Introduction to control of alimentary tract

Question 1

Role of ANS

Answer 1

The autonomic nervous system innervates the gut in terms of extrinsic parasympathetic (e.g. vagal) and sympathetic (e.g. splachnic) supply and then there is the intrinsic enteric nervous system.

parasympathetic will promote the digestive function and therefore digestive activity, secretions (gallbladder) and relax the rectum

sympathetic will inhibit digestive activity and stimulate the release of glucose via the liver

Question 2

Autonomic innervation of the gut

Answer 2

sight + smell of food have an effect on stomach -> send message down to the stomach via the vagal nerves therefore the stomach relaxes before the food gets there due to anticipation

as food make it way to the duodenum -> splanchnic nerve will promote activity within the GI tract

food making its way to the end of the stomach + food in SI -> contraction due to Ach and substance P/K
The contraxtion will be behind the bolus of food to move the food along so the other end needs to relac therefore VIP and NO will be released

Smooth muscle cells with receptors able to release neurotransmitters that will help to modulate peristaltic reflex

Question 3

Innervation of the gut

Answer 3

The gut is innervated by various parts of the nervous system.

There are vagus afferent fibres from the gut muscle and mucosa which travel to the brain and effect fibres which communicate with the ENS.

There is also communication from the spinal cord to the ENS by the splanchinic nerves. The splanchnic nerves carry afferent sympathetic neurones to the spinal cord and efferent neurones to the ENS.

The ENS doesn’t have to rely on the CNS, ENS can send its own signals to the gut muscle and mucosa

Question 4

ANS enables food storage in stomach

Answer 4

Empty stomach = 50ml
Receptive relaxation of fundus and body of stomach
Stomach accommodates 1.5L of food
Parasympathetic nerves
Vagus inhibitory fibres

Question 5

Relaxation of the reservoir (fundus) is mediated by reflexes and can be differentiated into 3 types and name one component mediator

Answer 5

receptive (mechanical stimulation of pharynx – mechanoreceptors, sight),

adaptive (vagal innervation (NO/VIP), tension of stomach)

feedback (nutrients, CCK).

Question 6

other mediators for the relaxation of the stomach

Answer 6

Briefly, the receptive, adaptive and feedback-relaxation of the stomach are mediated by non-adrenergic, non-cholinergic (NANC) mechanisms (i.e., inhibition involving NO, VIP, etc.) as well as by reflex chains involving release of noradrenaline. When the stomach is ready to receive the food, very early on, noradrenaline is released from the sympathetic nerve fibres which helps the stomach to relax. For a single best answer question, the correct answer will depend on the list of possible answers.

PACAP: pituitary adenylate cyclase (AC)-activating peptide (secretin family of peptides): isolated from pituitary and shown to stimulate adenylate cyclase AC activity in the anterior pituitary
High levels in brain, but also found in gut – myenteric and submucosal ganglia
Mediates neuronal regulation of gastric acid secretion; intestinal motility
Stimulates relaxation of colonic smooth muscle and stimulates pancreatic secretions; stimulates insulin and glucagon secretion in humans.

Question 7

Recap: functions of ANS enable storage of food in stomach

Answer 7

Vagus inhibitory fibres allow the release of NO and VIP

Mechanical stimuli within pharynx due to food in mouth called receptive relaxation which allows storage of food (stomach prepares)

Adaptive relaxation -> presence of food within the fundus allow relaxation to occur

Feedback -> if fat in food component, the partially digested food will make its way to the duodenum and the duodenum mucosa will pick it up that fat is there therefore it will feedback to higher centres of the brain (the vagus centre) which send a message down to say no need to put food here as CCK is release which allows adaptive relaxation to occur and you feel fuller for longer

CCK -> also promote the contraction of the gall bladder to allow bile duct secretion which will emulsify fat

Question 8

Effect of selective vagotomy

Answer 8

decreased accomodation of stomach + decreased compliance

Question 9

ANS enables movement of partially digested food into duodenum

Answer 9

Ripples of contraction move the food towards the antrum (thicker muscle layer) - where grinding will occur

Pyloric sphincter is often relaxed but closes upon arrival of peristaltic wave - don’t want too much food entering duodenum as its a smaller reservoir

Repulsion of chyme causes the opening of pyloric sphincter

Small partially digested material is squirted through the pyloric sphincter into duodenum

Repulsion of antral contents backwards towards the body allows mixing/grinding

Question 10

food enetring stomach and effect when entering duodenum

Answer 10

Cranial nerves + proximal contraction + motilin hormone are important for contraction

GRP (gastin releasing peptide) to release gastrin
CCK for contraction of gall bladder + relaxation of fundus and longer acccomadation is promoted

Whatever happens in the duodenum wtheter that is acid, protein or fat content, will feedback to the proximal end of the stomach -> duodenum is distended by the passage of the periodic squirting of partially digested food _> feedback will occur e.g. can’t cope no more due to detection of acid so controls how much bi-carbonate is produced

Question 11

What are horomones?

