NAS - Enteric Nervous System (Lecture 16)

ENS innit

What is the enteric nervous system?

• It is an intrinsic division of the visceral nervous system which innervates the GIT (and pancreas and biliary system) and is responsible for many important functions.

• Intrinsic means that it can function independently, however it does communicate with the brain and CNS for particular functions.

What are the sensory (afferent) neurones of the enteric nervous system like?

• Associated with mechanoreceptors and chemoreceptors.

• Mechanoreceptors detect physical presence of food in the GIT.

• Chemoreceptors detect chemicals released by the food.

What are the interneurones of the enteric nervous system like?

They are excitatory or inhibitory neurones that co-ordinate response.

What are the secretomotor (efferent) neurones of the enteric nervous system like?

• Innervate effectors which are either secretory glands or smooth muscles.

• Either excitatory or inhibitory, and use Ach or different types of peptides as neurotransmitter.

What coordinates the motility reflex and what does this reflex do?

The myenteric plexus coordinates this reflex.

This reflex allows peristalsis.

What is peristalsis?

The rhythmic movement of food along the GIT.

What are the multiple steps of the motility reflex?

1. A food bolus is detected in the GIT, by both mechanoreceptors (detect stretch of GIT wall) and chemoreceptors (detect food presence).

2. Afferent neurones deliver information from receptors to interneurones in the myenteric plexus.

3. Excitatory interneurones activate motor neurones of smooth muscle behind the bolus, stimulating contraction. This creates a pressure behind the bolus that pushes it along the GIT.

4. Inhibitory interneurones inactivate the motor neurones that innervate smooth muscle ahead of the bolus. This leads to the smooth muscle relaxing, dilating the tract ahead of the bolus to facilitate its movement along the tract.

5. Continuous contraction/relaxation of parts of the GIT in response to multiple food boluses leads to the rhythmic contraction of the GIT wall – peristalsis.

What coordinates the secretory reflex and what does this reflex do?

It is coordinated by the submucosal plexus.

This reflex allows the release of chemicals into the stomach and small intestine lumen to aid in food digestion.

What are the multiple steps of the secretory reflex?

1. Food presence is detected by the chemoreceptors and mechanoreceptors.

2. This information is delivered by sensory neurones to interneurones.

3. Interneurones activate secretomotor neurones which innervate exocrine glands, leading to release of chemicals (enzymes, mucus, acid, etc.) into the lumen of the stomach and small intestine to aid with digestion.

What are the multiple steps of blood flow reflex?

1. Presence of food is detected by the mechanoreceptors and chemoreceptors.

2. Information passed by sensory neurones to the interneurones in both the myenteric and submucosal plexuses.

3. Motor neurones release vasoactive neurotransmitters that dilate blood vessels that supply the GIT, to facilitate digestion.

How is the enteric nervous system integrated with the CNS?

• Autonomic afferents deliver sensory information from the enteric nervous system to the CNS. This is the reason why we are consciously aware of food in the stomach.

• The brain may supply motor neurones that also innervate parts of the GIT as part of the sympathetic or parasympathetic nervous system.

• For example, during the ‘fight-or-flight’ response, the sympathetic efferents from the CNS can override the peristalsis process to deliver blood to other parts of the body.

• The CNS can also control the anal sphincter muscles to consciously control defecation.

Does enteric function increase or decrease with age? Explain your answer.

It generally decreases with age as a result of death of neurones over time.

Give some examples of ENS-specific diseases.

• Chaga’s disease

• Achalasia

• Hirschsprung’s disease

What is Chaga’s disease?

• Insect bite delivers trypanosome parasite into the bloodstream.

• This parasite produces a toxin which kills ENS neurones, compromising ENS function.

• Due to its impact on the ENS, it stays longer in the body as it cannot be excreted or broken down by stomach acids as efficiently.

What is achalasia?

• Damage to enteric neurones of mouth and oesophagus, causing swallowing difficulties.

• This disorder is more common in older people, possibly as a result of loss of inhibitory neurones in the oesophagus.

What is Hirschsprung’s disease?

• Congenital absence of ENS neurones, usually in the distal colon.

• This leads to paralysis of smooth muscle in the distal colon, preventing its movement and excretion.

• Faeces builds up in the colon which causes it to expand.

• The faeces buildup can cause inflammation and infection due to the formation of large bacterial colonies.

• Also, can cause constipation due to inability of anal sphincter muscles to contract and relax.

Give some examples of general neurological disorders that could affect the enteric nervous system.

• Parkinson’s disease

• Peripheral neuropathies

• Myasthenia gravis

How is Parkinson’s disease linked to a disorder in the ENS?

• Pathogens entering the nervous system via the GIT, travelling to the brain via enteric neurones.

• This can cause death of neurones in the brain.

How is myasthenia gravis linked to a disorder in the ENS?

May affect ENS, due to possible production of ACh receptor antibodies which affect smooth muscle of GIT.

How are peripheral neuropathies linked to a disorder in the ENS?

Disorder of gut function

Describe a disorder which affects the gut-brain axis.

BS (irritable bowel syndrome):

• dysfunctional interaction between brain and ENS, leads to impaired ENS function.

• Associated with mental health conditions, especially those that involve increased anxiety.

• Symptoms include stomach pain, change in bowel habits (e.g. diarrhoea, constipation), bloating, excessive wind.

Give some examples of drugs which can target the enteric nervous system.

• Anti-diarrhoeal drugs such as loperamide

• Pain medication such as morphine and codeine is also possible

What do drugs that target the ENS exactly do?

• The ENS uses some opioids as neurotransmitter.

• Therefore, some drugs may target the opioid receptors.

• Morphine and codeine are also opioids, and they may activate opioid receptors in GIT which therefore reduces gut activity and may cause constipation as a side effect.