Introduction to the Function & Control of the Alimentary Tract

Question 1

Describe briefly the functions of the different components of the digestive tract

Answer 1

• Mastrication by jaw and teeth
• Lubrication by saliva from buccal cavity - saliva contains enzymes that break down carbohydrates
• Pharyngeal muscles propel materials into the oesophagus
• Air lock portal made in oesophagus- transport materials to the stomach
• Liver involved in detoxification and protein synthesis - secretion of bile
• Stomach acts as a reservoir and acid decontaminator - contains digestive enzymes and HCO3- to neutralise acids
• Gallbladder produces bile to help to emulsify lipids
• Pancreas secretes hormones and digestive enzymes
• Intestines then act to absorb water, digest food and absorb nutrients, ferment organic residue and reabsorb bile

Small intestine - enzymatic digestion and absorption of water, organic substrates, vitamins and ions

Large intestine - dehydration and compaction of indigestible materials in preparation for elimination

• The lower large intestine then stores faeces

Question 2

State the three most important stomach functions

Answer 2

1. Accommodation and storage
2. Mechanical and enzymatic breakdown
3. Slow delivery of chyme to duodenum

Question 3

Describe the two main places of storage in the GI tract

Answer 3

• Stomach
• Food is stored in the stomach during the first stages of digestion - it can remain there for up to one hour unmixed - hence it acts as a reservoir
• The fundus and body of the stomach which has a thinner muscle tone relaxes to allow a large volume of around 1.5 L of food storage
• Vagal reflex inhibits smooth muscle tone - stimulation of mechanoreceptors leads to fundic relaxation via VIP (vasoactive intestinal peptide) and NO (nitric oxide)
• The antral region of the stomach mixes/grinds food with gastric secretions which allows digestion
• Colon/rectum
• Storage of indigestive residues and faecal matter
• This is able to occur because this area of the stomach has thicker muscle

Question 4

Describe how gastric secretions are stored and which substances make up these gastric secretions and the functions of these substances

Answer 4

• The stomach stores 2-3 L of gastric juice for up to 24 hrs
• This gastric juice includes mucus, pepsinogen, intrinsic factors and lipases which aid in the digestion and absorption of food
• Mucus secreted by goblet cells and mucous neck cells - acts as a lubricant by acting as a barrier that protects the stomach/colon, especially from gastric acid
• Lipases convert triglycerides into fatty acids and glycerol
• Pepsin, secreted by chief cells or peptic cells as pepsinogen, aids in protein digestion
• HCl secreted by parietal cells are important in defence
• Intrinsic factors (secreted by parietal cells) needed for Vitamin B12 absorption

Question 5

Describe the paracrine secretions of the GI tract

Answer 5

• Often called local hormones
• They are secreted from the cells in the mucosa but differ from hormones because the chemical acts locally on adjacent cells via the interstitial fluid
• Example - somatostatin which inhibits gastrin release in the stomach

Question 6

Describe the exocrine secretions of the GI tract and where they come from

Answer 6

• Salivary glands - mucus needed in lubrication for mastication and speech - lipases
• Gastric glands - HCl , pepsin, mucus
• Pancreas - bicarbonate ions, enzymes e.g. amylase, lipase and carboxypeptidase
• Liver- bile, salts, bile salts
• Secretions from numerous glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection

Question 7

Describe the endocrine secretions of the Gi tract and where they come from

Answer 7

• Secretions called hormones synthesised by ductless glands enter the bloodstream and travel to their target tissues where they bind to specific receptors to elicit a response
• Gastrin- stomach (g cells in antrum) - binds to receptors on parietal cells and causes HCl secretion
• Secretin - duodenal mucosa - binds to receptors on pancreas and allows secretion of bicarbonate rich secretions from the pancreas
• Pancreozymin - cholecystokinin (CCK) - duodenal mucosa - promotes gallbladder contraction so allow bile into the stomach to aid digestion
• Insulin - by b cells of the pancreas

Question 8

Describe why absorption is needed and where it occurs

Answer 8

• For food to be of use nutrients 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 the small intestine
• Absorption of fluid occurs in the small intestine and colon
• Colon absorbs 90% of water reducing volume of 200ml of semi solid faecal matter
• Disorders of fluid secretion and absorption are important in the pathogenesis of diarrhoea

