How does the defecation reflex work, and what can affect it?

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When a person is first born, their defecation reflex is involuntary. As they age and undergo toilet training, they learn to control the urge to poop.

The defecation reflex begins in the colon. Muscles in the walls of the colon help move stool into the rectum through a process called peristalsis.

As the stool fills the rectum, the stretching of the walls activates receptors. This tells the brain it is time to have a bowel movement. The body allows a small amount of stool to pass into the anal canal, which helps a person determine if they need to pass stool or gas.

At this time, a person can use their voluntary control over the anal sphincter. They can either relax and have a bowel movement or hold their stool until an appropriate time. If the timing is unsuitable for a bathroom break, the walls of the rectum relax, and the defecation reflex temporarily goes away.

When a person gets to a restroom, they use their abdominal muscles to bear down and increase pressure on the stool. Simultaneously, they relax their external anal sphincter.

This coordinated action helps release the stool from the body.

What triggers the defecation reflex?

The primary trigger for the defecation reflex is when stool accumulates in the rectum, stretching the rectal walls. This stretching sends signals to the brain, letting the body know it is time for a bowel movement.

The two types of defecation reflex are:

• Intrinsic reflex: This reflex occurs automatically when the rectum fills with stool. The body initiates this response without the person having to think about it. The rectal muscles contract while the internal anal sphincter relaxes. This allows a small part of the stool to pass through.
• Parasympathetic reflex: This reflex occurs voluntarily when a person decides they are ready to use the restroom. They consciously relax their external anal sphincter and bear down with their abdominal muscles. This allows the stool to pass.

Several medical conditions can affect the defecation reflex, leading to irregular bowel habits. Some people may experience constipation, diarrhea, or a combination.

Sometimes, people have fecal incontinence, which is when they lose voluntary control over their defecation reflex.

Constipation

Constipation is when a person has three or fewer bowel movements per week or has to strain excessively to pass stool. The stools become hard, dry, and difficult to pass from the rectum.

Sometimes, a person with constipation may continue feeling the defecation reflex even after having a bowel movement.

Conditions that may cause constipation include:

• dietary and lifestyle choices
• irritable bowel syndrome (IBS)
• disorders of the brain and spine
• spinal cord or brain injuries
• medication side effects
• diabetes
• hypothyroidism
• diverticular disease
• anatomical problems of the digestive tract

Diarrhea

Diarrhea is when a person has loose, watery stools three or more times a day.

A person with diarrhea may experience a defecation reflex that feels urgent. Sometimes, they cannot maintain voluntary control of their bowel movement.

Medical conditions that may cause diarrhea include:

• infections
• travelers diarrhea
• side effects of medications
• food allergies or intolerances
• celiac disease
• Crohn’s disease
• IBS
• small intestinal bacterial overgrowth (SIBO)
• ulcerative colitis

Fecal incontinence

Fecal incontinence is when a person cannot control their bowel movements when they have the defecation reflex. It is most common in older adults due to the anal sphincter losing tone. However, it can happen at any age due to:

• spinal cord injuries
• spinal cord defects
• malformations in the rectum or anus
• injury after giving birth vaginally

A person may struggle with their defecation reflex and bowel movements. However, there are steps they can take to improve their bowel health, including:

• Dietary changes: Eating plenty of fiber daily helps soften the stool and promotes regular bowel movements. Foods high in fiber include fruits, vegetables, and whole grains.
• Drinking water: Staying hydrated helps the digestive system function properly and helps prevent stool from drying out and causing constipation.
• Lifestyle modifications: Regular physical activity such as walking or other exercise can help stimulate bowel movements.
• Bowel training: Trying to have a bowel movement at the same time each day can help a person develop regular bowel movements.
• Over-the-counter (OTC) treatments: Some treatments may help a person have a bowel movement. These include laxatives, stool softeners, and enemas. However, they should consult their doctor before using these treatments regularly.

People should contact a doctor if OTC treatments are not working. Those with diarrhea should contact a doctor if it lasts for 2 or more days.

There are several situations where a person may need medical intervention, such as:

• blood in the stool
• persistent symptoms
• sudden changes in bowel habits
• severe abdominal pain
• severe rectal pain

The defecation reflex is a natural and essential process that allows waste material to leave the body. It starts when feces passes into the rectum. This activates involuntary receptors telling the brain that a stool is present. A person then completes the defecation reflex when they voluntarily relax their anal sphincter and have a bowel movement.

Several factors can affect the defecation reflex. These include diet, lifestyle, and medical conditions. Eating a diet high in fiber, staying hydrated, and getting regular physical activity can promote regular defecation.

People experiencing persistent or severe symptoms should consult with their doctor for proper guidance.