Function and Control of the Alimentary Tract

Question 1

What are the two main storage spaces of the digestive system?

Answer 1

A) Stomach- food store here during first stage of digestion; may remain there for 1hr unmixed (acts as a reservoir)
Fundus and body of stomach (thinner muscle tone) relaxes, allowing large volume (1.5L) of food storage
Vagal reflex inhibits smooth muscle tone- mechanoreceptors -> fundic relaxation
- Vasoactive intestinal peptide (VIP) and nitric oxide (NO) are very important for the fundus relaxation
Antral region mixes/grinds food with gastric secretions -> digestion
B) Colon/rectum- storage of indigestive residues and faecal matter

Question 2

What gastric secretions does the stomach store?

Answer 2

Stomach stores 2-3 litres of gastric juice/24hr (mucus, intrinsic factor, lipase) which help in digestion and absorption of food
Mucus (secreted by goblet cells and mucus neck cells)- acts as a lubricant by acting as a barrier that protects the stomach and colon especially from gastric acid (prevents trauma)
Lipase- converts triglycerides to fatty acids and glycerol
Pepsin (secreted by chief cells or peptic cells as pepsinogen)- protein digestion
HCl (secreted by parietal cells)- important in defence and for the conversion of pepsinogen to pepsin
Intrinsic (secreted by parietal cells)- for vitamin B12 absorption

Question 3

What are paracrine secretions?

Answer 3

Often called “local hormones” so not endocrine secretion
Secreted from cells in the mucosa, but unlike hormones, the chemical acts locally on adjacent cells via the interstitial fluid
Example- somatostatin inhibits gastrin release in the stomach
So the cell that secrets somatostatin (D cell) is very close to the cell that secretes gastrin (G cells)
Gastrin is a hormone that travels in the blood to target cells

Question 4

What are exocrine secretions?

Answer 4

Salivary glands- mucus (lubrication for mastication and speech) lipase
Gastric glands- hydrochloric acid, pepsin, mucus
Pancreas- bicarbonate ions, enzymes (e.g. amylase, lipase, carboxypeptidase)
Liver-bile salts, bile acids which then gets secreted and gets stored into the gall bladder if not feeding or, if feeding, goes straight to the duodenum (provided there are no gall stones)
Secretions from numerous glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection

Question 5

What are endocrine secretions?

Answer 5

Secretions called ‘hormones’ synthesised by ductless glands enter the blood stream, travel to their target tissue(s) where they bind to specific receptors to elicit their effects
Gastrin- secreted by the stomach (G cells in antrum)
Secretin- secreted by the duodena mucosa because the duodenum is able to sense that the chyme is too acidic thereby needing more bicarbonate so the secretin will travel through the blood to the pancreas and tells it to secrete bicarbonate rich secretion so the chyme can be neutralised
Pancreozymin-cholecystokinin (CCK)- duodenal mucosa, gets secreted in response to a lipid filled food
It travels to the gall bladder, causes gall bladder contraction and releases bile salts along with secretions from the pancreas to help neutralise and digest the chyme
Insulin- pancreas (b-cells)
Exocrine, endocrine and paracrine secretions allow active digestion and control of digestion, gastric motility (and energy homeostasis)

Question 6

Where are nutrients absorbed?

Answer 6

For food to be of use to the body, the nutrients resulting from digestion must be transported across the intestinal epithelium into the blood (e.g. glucose, amino acids) or lymph via lacteals (fats/lipids)
Absorption occurs mainly in small intestine
Absorption of fluid occurs in the small intestine and colon
Colon absorbs 90% of water, reducing volume to 200ml of semi-solid faecal matter
Disorders of fluid secretion and absorption are important (together with motility) in the pathogenesis of diarrhoea

Question 7

What movement goes on in the digestive system?

Answer 7

Storage e.g. proximal stomach, descending colon
The movement of the muscular wall (mostly smooth muscle except extreme ends of the upper oesophagus/rectum) allows:
Movement from one region to another (law of gut); mass evacuation
Mechanical degradation e.g. gastric antrum- grinding
Mixing lumen contents e.g. small intestine- peristalsis
Transports of nutrients, water and of urea and electrolytes
Digestion and absorption

Question 8

How do drugs and other products of normal metabolism leave the body?

Answer 8

Drugs and some products of normal metabolism may leave the body in:
Saliva
Bile though bile can get reabsorbed
Faeces
(Vomit)
Indigestible food residues (e.g. tomato skin) leave the body in the faeces

Question 9

What defence mechanisms does the digestive system have?

Answer 9

The gut is unsterile as it is open to external environment and the following help to protect the gut:
1. Sight, smell and taste alerts us to harmful food substances
2. Vomit reflux
3. Acid in stomach (HCl) kills most harmful bacteria
4. Mucus secretions
5. Natural bacterial flora prevents colonisation of harmful bacteria
6. Aggregation of lymphoid tissue (e.g. Peyer’s patches) able to mount a response to food-borne antigens- analyse and respond to pathogenic microbes
Peyer’s patches- located in the lamina propria layer of the mucosa and extending into the submucosa of the ileum

Question 10

How is the gut innervated?

Answer 10

Splanchnic nerves- the paired nerves that principally innervate the internal organ
We call them paired nerves because we have this nerve that innervates the gut mucosa and sends sensory information to the spinal cord
There is an integration of message there that gets passed to the enteric nervous system (the little brain of the gut) where it makes sense of that and an appropriate action is taken
Also you have these long nerves that innervate the alimentary tract, and these send sensory information to the brain (long nerves because they move from the gut to the brain)
Then you have the efferent fibres which take the message from the brain and pass it to the enteric nervous system where they make sense of it
When message is coming from the sensory nerves to the brain its called afferent, from the brain to the ENS its efferent

Question 11

How does autonomic innervation control movement in the stomach?