Question 9

Describe motility within the GI tract (what is its importance)

Answer 9

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

Question 10

Describe how the GI tract excretes materials

Answer 10

• Drugs and some products of normal metabolism may leave the body in saliva, bile, faeces or vomit
• Indigestible food residues leave the body in faeces

Question 11

Describe why the GI tract needs defence mechanisms

Answer 11

• Like the skin and airway the gut epithelium is an interface with the contaminated outside world (exposed to external environment)
• The intestine is the largest mucosal surface in the body and is probably exposed to the heaviest burden of environmental antigens
• Like the skin if there is a breach in the barrier toxins will enter the blood
• It is also the largest lymphoepithelial organ

Question 12

State the defence mechanisms the GI tract has

Answer 12

1. Sight, smell and taste alerts us to harmful food substances
2. Vomit reflex
3. Acid in the stomach kills most harmful bacteria
4. Mucus secretions
5. Natural bacterial flora prevents colonisation of harmful bacteria
6. Aggregation of lymphoid tissue able to mount a response to food borne antigens - analyse and respond to pathogenic microbes
7. Peyer’s patches - located in the lamina propria layer of the mucosa and extending into the submucosa of the ileum

Question 13

Describe the metabolic functions of the liver

Answer 13

• The liver is a major metabolic organ in the abdominal cavity and weighs about 1.3kg in an adult
• It is involved in carbohydrate, nitrogen and lipoprotein metabolism as well as production of bile from cholesterol

Question 14

Summarise the two main types of nerves involved in innervation of the gut and how these nerves innervate the gut

Answer 14

• Food intake initiates dissension of the stomach which stimulates stretch receptors and in turn stimulates nerves
• Splanchnic nerves send signals to the spinal cord which then either directly signals or send messages to the brain which then sends signals to the enteric nervous system
• Vagal nerves also send signals to the brain via afferent vagal nerves , the brain then sends signals to the enteric nervous system via efferent vagal nerves
• There are also shorter nerves which directly innervate the gut via the enteric nervous system without input from the spinal cord or brain

Question 15

Briefly describe the effects that the parasympathetic and sympathetic nerves have on the GI tract

Answer 15

• Parasympathetic input -
• Stimulated digestive activity
• Stimulates the gallbladder
• Relaxes the rectum
• Sympathetic input -
• Inhibits digestive activity
• Stimulates glucose released by the liver

Question 16

Describe the dynamic nature of innervation of the gut - how there is relaxation and contraction simultaneously

Answer 16

• The gut must be stimulated to contract and relax in different places alternately to ensure food moves down the digestive tract
• When there is a bolus of food in the digestive tract moving along it the area behind that bolus must contract to allow the food to move into the duodenum and then the rest of the tract
• But the area ahead of the bolus of food must be relaxed to ensure there is accommodation of that food
• The ascending contraction occurs because of acetylcholine released at motor neurons
• The descending relaxation occurs because of VIP/NO released at motor neurons

Question 17

Describe the role of the pyloric sphincter in the movement of food through the GI tract

Answer 17

• Ripples of contraction move the food towards the antrum (thicker muscle layer)
• Pyloric sphincter is often relaxed but closes upon arrival of peristaltic wave
• Repulsion of chyme causes the opening of pyloric sphincter
• Small partially digested material is squirted through the pyloric sphincter into the duodenum
• Repulsion of antral contents backwards towards the body allows mixing/ grinding
• There is overall a sieving effect where viscous and solid matter is retained in the stomach

Question 18

Describe the role of the fundus in stomach contraction

Answer 18

The fundus of the stomach acts as a pacemaker zone as it is able to undergo spontaneous depolarisation - can initiate peristalsis which then sweeps across the stomach wall

Question 19

What are all the hormones produced in the gut?