Answer 11

Hormones are carried in the blood from their site of production to their target site

All hormones produced by the gut are peptides (sequence of amino acids)

Question 12

Neurocrine communication

Answer 12

sensory neuron -> interneuron -> secretomotorneuron to secrete neurotransmitter e.g. Ach where it binds to a receptor and get an effect

Ach -> contraction + secretion as it promotes acid secretion

Question 13

Endocrine communication

Answer 13

The Ach secretiob initiated by sight of food, smell of food, taste of food and food in stomach -> neuronal process therefore gastrin-mediated + Ach-mediated acid secretion

Secretomotor neurons release Ach which binds to receptos on Parietal cells (musacarinic receptors). Ach can bind to ECL cells to promote histamine secretion too. The histamine will bind to H2 receptors on pareital cells to release Hcl

GRP causes the secretion of gastrin from G cells which will travel in the blood. It can have an effect on the parietal cells directly or it can bind to target cells on the ECL cells.

When gastrin binds to this, it promotes the secretion of histamine from those cells which promote acid secretion from parietal cells as histamine has receptors (H2) which mediate acid secretion

Question 14

Other endocrine hormones that have an effect

Answer 14

oestradiol and progesterone can reduce the gut transition time

Question 15

Paracrine control and example

Answer 15

paracrine agents go via the interstitial fluid to adjacent cells

When too much acid production from G cell, the D cell will secrete somatostatin which will inhibit the G cell. This is dependent on high levels of H+ as H+ will bind to the D cell to secrete somatostatin

Question 16

Neural control

Answer 16

There is both parasympathetic and sympathetic supply to the gut.

A large amount of parasympathetic is from the vagal nerve (vagal afferents and efferents) as well as there being spinal afferents and efferents from the sympathetic system (mainly splanchnic nerves). (afferents going towards CNS, efferents towards the gut).

Vagus nerve afferents innervate the lower oesophagus, fundic region and duodenal area.

Question 17

What is the vago-vagal reflex?

Answer 17

It is reflex circuit within the GIT

A reflex in which both the afferent (“sensory”) and efferent (“motor”) axons are in the vagus nerve trunk

Pathway is via the brain stem (medulla)

The vago-vagal reflex is active during the relaxation of the stomach in response to swallowing food.

Question 18

What is the vago-vagal pathway?

Answer 18

Control of accommodation
Coordination of contraction/relaxation of GIT

Food when consumed will stimulate the mechano receptor which will stimulate the vagal afferent fibres. The brain will decide it wants to increase motility/secretion

Vagal afferents from the gut enter into the medulla -> NTS -> DMVN -> vagal efferent

The short post-gangilionic vagal nerve releases Ach at the target tissue of the gut, which effects secretion and motility of the gut.

Question 19

Enteric nervous system or local reflex

Answer 19

Two nerve fibres are intrinsic to the gut:
Myenteric plexus (Auerbach’s plexus): motor function
Submucosal plexus (Meissner’s plexus): intestinal secretions

Reflexly regulate GI function entirely within the wall of the gut
Connected to CNS by parasympathetic and sympathetic fibres, but can function autonomously without these connections

Those effects are mediated entirely by the enteric nervous system (the third component of the ANS)-has a similar number of neurons (100 million) as the spinal cord

Neurotransmitters: ACh, NA, 5-HT, GABA, ATP, NO

Question 20

Auerbach’s plexus and Meissner’s plexus

Answer 20

Auerbach’s plexus has both parasympathetic and sympathetic input whereas the Meissner’s plexus has only parasympathetic fibres and provides secretomotor innervation to the mucosa nearest the lumen of the gut

submucosal/meissner plexus surrounds the submucosa and is beneath the circular muscle -> closer to lumen for secretions

auerbach/myentric plexus is beneath the logituduinal muscle and between that and circular muscle -> for motor control

Question 21

Auerbach’s plexus (myentric plexus)

Answer 21

Most prominent plexus
Cholinergic innervation ↑ gastric motility and secretion
Adrenergic stimulation ↓ gastric motility and secretion

Question 22

Colo-colonic reflex

Answer 22

presence of food or food products/distension of the stomach increase motility of the colon in response – again it is due to stretch in the stomach and by-products of digestion in the small intestine

Question 23

Metabolic – type of food eaten

Answer 23

Rate of emptying is dependent upon the material’s ability to be absorbed

Carbohydrates emptied quickly into duodenum
Proteins – slow emptying
Fatty foods – even slower due to emulsification in duodenum which is a time dependent process

Fatty acids in the duodenum → ↓ in gastric emptying by increasing the contractility of pyloric sphincter therefore accomodation will continue to occur

Question 24

Peristalsis

Answer 24

Slower in large intestine compared to small intestine

Wave of propulsive contractions moves contents of gut towards the anus

Distension initiates contraction -> caused by the presence of a bolus of food -> mechano-receptors have vagal innervation to allow peristalsis to occur

Vagal inhibitory and excitatory fibres control movement (control the contraction and relaxation of gut smooth muscles) -> in a coordinated fashion (longtidunal muscle contract + circular muscle relax on the recieving end but the opposite happens on other end so coordinated action)