Answer 11

Neurotransmitters that are important for the contractile responses are acetylcholine or Substance P which pushes the bolus of food across
Relaxation is caused by vasoactive intestinal peptide (VIP) and nitrous oxide
For example if the gut needs to contract or relax, for example peristalsis needs to occur, you will have contractile responses and relaxation responses
You can not have contraction going on continuously otherwise this are will be too narrow for the bolus of food to move along

Question 12

How does the ANS enable movement of food into the duodenum?

Answer 12

3.Ripples of contraction move the food towards the antrum (thicker muscle layer than fundus)
(Pyloric sphincter is often relaxed but closes upon arrival of peristaltic wave)
4.Repulsion of chyme causes the opening of pyloric sphincter
1.Small partially digested material is squirted through the pyloric sphincter into duodenum
2.Repulsion of antral contents backwards towards the body allows mixing/grinding
Sieving effect: viscous and solid matter are retained in the stomach

Question 13

What is the importance of endocrine control in the digestive system?

Answer 13

Hormones are carried in the blood from their site of production to their target site
All hormones are produced by the gut are peptides
Sensor cell secretes hormone, travels in blood to bind to the target cell
Neurocrine control like NANC
Sensory neuron releases hormone eventually from secretomotor neuron to target cells e.g. ACh

Question 14

What are the gastric-mediated effects of the digestive system?

Answer 14

G cells secrete gastrin
There is a parasympathetic neuron which releases a gastrin releasing peptide to get G cell to secrete gastrin
Gastrin travels within the circulation
It can act directly on the cells that lead to acid secretion, the parietal cells releasing HCl
It can also bind to receptors on enterochromaffin cells (ECL cells) and induces those cells to secrete histamine
Histamine then goes on to bind to receptors on the parietal cells
The specific receptors that it acts on on the parietal cells are called H2 receptors
You can have the parasympathetic neuron mediating or initiating the release of acetylcholine which can act on the parietal cells via muscarinic receptors but can also act on the ECL cells
These mechanisms are all initiated by the presence of food in the stomach

Question 15

What are the parts of the stomach?

Answer 15

Angle of His
Angular notch
Pyloric antrum
Pyloric sphincter
Chyme passes through the pyloric canal into the duodenum
The fundus has got a pacemaker zone so it has a basic electrical rhythm BER and peristalsis can start there and sweep along the length of the stomach

Question 16

What is the importance of paracrine control in the digestive system?

Answer 16

The importance of paracrine control, at the level of somatostatin, is that when there is hypersecretion of acid there is hyper acidity which could cause us ulcers
That hyper acidity stimulates the D cells
The D cells release somatostatin
The somatostatin, as a paracrine factor, travels a short distance through interstitial fluid and acts on the G cells to inhibit it so that no more gastrin or less gets released

Question 17

What is the vago-vagal reflex?

Answer 17

It describes a type of reflex in which both the afferent (“sensory”) and efferent (“motor”) axons are in the vagus nerve trunk
It is reflex circuit within the GIT
Pathway is via the brain stem (medulla)
Describes reflex control of responses to gut stimuli via the (nucleus tractus solitarius) NTS and DMVN (dorsal vagal complex in the brain)
Thus the vago-vagal reflex is active during the receptive relaxation of the stomach in response to swallowing – reflex goes from stomach to brain and then back to stomach → active relaxation of smooth muscles of the stomach
It also promotes motility and acid secretion

Question 18

What is the vago-vagal reflex pathway? (+e)

Answer 18

Vagal afferent signal is sent to the NTS, gets processed and sent out of the DMVN as a vagal efferent signal back to the stomach
An example
A message gets sent to the brain stem, send down the pre-ganglionic vagal efferent fibres
And you have this interneuron, the short post-ganglionic nerve
Both release acetylcholine in this case (as Ach mediates secretion)

Question 19

Which nerve fibres are intrinsic to the gut?

Answer 19

Two nerve fibres are intrinsic to the gut:
1. Myenteric plexus (Auerback’s plexus)- motor function
2. Submucosal plexus (Meissner’s plexus)- intestinal secretions
These two reflexively regulate GI functions 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)
It has a similar number of neurons (100 million) as the spinal cord
Neurotransmitters- Ach, NO, NA, 5-HT, GABA, ATP

Question 20

Where are the myenteric and submucosal plexus?

Answer 20

You can see that the submucosal plexus is very close to the lumen of the gut, because its important for secretions
The myenteric plexus is quite important for contractions so its closer to the longitudinal muscle

Question 21

What are the Auerbach’s and Meissner’s plexus for? (from where do they receive input)

Answer 21

Auerbach’s and Meissner’s plexuses are in wall of colon- receive input from both parasympathetic and sympathetic nervous system
Parasympathetic activity → ↑ contraction of proximal colon
Sympathetic activity → ↓ colonic movement
Colo-colonic reflex- food/food products ↑ motility

Question 22

What does the Auerbach’s plexus do?

Answer 22

Most prominent plexus
Cholinergic innervation ↑ gastric motility and secretion
Adrenergic stimulation ↓ gastric motility and secretion
In general you have more adrenergic stimulation

Question 23

How does metabolic control with the 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 (because it must be emulsified first)
Fatty acids in the duodenum → ↓ in gastric emptying by increasing the contractility of pyloric sphincter, so that material can not pass from the stomach into he duodenum

Question 24

What is peristalsis and how does it work?

Answer 24

Slower in large intestine compared to small intestine
Wave of propulsive contractions moves contents of gut towards the anus
Distension initiates contraction
Vagal inhibitory and excitatory fibres control movement