Lecture 13: Enteric Nervous System 2

Question 1

What is segmentation (mixing)?

Answer 1

local constrictions alternating with relaxations

depends on ENS

Question 2

What is peristalsis (propulsion)?

Answer 2

strong constrictions that propagate anally along the intestine (or colon) preceded by a relaxation
depends on ENS

Question 3

What is retropulsion?

Answer 3

peristalsis-like contractions that propagate orally

Question 4

What is receptive relaxation?

Answer 4

relaxation of smooth muscle to prepare a segment for arrival of content

Question 5

In which parts of the gastrointestinal tract does receptive relaxation occur?

Answer 5

stomach

small / large intestine

Question 6

What are the three phases of the migrating motor complex that appear to propagate from either gastric antrum or the proximal duodenum to the ileo-caecal junction?

Answer 6

phase 1 - quiescence lasting about an hour at any one site
phase 2 - irregular contractions that build to…
phase 3 - strong, rhythmic contractions lasting 10-15 min

Question 7

What does the migrating motor complex clear? How often does this occur?

Answer 7

debris and bacteria from small intestinal lumen

repeats every 90 - 120 min

Question 8

What happens during feeding?

Answer 8

migrating motor complex stops clearing debris and bacteria from small intestinal lumen

Question 9

What are intrinsic sensory neurons immunoreactive for and what do they contain?

Answer 9

calbindin

acetylcholine and substance P

Question 10

What do excitatory motor neurons contain?

Answer 10

acetylcholine and substance P

Question 11

What do inhibitory motor neurons contain?

Answer 11

nitric oxide synthase (NOS)

Question 12

What are ascending interneurons immunoreactive for? What are descending interneurons immunoreactive for?

Answer 12

calretinin
NOS, somatostatin or 5-HT
(both are cholinergic)

Question 13

What happens when intrinsic sensory neurons are activated at one site?

Answer 13

leads to simultaneous excitation of local excitatory motor neurons, local inhibitory motor neurons, ascending interneurons and descending interneurons

Question 14

Where do excitatory motor neurons project? Where do inhibitory motor neurons project?

Answer 14

excitatory motor neurons project orally while inhibitory motor neurons project anally

Question 15

What has modelling shown about segmentation and anal propagation?

Answer 15

that segmentation and anal propagation can come from a polarised circuit

Question 16

What is the basic neural circuit involved in Bayliss and Starling’s law of intestine initiated by?

Answer 16

activation of intrinsic sensory neurons (ISNs) at one site

Question 17

What can activation of ISNs stimulate?

Answer 17

local interneurons and motor neurons whose projections are polarised

Question 18

Where do excitatory motor neurons project and what do they mediate? Where do inhibitory motor neurons project and what do they mediate?

Answer 18

excitatory motor neurons project orally to mediate contractions
inhibitory motor neurons project anally to mediate relaxations below the stimulation

Question 19

What accounts for the propagation of the motor patterns as well as motor neurons?

Answer 19

interneurons extend the range of this polarisation by contacting other interneurons

Question 20

How many classes of descending interneurons have been identified and where do they project?

Answer 20

three classes

project anally

Question 21

What are some descending interneurons connected to and what does this allow?

Answer 21

some are connected to ascending interneurons and excitatory motor neurons and this allows anally directed constriction to follow relaxation, hence allowing propagation of motor patterns

Question 22

What do immunofluorescence studies show?

Answer 22

that descending interneurons do make synapses with ascending interneurons (and excitatory motor neurons)

Question 23

What will a local disruption in the circuit produce?

Answer 23

stationary contractions on either side of the disruption

Question 24

How does chemical composition of the contents of the gut influence behaviour of the gut?

Answer 24

high nutrient contents favour segmentation over propulsion

Question 25

How does volume of the contents of the gut influence behaviour of the gut?

Answer 25

large volumes trigger propulsive contractile patterns and receptive relaxation

Question 26

How does viscosity and texture of the contents of the gut influence behaviour of the gut?

