L20. Neural and Hormonal Control of Gastrointestinal Function 1

Question 1

What are the basic functions of the GIT? [5]

Answer 1

1. Digestion
2. Absorption
3. Excretion of waste
4. Prevention of invasion by pathogens
5. Contains the microbiome

Question 2

The gut coordinates widely separated regions in a complex manner. What are the key regulatory requirements/functions of the gut? [3]

Answer 2

1. Controls intestinal smooth muscle contractions for peristalsis
2. Regulation of the secretion of digestive enzymes
3. Control of re-absorption of water and electrolytes

Question 3

There are many interacting control systems with some redundancies in function. Why is this so?

Answer 3

There needs to be failsafes built in to ensure that this system doesn’t succumb to damage that it is so susceptible to (in direct contact with the outside environment)

Question 4

What are the two main interacting control systems of the gut?

Answer 4

Local (enteric) and endocrine signals

Question 5

Describe the local control of the gut

Answer 5

1. The enteric nervous system: controls secretion of salt and water and is pre-programmed (the brain has no impact on what, only how much)
2. Interstitial cells of Cajal: modified excitable cells that produce pacemaker, rhythmic changes in membrane potential of smooth muscles (motor activity/motility)

Question 6

Describe the endocrine control of the gut

Answer 6

Signals from the intestinal mucosa to and from the ANCILLARY ORGANS like the brain, pancreas and gall bladder:

• secretion of enzymes
• food seeking behaviour and appetite

Question 7

What is the difference between the mucosa of the small intestine vs. the colon?

Answer 7

Small intestine has villi in the mucosa extending out and pits going in while the Colon has a relatively flat mucosal surface with deep pits/crypts

Question 8

Describe the basic structure of the enteric nervous system in the gut wall (in relation to the muscles)

Answer 8

Longitudinal muscle and circular muscle layers of the gut wall.
Between the two muscle layers is the MYENTERIC PLEXUS
And under the circular muscle (between it and the submucosa) is the SUBMUCOSAL PLEXUS

Question 9

What is the myenteric plexus responsible for?

Answer 9

Stimulation leads to:

• increased tone of the gut wall
• increased intensity of the rhythmical contractions
• a slight increase in rate of the rhythm of contraction
• an increased velocity of peristalsis
• inhibition of contraction of the pyloric and the ileocecal valves

Question 10

What is the submucosal plexus primarily responsible for?

Answer 10

concerned with the control of functions within the inner walls of each gut segment i.e. local absorption, secretion, contraction

Question 11

Describe the Interstitial Cells of Cajal (ICC)

Answer 11

Specialised cells in both plexuses
Form a 2D sheet of cells interconnected by gap junctions forming an ELECTRICAL SYNCYTIUM
- they generate large depolarising potentials with rhythms
- electrically coupled to the muscle layers

Question 12

What are the mucosal Enteroendocrine (EE) cells?

Answer 12

Several subtypes exist
Endocrine cells localised and specialised to the gut
Contain and secrete many mediators including serotonin, CCK, somatostatin, Glucagon-like peptides, pancreating polypeptide

Question 13

What are some examples of enteroendocrine cells?

Answer 13

Enterochromaffin cells: secrete serotonin
G cells: secrete gastrin
D cells: secrete somatostatin
Enterochromaffin-like cells: histamine

Question 14

Where to the enteroendocrine cells lie in the mucosa?

Answer 14

Span the apical surface of the epithlium at the level of lumen. Activation at one end (lumen) leads to release of their contents at the basolateral surface to act in an endocrine or paracrine manner

Question 15

What are the 3 major interacting control reflexes acting on the gut? (one is a subset of the other)

Answer 15

1. Vago-vago reflex
2. Intestino-intestinal reflex

Vago-vago is a subtype of the intestino-intestinal reflex

3. CNS control

Question 16

Describe the vago-vago reflex

Answer 16

Pathway that coordinates the movement in the upper GI tract through the vagus nerve

Activation at the peripheral level by sensory stimuli
= primary afferent neurons via vagus to the brain to activate neurons
= descending signals via the dorsal motor vagal pathway produces efferent control of the gut

Question 17

What kind of the reflexes are controlled by the vago-vagal pathway?

