ani muscle is a complex, funnel-shaped structure, situated on either side of the pelvis. It is the largest component of the pelvic floor and provides support to the pelvic visceral structures. The levator ani is formed from three muscle components: the pubococcygeus, the iliococcygeus, and the puborectalis.
The pubococcygeus muscle is a hammock-like muscle found in both sexes, stretching from the pubic bone to the coccyx (tail bone). It forms the floor of the pelvic cavity and supports the pelvic organs. In females, the pubococcygeus supports the vagina, while in males, it supports the prostate. The pubococcygeus also controls the flow of urine and helps reduce urinary incontinence.
The iliococcygeus is a thin sheet of muscle that traverses the pelvic canal, connecting with the superior surface of the sacrum and coccyx. It arises from the inner side of the ischium (the lower and back part of the hip bone) and attaches to the coccyx and anococcygeal body.
The puborectalis muscle forms a U-shaped sling around the lower rectum. It acts in association with the internal and external anal sphincters during the process of defecation. Relaxation of the puborectalis muscle, along with the anal sphincters, allows feces to pass out of the rectum.
The levator ani muscles work together with the puborectalis muscles to facilitate complete fecal expulsion. They are also involved in proper sexual functioning, defecation, and urination, allowing various structures to pass through them.
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How constipation impacts faecal flow
Constipation is defined as reduced defecation frequency, generally three or fewer times a week. It is characterised by stool hardening, which makes it difficult to pass faeces and forces individuals to strain during defecation. This can be caused by a low-fibre diet, certain medications, disorders of nerves and muscles involved in defecation, and the prolonged suppression of the defecation urge.
Constipation can lead to faecal impaction, a severe form of constipation where there is a large lump of dry, hard stool stuck in the rectum. This occurs when constipation is left untreated, causing a buildup of faeces that cannot be passed naturally. Faecal impaction can result in abdominal pain, a swollen abdomen, and the urgent need to defecate without being able to do so. It can also cause gastrointestinal pressure from waste buildup, leading to discomfort and pain in the abdomen and lower back.
The impact of constipation on faecal flow can be understood through the process of defecation. Defecation is the act of expelling faeces from the digestive tract via the anus, requiring coordination between the gastrointestinal, nervous, and musculoskeletal systems. Normally, the process begins with mass movement from the colon to the rectum, triggering the defecation reflex. This involves rectal contraction and internal and external anal sphincter relaxation, allowing faeces to be pushed out by smooth muscles. However, constipation can disrupt this process by slowing down colonic motility and hardening the stool, making it difficult for the rectum to contract and the anal sphincters to relax sufficiently for faecal expulsion.
Additionally, constipation can impact the voluntary control of defecation. While the inner anal sphincter is involuntary, the outer anal sphincter can be controlled voluntarily to initiate or prevent the passage of stool. In cases of constipation, the outer anal sphincter may be voluntarily tensed to hold back hardened stool, further contributing to the buildup of faeces in the rectum. This can lead to faecal impaction if left untreated.
To summarise, constipation impacts faecal flow by slowing down colonic motility, hardening stool, and disrupting the normal process of defecation. It can also affect the voluntary control of defecation, as individuals may tense their outer anal sphincter to hold back hardened stool. Untreated constipation can progress to faecal impaction, causing abdominal pain and discomfort due to waste buildup. Therefore, it is important to address constipation promptly and make necessary dietary and lifestyle changes to restore normal faecal flow.
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Frequently asked questions
Defecation is the act of expelling faeces from the digestive tract via the anus.
The internal and external anal sphincters, the levator ani, and the puborectalis muscles are involved in defecation. The pelvic floor muscles also play a crucial role in controlling bowel movements.
The internal anal sphincter (IAS) is made of smooth muscle and keeps the anus closed until we are ready to have a bowel movement. We do not have voluntary control over this muscle.
The external anal sphincter (EAS) is made of striated muscle, similar to the pelvic floor muscles. We have voluntary control over the EAS, allowing us to hold on in case of diarrhoea or gas.
You can locate your pelvic floor muscles by squeezing the vaginal opening, urethra, and anus. You should feel muscles pull inward and upward. These are your pelvic floor muscles.
