What happens in your brain when you poop?

Updated

The sacral spinal cord triggers the defecation reflex, the insula creates the urge, the prefrontal cortex decides if it's appropriate, and the motor cortex relaxes the external sphincter.

Brainstem, Insula, Prefrontal Cortex, Motor Cortex lit up on the Brain Atlas 3D brain: what happens in your brain when you poop
Figure. The brain regions that light up when you poop, in order: Brainstem (the reflex), then Insula (the urge), then Prefrontal Cortex (the decision), then Motor Cortex (the release).

Watch it light up on the 3D brain

Brain regions that light up

OrderRegionWhat it does here
1BrainstemRectal fullness triggers the sacral spinal cord's defecation reflex, which relaxes the internal anal sphincter.
2InsulaThe insula registers the sensation of rectal pressure, creating the conscious urge to defecate.
3Prefrontal CortexThe prefrontal cortex evaluates the social context and decides to permit or suppress the urge.
4Motor CortexThe motor cortex sends signals to relax the external anal sphincter and increase abdominal pressure.

1. Brainstem: the reflex

Step 1 of what happens in your brain when you poop: Brainstem lit on the Brain Atlas 3D brain
Figure 1. Brainstem, the reflex: Rectal fullness triggers the sacral spinal cord's defecation reflex, which relaxes the internal anal sphincter.

The brainstem integrates sensory signals from the rectum and sends parasympathetic signals to the colon and rectum. This causes peristaltic waves and relaxation of the internal anal sphincter, an involuntary smooth muscle. This reflex can occur without conscious input, but higher brain centers normally modulate it.

2. Insula: the urge

Step 2 of what happens in your brain when you poop: Brainstem, Insula lit on the Brain Atlas 3D brain
Figure 2. Insula, the urge: The insula registers the sensation of rectal pressure, creating the conscious urge to defecate.

The insula is the primary cortical area for interoception, the sense of the body's internal state. It receives signals from the rectum via the thalamus and generates the subjective feeling of needing to go. This awareness is essential for deciding when and where to defecate.

3. Prefrontal Cortex: the decision

Step 3 of what happens in your brain when you poop: Brainstem, Insula, Prefrontal Cortex lit on the Brain Atlas 3D brain
Figure 3. Prefrontal Cortex, the decision: The prefrontal cortex evaluates the social context and decides to permit or suppress the urge.

The prefrontal cortex is involved in executive functions including decision-making and impulse control. It weighs social appropriateness, availability of a toilet, and other factors. If conditions are favorable, it sends signals to allow the reflex to proceed; if not, it inhibits the brainstem and signals the motor cortex to contract the external sphincter to maintain continence.

4. Motor Cortex: the release

Step 4 of what happens in your brain when you poop: Brainstem, Insula, Prefrontal Cortex, Motor Cortex lit on the Brain Atlas 3D brain
Figure 4. Motor Cortex, the release: The motor cortex sends signals to relax the external anal sphincter and increase abdominal pressure.

The motor cortex controls voluntary skeletal muscles via the corticospinal tract. It projects to the pudendal nerve to relax the external anal sphincter, and to abdominal muscles to contract, raising intra-abdominal pressure. This, combined with the brainstem reflex, allows stool to pass.

Good to know

  • The internal anal sphincter is smooth muscle under autonomic control, while the external anal sphincter is skeletal muscle under voluntary control.
  • The defecation reflex can be voluntarily overridden by higher brain centers, allowing continence.
  • The Valsalva maneuver increases intra-abdominal pressure to aid defecation.

Pooping vs Holding It In

When holding it in, the prefrontal cortex actively inhibits the brainstem reflex and the motor cortex contracts the external sphincter. When pooping, that inhibition is released, the sphincter relaxes, and abdominal muscles contract.

Test yourself

  1. Which brain region is primarily responsible for the conscious urge to defecate?

  2. The internal anal sphincter is controlled by which part of the nervous system?

  3. If you decide to delay defecation, which region is most active in suppressing the urge?

0 of 3 answered · 0 correct

Simplified: many regions work together at once, and this shows the main players in the order they act.