Extrinsic and enteric nervous systems

A person’s gut is controlled by two different nervous systems: the extrinsic nervous system and the enteric nervous system.

Extrinsic nervous system

The extrinsic nervous system is the nervous system that is external to the bowel and it consists of autonomic, sensory and motor nerves. In this step we focus on the autonomic nervous system.

Autonomic nervous system

The autonomic nervous system controls the gastrointestinal tract. It works on the smooth muscle and its action is involuntary. It is comprised of a complex system of parasympathetic and sympathetic nerves:

• Parasympathetic nerves – stimulate motility ie ‘makes you poo’
  • The parasympathetic nerves originate in the medulla with the brain
  • Innervation of ascending colon and half transverse colon is via the vagus nerve
  • Innervation of the descending colon, rectum and anus is via the sacral nerve roots S2-S4 though the pelvic nerves, these para sympathetic nerves stimulate the rectum and anal canal to contract and the internal anal sphincter to relax, this allows faeces to pass though the anus
• Sympathetic nerves – inhibit motility ie ‘stops your poo’
  • The sympathetic nerves originate lower down the spine, between the 10th thoracic and 3rd lumbar segments
  • Innervation from the lumbar spine T11- L2 via the hypogastric nerve, these Sympathetic nerves stimulates the rectum and anal canal to relax and the anal sphincter to contract, to maintain continence

Sensory and motor innervation of the bowel and pelvic floor is through the vagus nerve, the nervi erigentes (pelvic splanchnic nerves), direct sacral root branches and the pudendal nerve.

Mixed nerves supply the somatic voluntary muscles of the pelvic floor and the external anal sphincter.

The above demonstrates the complexity of the extrinsic nervous control of the bowel and pelvic floor.

Enteric nervous system

The enteric nervous system or intrinsic nervous system is the internal nervous system of the gut and is embedded in the wall of the gut, it begins at the oesophagus and extends to the anus.

The enteric nervous system is composed of the submucosal (also known as the Meissner) and myenteric (also known as the Auerbach) plexuses, which largely regulate segment-to-segment movement of the gastrointestinal tract.

The structure is illustrated in the diagram below, this further demonstrates the complexity of the nervous system of the bowel.

^{Figure 4.7: Illustration of neural control of gut wall by sympathetic, parasympathetic and enteric nervous system. © Boumphreyfr CC BY-SA 3.0}

The enteric nervous system coordinates gut secretion, blood flow and muscular activity.

The enteric nervous system gives the colon the ability to produce peristalsis, a wave-like flow of contraction that pushes the stool, in stages, towards the rectum and anus.

It is also responsible for the gastro-colic reflex. This reflex is very important for bowel control and will be discussed in the next step.

Clinical practice points.

There are some important clinical practice points relating to the bowel physiology.

1. The enteric nervous system normally communicates with the central nervous system though the parasympathetic (vagus nerve) and the sympathetic nervous systems but it is also able to function independently.

   If the vagus nerve is severed the enteric nervous system can continue to function itself without input from the central nervous system. This can be seen in some individuals who have severe nerve damage but who still have some gut motility. (This is different to the nerve control for the bladder, which can become atonic).
2. A neurogenic bowel can result from damage to any component of the central nervous system and/or the extrinsic and enteric innervation of the bowel. A neurogenic bowel can result from:
   • a neuropathological process eg a spinal cord injury
   • unrelated factors to a neurological disease eg low dietary fibre, insufficient fluid intake, difficulty accessing the toilet
   • a combination of a neuropathology and unrelated factors

3. Damage to the frontal lobe of the brain can lead to emotional disturbance, which can lead to reduced awareness a lack of voluntary control of the pelvic floor or lack of awareness of the call to stool.

4. If the sensory messages to the brain are impaired, the call to stool may be diminished and this can lead to constipation and/or faecal impaction.

5. If there is no stool in the rectum there is no sensation of the need to open bowels.

6. If the call to stool is ignored repeatedly and stool remains in the rectum it will become stretched and there will be a loss of sensation.

   This explains why individuals with constipation and overflow incontinence have no awareness of their faecal leakage.

© This work is a derivative of a work created by Boumphreyfr, licensed under CC BY-SA 3.0 International Licence adapted and used by the Association for Continence Advice.