W7: Enteric NS

Question 1

What is the enteric NS sometimes referred to as?

Answer 1

Parasympathetic

Intramural plexus - subclassification of parasympathetic

Question 2

The ENS forms the intrinsic innervation of the GIT. What does intrinsic mean?

Answer 2

All of the basic elements necessary for function and neural control of function are present within the GIT, and do not need external influence. (Brain can influence but does not need to)

Question 3

What parts of the GIT are controlled by the ENS?

Answer 3

Oesophagus to rectum.

Pancreas and biliary system is partly control but complex and poorly understood

Question 4

What is the ENS made from?

Answer 4

A large network of neurones

Question 5

Where is the ENS located?

Answer 5

In the wall of the GIT

Question 6

GIT ENS histology, what are the green stain circles and processes?

Answer 6

Green stain circles = nuclei of neurones

Long processes emanating from the collections of cell bodies = axons

Question 7

What are the two major plexuses of ganglion cells and fibre bundles in the GIT and where are they found?

Answer 7

Submucosal (Meissner’s) plexus - found in stomach and intestines only
Myenteric (Auerbach’s) plexus - full length of the GIT

Question 8

Within the GIT wall, where is the myenteric plexus found?

Answer 8

Between the two layers of smooth muscle - longitudinal (outside) and circular (inside)

Question 9

Within the GIT wall, where is the submucosal plexus found?

Answer 9

Lies internally to the circular muscle and adjacent to the submucosa

Question 10

What are all the layers of the GIT wall from outside to inside?

Answer 10

Longitudinal muscle, submucosal plexus, circular muscle, myenteric plexus, submucosa, mucosa, lumen

Question 11

What afferent neurones are found in the plexuses?

Answer 11

Mechanoreceptors and chemoreceptors

Question 12

What interneurones are found in the plexuses?

Answer 12

Large number of interneurones.

Excitatory or inhibitory.

Question 13

What type of efferent neurones are found in the plexuses and their neurotransmitters?

Answer 13

Excitatory or inhibitory

Many neurotransmitters including ACh, various peptides etc

Question 14

What are the effectors of the plexuses?

Answer 14

Motor and secretory effectors, e.g., smooth muscle and glands

Question 15

What type of reflexes do the ENS plexuses cause?

Answer 15

Short reflexes, involving only neurones within the GIT.

Question 16

What are the functions of enteric reflexes?

Answer 16

To control motility, secretion, blood flow to the gut

Question 17

Is peristalsis a motor or secretory reflex?

Answer 17

Motor

Question 18

What is peristalsis?

Answer 18

The movement of food in a rhythmic fashion along the GIT

Question 19

What plexus is used in peristalsis?

Answer 19

Myenteric

Question 20

What do the sensory neurones detect in peristalsis and how is information passed on?

Answer 20

Sensory neurones detect the contents of the lumen, and the detection of stretch in the lumen in response to the presence of food.
Information distributed via the interneurones to various motor efferents that can interact with the muscle layers.

Question 21

What needs to happen to propel the food bonus along the GIT?

Answer 21

Contraction of muscle behind the bolus to constrict the diameter of the lumen and applying pressure, forcing the bolus forwards. And relaxation of muscle in front of the bolus.

Question 22

What does the bolus stimulate?

Answer 22

Bolus stimulates the mucosal sensory receptors (either mechanical or chemical). Information is transmitted upto the myenteric plexus.

Question 23

What 2 major effects are produced by the interneurones in peristalsis?

Answer 23

Behind the bolus via excitatory interneurons: we can cause an increase in activity in excitatory efferents causing smooth muscle contraction, which will decrease the size of the lumen, so the bolus can be squeezed forward.
In front of the bolus, inhibitory interneurons and inhibitory efferents will allow relaxation of the muscle in front. So rhythmic movement of the bonus down the GIT.

Question 24

What plexus does the secretory reflex use?

Answer 24

Submucosal

Question 25

Describe the reflex arc of the secretory reflex.