Answer 19

Peptides

Question 20

Describe gastrin mediated effects

Answer 20

• Gastrin is secreted by cells and then the gastrin is secreted into the blood
• The secretion of gastrin is usually under neuronal control via parasympathetic system - gastrin releasing peptide stimulates gastrin release

Gastrin can act in two ways:
- It can act directly on its target cells the parietal cells to aid release of HCl
- It can also act on enterochromaffin like cells , neuroendocrine cells found in gastric glands of the gastric mucosa
- The ECL cells secrete histamine which triggers HCl release from parietal cells as parietal cells have histamine receptors (H2 receptors)
- Acetylcholine released by neurons can also act on ECL or parietal cells to aid HCl release by binding to muscarinic receptors

Question 21

Describe paracrine control of the alimentary tract

Answer 21

• Somatostatin release is controlled by hyperacidity
• When there is a high [H+], D cells secrete somatostatin
• The somatostatin travels through interstitial fluid and inhibits G cells to inhibit gastrin which then stops gastrin from stimulating acid release

Question 22

State the 2 intrinsic neuronal plexuses of the gut

Answer 22

1. Submucosal meissner plexus - regulates the digestive glands
2. Myenteric auerbach plexus - primarily connected with gut motility

Question 23

Describe the nervous control of the GI tract

Answer 23

• Intrinsic control -
• Nerve plexuses near the GI tract initiate short reflexes
• Short reflexes are mediated by local enteric plexuses
• There is a stimulus which stimulates chemoreceptors, osmoreceptors or mechanoreceptors which leads to stimulation of loca enteric nerve plexus which impacts smooth muscles or glands to change contraction of secretory activity of the gut
• Extrinsic controls -
• Long reflexes arising within or outside the GI tract
• Involves CNS centres and extrinsic autonomic nerves
• Parasympathetic reflexes
• There is a stimulus which stimulates chemoreceptors, osmoreceptors or mechanoreceptors which then signals to the central nervous system which sends signals to the local enteric plexus to then affect the smooth muscle or glands of the gut to change contraction/secretory activity

Question 24

Describe the vago vagal reflex including the reflex pathway

Answer 24

• It describes a type of reflex in which both the afferent (sensory) and efferent (motor) axons are in the vagus nerve trunk
• It is a reflex circuit within the GI tract and has a pathway via the medulla of the brain stem
• Describes a reflex control of responses to gut stimuli via the NTS and DMVN (dorsal vagal complex) in the brain
• Thus the vago vagal reflex is active during the receptive relaxation phase of the stomach in response to swallowing - and the reflex goes from the stomach to the brain and then back to the stomach - there is active contraction and relaxation of the smooth muscles of the stomach
• It also promotes motility and acid secretion

Vago vagal reflex pathway:

• There is a signal sent via an afferent vagal neuron to the NTS (nucleus tractus solitarius - main site of termination of vagal afferents) which then signals the DMVN (dorsal motor vagal nucleus- main site of origins of vagal efferents supplying the gut) both in the brain stem
• A signal is then sent down a vagal efferent nerve back to the stomach - acetylcholine release from these neurons can mediate both secretion and motility in the stomach

Question 25

Describe the enteric nervous system/local reflexes

Answer 25

1. Myenteric plexus - motor function
2. Submucosal plexus - intestinal secretions function

• Reflexly regulate GI functions entirely within the wall of the gut
• Connected to the CNS by parasympathetic and sympathetic fibres but can function autonomously without these connections
• Those effects are mediated entirely by the enteric nervous system - it has a similar number of neurons as the spinal cord
• It uses the neurotransmitters acetylcholine and nitric oxide

Question 26

Describe what cholinergic and adrenergic stimulation does to gastric motility and secretion in the myenteric plexus

Answer 26

• Cholinergic innervation increases gastric motility and secretion
• Adrenergic stimulation decreases gastric motility and secretion

Question 27

Describe how the type of food eaten impacts the rate of emptying of the stomach

Answer 27

Rate of emptying is dependent upon the materials ability to be absorbed
- Carbohydrates empties quickly into duodenum
- Proteins have slow emptying
- Fatty food are even slower
- Fatty acids in the duodenum decrease gastric emptying by increasing the contractility of pyloric sphincter

Question 28

Describe what peristalsis is

Answer 28

This is a wave of coordinated relaxation and contraction that goes across the stomach smooth muscle and throughout GI towards the anus

Question 29

Where is peristalsis slower?

Answer 29

Slower in the large intestine compared to the small intestine

Question 30

What is peristalsis stimulated by and what does it cause?

Answer 30

Wave of propulsive contractions moves contents of the gut towards the anus
Distension initiates contraction

(Vagal inhibitory (VIP/NO) and excitatory fibres (ACh) control movement)