Answer 26

low viscosity contents move rapidly, but do not deform the mucosa high viscosity contents move slowly and produce mechanical stimulation of mucosa

Question 27

Which neurons influence behaviour of the gut?

Answer 27

chemoreceptive and mechanoreceptive intrinsic sensory neurons

Question 28

What does the mucosal epithelium contain which may act as sense cells?

Answer 28

many types of endocrine cells

Question 29

What are the characteristics of enteroendocrine cells?

Answer 29

>10 different types, have apical membrane exposed to lumen, basal membrane exposed to lamina propria

Question 30

Which type of cell do enteroendocrine cells make up a significant proportion?

Answer 30

enterochromaffin (EC) cells

Question 31

What do enterochromaffin cells synthesise, store and release?

Answer 31

serotonin

Question 32

What do both 5-HT enterochromaffin cells and PYY enteroendocrine cells have?

Answer 32

neuropods

Question 33

What is the role of neuropods?

Answer 33

contact terminals of sensory neurons (afferents) and can get input from efferent enteric or extrinsic neurons

Question 34

What do neuropods have proteins involved in?

Answer 34

proteins involved in transmitter release | also receptors for efferent transmitters

Question 35

What does blocking of 5-HT uptake with an SSRI result in?

Answer 35

increases segmenting motor activity in the jejunum

Question 36

What does blocking of 5-HT receptors in the mucosa result in?

Answer 36

reduces nutrient induced segmenting motor activity

Question 37

What does releasing 5-HT from mucosa with cholera toxin rapidly increase?

Answer 37

propulsive motor activity

Question 38

Blocking 5-HT uptake with an SSRI increases segmenting motor activity in the jejunum because...

Answer 38

serotonin can act on enteric neurons

Question 39

What can amino acids excite via release of 5-HT and/or ATP?

Answer 39

local reflexes

Question 40

What is mucosal 5-HT an example of?

Answer 40

a sensory mediator but it is not also a circulating hormone

Question 41

Why is secretion of water and salt essential for digestion?

Answer 41

both digestion of macronutrients and the turbulent flow that brings nutrient molecules to the absorptive epithelium depend on there being adequate amounts of water in the lumen

Question 42

What is the volume of water transported across the mucosa per day equivalent to?

Answer 42

the whole blood volume of an animal

Question 43

How is secretion controlled by neurons?

Answer 43

Cl- ions are transported into lumen from enterocytes in mucosal crypts via CFTR channels in the apical membrane -> water and Na+ ions follow passively

Question 44

What is secretion enhanced by?

Answer 44

vasodilation of mucosal and submucosal blood vessels

Question 45

What are the two types of secretomotor neurons in the submucosal plexus and how do they work?

Answer 45

cholinergic: release ACh onto M3 muscarinic receptors to increase Cl- concentration gradient non-cholinergic: release vasoactive intestinal peptide (VIP) to activate adenylyl cyclase which results in cAMP opening CFTR channels

Question 46

What are both types of secretomotor neurons regulated by?

Answer 46

enteric circuits

Question 47

______ is electroneutral, while ______ is electrogenic.

Answer 47

absorption, secretion

Question 48

How is absorption controlled by enterocytes?

Answer 48

Na+ is transported from lumen into enterocytes (via NHE3 sodium hydrogen exchanger) at tips of villi (SI) and mucosal surface of colon

Question 49

When does diarrhoea occur? What are the two circumstances in which this occurs?

Answer 49

when secretion of water into the lumen exceeds absorption of water from the lumen -> can be due to increased secretion in the duodenum and jejunum or increased transit speed through the ileum and colon

Question 50

How does cholera toxin cause diarrhoea?

Answer 50

acts via neurons in duodenum and jejunum -> hypersecretion that swamps absorption in ileum and colon

Question 51

What is the mechanism by which cholera toxin causes diarrhoea?

Answer 51

acts by activating adenylyl cyclase | releases 5-HT from EC cells -> leads to increased propulsive contractions as well as hypersecretion

Question 52

Need ____ exposure to cholera toxin to produce ____, so ____ changes in affected neurons are relatively ____.

Answer 52

prolonged, hypersecretion, plastic, slow