Answer 25

Presence of food stimulates sensory receptors via the integrating centre where the interneurones reside, in this case the submucosal plexus. Some information from the myenteric plexus can get to the submucosal plexus too.
Efferent neurones are activated = secretory neurones
These produce secretions, enabling processing of the food

Question 26

What does the submucosal plexus do in the secretory reflex?

Answer 26

Interneurons of the submucosal plexus can process the info and allow the efferent neurones to interact with secretory cells which produce their secretions into the lumen, to help with their digestion or lubrication processes.

Question 27

What plexus is involved in the control of blood flow?

Answer 27

Submucosal and myenteric

Question 28

What do the efferent neurones innervate in the control of blood flow?

Answer 28

Some efferent neurones innervate the blood vessels and produce vasoactive transmitters = influence the contraction state of the smooth muscle within the vasculature.

Question 29

How is the control of blood flow information integrated with the CNS?

Answer 29

Enteric NS sends info to the brain via afferents which also collect other parts of the sympathetic and parasympathetic information from the viscera.
ENS <-> SNS/PNS <-> CNS

Question 30

Why is the control of blood flow a long reflex?

Answer 30

Long reflex = the sensory information detected in the enteric is transmitted up to the brain and the brain can respond and cause changes in the enteric function. So not confined to the GIT wall.

Question 31

How does the CNS influence vary along the GIT?

Answer 31

Brain has little ability to override the movement of food down oesophagus etc.
But has much influence on the activity of the sphincters, e.g., controlling defecation. Very young babies cannot do this (pressure of faeces activates the short reflex to open the sphincter).

Question 32

What happens to the ENS with age?

Answer 32

With age the ENS does not function as well. Potentially due to a decrease in the number of neurones, so intrinsic reflexes are partly compromised. Might influence appetite and nutrition status.

Question 33

What is Chaga’s disease?

Answer 33

Caused by a Trypanosome parasite, which is transferred into human blood when these insects bite. It produces a toxin which can cause the death of neurones in the ENS. And then by shutting down the ENS the parasite can be maintained within the body and not eliminated as easily.

Question 34

What is achalasia?

Answer 34

Individuals have difficulties with swallowing. Affects the oral cavity, what is happening in their oesophagus, and through the sphincter thus affects nutrition. Due to the loss of inhibitory neurones within the myenteric plexus of the oesophagus or the sphincter at the entrance to the stomach. Could be autoimmune.

Question 35

What is Hirschprung’s disease?

Answer 35

Megacolon. Congenital absence of ENS neurones. The ENS neurones that are missing are found in the distal colon. So food is passed through the GIT through peristalsis and gradually gets digested until waste. When this waste bolus gets as far as the distal colon, it is unable to undertake peristalsis and is constricted so faeces cannot pass through it, so faeces build up. These are a breeding ground for bacteria so can cause colitis (inflammation). Abolition of rectoanalreflex in babies. Treatment is colostomy.

Question 36

How can Parkinson’s affect the gut?

Answer 36

Some pathogen that causes death of neurones in the brain might enter the nervous system via the GIT, and travel via enteric neurones and through sympathetic and parasympathetic afferents all the way to the brain.

Question 37

How is MG related to ENS dysfunction?

Answer 37

Possible to see antibodies against the ACh receptors associated with smooth muscle too.

Question 38

How do peripheral neuropathies the ENS?

Answer 38

Any kind of disruption to the peripheral NS could include the enteric.

Question 39

What is IBS?

Answer 39

Dysfunctional interaction between the brain and ENS, such that ENS function is disrupted. Particularly associated with mental health problems, e.g., anxiety.

Question 40

Complexity of IBS means there are lots of potential neurotransmission processes to target with a variety of drugs. There are neurones within the enteric that release endogenous opioids as their neurotransmitters so are opioid receptors too. Describe some drug action.

Answer 40

Good: opioid receptors are able to be targeted to change the function of the ENS and thus GIT. E.g., anti-diarrhoeal agents reduce gut motility by acting at opioid receptors.
Bad: many analgesias e.g., morphine, codeine, are also opioids, so when taken these can affect opioid receptors in the GIT, causing a reduction in gut activity, so associated with constipation.