Answer 17

Swallowing
Acid secretion in the stomach
Peristalsis of the oesophagus
Contraction of the stomach and duodenum

Question 18

What happens to vagal influence moving down the gut tube?

Answer 18

It wanes progressively

It only controls about 0.5 of the 6m long gut tube

Question 19

Describe the intestino-intestinal reflexes

Answer 19

They run out of the gut into the CNS and then back down into the intestine via vagal or other means

eg. Dorsal root ganglia and spinal cord

Question 20

What are the major differences between the vagal pathway vs. the spinal nerve pathway?

Answer 20

VAGUS:
excitatory
tends to deal with day to day monitoring

SPINAL:
inhibitory
deal with nociception (irritation/pain)

Question 21

A class of intestino-intestinal reflexes involves cell bodies in the enteric nervous system (embedded in the gut wall) projecting out into what pre-vertebral sympathetic ganglia?

Answer 21

Celiac

Inferior mesenteric ganglia

Question 22

What is the general trend of the intestino-intestinal reflex pathways in terms of synaptic connections and sympathetic outflow?

Answer 22

Outflow goes BACK to inhibit various parts of the intestine

The signalling cells and controlling cells lie distally to the effector cells and the neurons they innervate

Question 23

Give a basic example of an intestino-intestinal reflex pathway

Answer 23

Distention or strong stimuli sensed by the colon leads to inhibition of motor activity in the small intestine (distal to proximal direction)

Tells the earlier parts to slow down as the later parts are not ‘ready’ for more.

Question 24

What are the CNS control pathways involved in?

Answer 24

Anticipation, mood, activity (mediated through cognitive function)

Question 25

How do vagal and sympathetic input modulate the enteric neural circuits of the enteric nervous system?

Answer 25

The produce a complex motor output INDIRECTLY (don’t act on the muscle but rather controls the amount of activity of the intrinsically programmed system)

Question 26

To be full of flavour and taste, what must the food have?

Answer 26

Significant amounts of fat
Free amino acids to excite umami receptors
sugar or sweeteners to be aromatic

Question 27

What is the cephalic phase?

Answer 27

A reflex triggered by sight, smell and taste (and thought) of food

Causes salivation, gastric acid and pepsin secretion and relaxation IN ANTICIPATION of food.

Question 28

Through what does the cephalic phase work through?

Answer 28

The vagus nerve

Question 29

What is the major function of the cephalic phase?

Answer 29

To set up the stomach to store food and begin the barrier function of the GIT

If this didn’t occur, there is less acid secretion and the stomach isn’t ready to cope with the inflow of food.

The acid is also very important to killing bacteria in food

Question 30

How is primary peristalsis of the oesphagus controlled?

Answer 30

It is entirely vagal (neural)

Question 31

What are the four main mediators that interact in regulating acid secretion?

Answer 31

1. ACh from enteric neurons excited by vagus
2. Gastrin from G cells in antrum and duodenum
3. Histamine from ECL cells
4. Somatostatin inhibition from D cells

Question 32

What happens when food enters the stomach?

Answer 32

Distention caused by contents is sensed and activates ENTERIC and VAGO-VAGAL REFLEXES that secrete more acid and pepsin

Question 33

How do the Interstitial cells of cajal act in this stomach phase of digestion?

Answer 33

They’re activity propagates from the corpus (body) to the antrum causing ripples of constriction to propagate food towards the pylorus

Question 34

What happens to food in the antrum?

Answer 34

Triggers a reflex INHIBITION of acid secretion in the body (triggers D cells to release somatostatin)

Strong constrictions from the ICC prortentials drive cells to the CLOSED sphinchter.

Question 35

What happens to the content of the food in the stomach?

Answer 35

Acid, protease and mechanical activity causes components of food to separate fat out

The fat floats to the top of the stomach in the fundus