Enteric nervous system

Question 1

Enteric nervous system

Answer 1

The third division of the ANS, forming the intrinsic innervation of the GI tract, from the oesophagus to the rectum (and the pancreas and biliary system). Comprised of a network of 80-100 million neurones.

Question 2

Enteric nerve plexuses

Answer 2

• myenteric plexus; between the longitudinal and circular layers of smooth muscle, found in the stomach and small intestine, responsible for peristalsis
• submucosal plexus; deep to the mucosa of the epithelium, found along the entire GI tracts, responsible for glandular secretion

These nerve plexuses are extensively interconnected:

Question 3

Types of neurones in the ENS

Answer 3

• afferent (sensory) neurones eg. mechanoreceptors, chemoreceptors
• interneurones; excitatory and inhibitory
• efferent (secretomotor) neurones; excitatory and inhibitory
• effectors; motor and secretory

These form a reflex pathway.

Question 4

Short reflexes

Answer 4

Most of the action of the ENS is conducted via reflexes which only involve neurones in the GI tract, known as short reflexes. These control motility, secretion, and blood flow to the gut.

Question 5

Motor reflex

Answer 5

The myenteric plexus is responsible for peristalsis, which moves a bolus of food along the GI tract in a caudal direction. Distension of the bowel stimulates sensory receptors in the mucosa, bringing about a response integrated by interneurones. Motor neurones cause smooth muscle to contract behind the bolus and relax in front of it.

Question 6

Secretory reflex

Answer 6

The submucosal plexus is responsible for glandular secretion. A bolus stimulates mechano- and chemoreceptors in the mucosa, bringing about a response integrated by interneurones. Secretory neurones stimulate the secretion of mucous, enzymes and stomach acid.

Question 7

Control of blood flow

Answer 7

Both the myenteric and submucosal plexuses are involved in the control of blood flow to the GI tract via the release of vasoactive neurotransmitters.

Question 8

Gut-brain axis

Answer 8

The ENS is integrated with the CNS via sympathetic and parasympathetic neural pathways. These are known as long reflexes. The influence of the brain on the ENS varies along the GI tract.

Question 9

Chaga’s disease

Answer 9

Trypanosome parasites transmitted into the bloodstream via an insect bite produce a toxin which can cause dysfunction/death of neurones, including those of the ENS. Prevalent in South America, particularly in rural areas.

Question 10

Achalsia

Answer 10

A condition affecting the oesophagus and spinchter, resulting in difficulty swallowing. Appears to be due to the loss of inhibitory interneurones in the myenteric plexus. More common in the elderly.

Question 11

Hirschprung’s disease

Answer 11

A congenital absence of ENS neurones in the distal colon, causing a build-up of faeces in the constricted area. Results in a distended colon, inflammation (colitis) and abnormal defecation due to the absence of the rectoanal reflex. May be surgically treated with a colostomy (removal of part of the colon).

Question 12

Other ENS diseases

Answer 12

General neurological diseases which affect the ENS include peripheral neuropathies and myasthenia gravis. Parkinson’s disease may be caused by a pathogen which enters via the ENS, and can result in disrupted gut function. Disruption of the gut-brain axis causes dysfunctional interaction between the CNS and ENS, such as in irritable bowel syndrome (IBS).

Question 13

ENS drugs

Answer 13

The varied nature of ENS neurotransmission provides many targets for drugs, such as opioid receptors. Anti-diarrhoeal medications target these on purpose, whereas opioid analgesics (eg. morphine) result in constipation as an unintended side effect.