Lecture 25- Enteric nervous system I

Question 1

What is the ENS?

Answer 1

• ENS is most complex nervous system outside the brain

– Sometimes called the “Little Brain”

• controls the GIT
• it is embedded in the GIT

Question 2

What are the behaviour that ENS can produce like?

Answer 2

• ENS can produce complex behaviours without input from brain

– Allows general rules for behavioural regulation to be inferred from a relatively simple system

-basically invertebrate nervous system within a vertebrate nervous system

Question 3

What are the neurotransmitters found in the ENS?

Answer 3

• Most neurotransmitters and their receptors are found in ENS

– Can study roles of these molecules in intact circuits

-can study it in a dish as it doesn’t need any outside input -lot of discoveries made in the ENS as easier to access

Question 4

What is the definition of the ENS?

Answer 4

• Network of neuron cell bodies, processes and axons in wall of gastrointestinal tract

– Largest part of autonomic nervous system

• Includes axons and terminals of visceral sensory neurons, sympathetic neurons and parasympathetic neurons that innervate the GI tract

• lot of cells and nerve terminal
• output includes the axons in the muscle

Question 5

How long is the ENS?

Answer 5

• ENS runs the entire length of the GI tract

– I.e. from back of mouth to anus >7m

-in a guinea pig it is about 1 m

Question 6

How many neurons are there in ENS?

Answer 6

• In human contains about 300,000,000 neuron cell bodies (> number of neurons in spinal cord)

– Guinea-pig 10,000,000 to 30,000,000 neurons

Question 7

What are the cell bodies of the ENS neurons grouped into?

Answer 7

• Cell bodies grouped into ganglia (clusters of neurons)

Question 8

What are the two ganglionated plexuses in the ENS?

Answer 8

– Myenteric (Auerbach’s) plexus

– Submucosal (Meissner’s) plexus

• many of the glia found in ganglia as well (the lumps)
• in the gut the ganglia are smaller but very distictive -they are organised in two layers’next to muscle (myenteric plexus) and mucosa( meissner’s plexus)

Question 9

What are the layers of the small intestine (ENS)?

Answer 9

• within the villi have the nerouns extrinsic and intrinsic submocosal plexus= contains neuron cell bodies and ganglia conneted in nerve tracts and arterier and arterioles, capillaries come out into the mucosa
• clumps of neuron in submocosal = about 20 in a group
• then another layer of smooth muscle= circular muscle= genrates the force of the contraction= the contractile activity= innervated by myeteric plexus neurons (in human 300+ in a ganglion)
• longitudinal muscle=not well understood -same like this almist along the whole length

Question 10

Is the submucosal plexus present in the stomach?

Answer 10

• Submucosal plexus present in stomach, but not ganglionated.

Question 11

Where does the ganglionated submucosal plexus begin?

Answer 11

• Ganglionated submucosal plexus begins at pylorus -no obvious reason for this

Question 12

Where does the ganglionated myenteric plexus begin?

Answer 12

• Ganglionated myenteric plexus begins at upper oesophageal sphincter

Question 13

What do several non-ganglionated plexuses do?

Answer 13

• Several non-ganglionated plexuses

– Innervate muscle, mucosa, blood vessels

Question 14

What is the innervation of the GI tract?

Answer 14

• Sympathetic nerves innervate whole length of GI tract

-these modulate the ENS circuits

Question 15

What parts of the GI tract are innervated by the parasympathetic nerves?

Answer 15

• Parasympathetic nerves innervate upper (oesphagus, stomach, duodenum) and lower (colon, rectum, anus), but not mid (jejunum, ileum) GI tract

Question 16

Which parts of the GI tract are innervated by the extrinsic sensory neurons?

Answer 16

• Extrinsic sensory neurons (visceral primary afferents) innervate the full length

Question 17

What is the GI tract innervation like?

Answer 17

• the anatomic division= vagus controls the supply to the upper part of the GIS and has direct point to point control, whereas the sympathetic control= operated by altering the function of the ENS , the sympathetic = controls most of it
• there is not much of central control, can de-innervate the gut and get pretty complete function -treat hiccups by cutting vagus and they can still digest food
• but vagus is important! it is the route via which bacteria etc. can be affected by the CNS, in the control of the upper gut as well,

Question 18

What are the extrinsic primary afferent that supply the GI tract?

Answer 18

• Large supply to upper GI tract run in vagus nerve – 90% of vagal axons are primary afferents

• so they carry information up to the brain or up to the spinal cord
• unusual function, since they branch heavily and have neurotransmitters in terminals= can activate one neuron and another branch of it will activate another part of the gut = monosynaptic reflex= activate terminal and get output locally, neuronal reflex= prominant in the gut
• Dorsal root ganglion cell afferents run in splanchnic and pelvic nerves
• Functions and central terminations of vagal afferents and dorsal root afferents are distinctively different

Question 19

What are the functions of the ENS?

Answer 19

• Control of movement of intestinal content
• Regulation of water and electrolyte transport across the mucosa

– Movement of water and salt into the body from the intestinal lumen – absorption

– Movement of water and salt from body into the lumen – secretion

• Contributes to control of acid secretion in stomach, mucus secretion along length and bicarbonate secretion in duodenum

• state of the bowel become more and more important, get constipated more
• water and salt regulation= often in disease this goes badly, cholera, typhoid= breakdown in neural control of this

Question 20

What are the intestinal movements like in the GI tract?

Answer 20

• Contractions only seen if nutrient is in lumen
• Contractions abolished by tetrodotoxin, which blocks neural action potentials, but not those of smooth muscle
• Contractions blocked by antagonists of nicotinic or muscarinic acetylcholine receptors
• drugs that block acetylcholine, block the effect, at two different receptors, the nicotinic receptor (NMJ one) and the muscarinic AChRs (slows down the heart)
• decaonic acid= if not there the gut is not moving but with it it contracts
• the constrictions are localised,liquid moves, propelled by peristalsis, the other stationary constrictions that don’t move= squirt liquid in both directions and create turbulent flow= create mixing= two fundamental patterns= localised (segmentation) and peristalis
• all these contractions are abolished by tetradotoxin= japaned puffer fish poison= TTX= second most powerful biological poison, kills you in about 2 hours= turns off your ability to breathe
• blocks the ENS (via blocking potassium channels) so smooth muscle still moves as have calcium dependent channels= but that means when the conracticons of stop when TTX is there measn that neurons needed for contractions

Question 21

What can we learn from the video of the in vitro gut movement?

Answer 21

• Contractions of intestinal smooth muscle do not depend on the brain or spinal cord
• Contractions can be confined to a local region or sequentially cover the whole segment
• Some contractions apparently move along the segment – propagate
• Jejunum contains entire circuit for generation of a complex behaviour

– Circuit involves two types of cholinergic neurons -the gut produces complex behaviour without any intervention from other parts of the body, can have localised contraction or peristaltic contractions= the system switches from one to the other

• there is retropulsion= also vomiting
• the duodenum contains ENS and that is capable to produce complex behaviour has everything for it, need to be able to detect nutrients,there have to be two classes of ACh, one receives inputs and onather type gives input to muscle

Question 22

What are the type so of neurons that must be in the ENS?

Answer 22

• Intrinsic sensory neurons – Sensitive to distension – Sensitive to mucosal deformation – Sensitive to luminal nutrients
• Excitatory motor neurons – To circular muscle – To longitudinal muscle
• Inhibitory motor neurons – To circular muscle – To longitudinal muscle
• Interneurons – Running orally, ascending – Running anally, descending
• Secretomotor neurons