An introduction to the functions and control of the alimentary tract Flashcards
What are digestive functions of the stomach
Digestive functions of stomach:
1) Accommodation and storage
2) Mechanical and enzymatic breakdown
3) Slow delivery of chyme to duodenum
Explain storage in the stomach
stomach: food stored here during first stage of digestion
Fungus and body of stomach relaxes and allows large volume of food storage
-vagal reflex inhibits smooth muscle tone- mechanoreceptors —-> fundic relaxation
Explain storage in the colon/rectum
colon/rectum: storage of indigestive residues and faecal matter
How does the stomach accommodate food in the stomach
by using VIP and NO to relax the walls of the stomach to expand
Explain the storage of gastric secretions
-stomach stores 2-3 litres of gastric juice
Mucus; 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; protein digestion
HCl; important in defence
Intrinsic; for vitamin B12 absorption
Explain paracrine secretions
Paracrine secretions:
-often called ‘local hormones’
-secreted from cells in the mucosa, the chemical acts locally on adjacent cells via the interstitial fluid
Explain exocrine secretions
exocrine secretions:
-salivary glands (mucus and lipase)
-gastric glands (hydrochloric acid, pepsin and mucus)
-pancreas (bicarbonate ions, enzymes)
-liver (bile salts, bile acids)
-secretions from numerous glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection
Explain endocrine secretions
-secretions called ‘hormones’ are synthesised by ductless glands that enter the blood stream, travel to their target tissues where they bind to specific receptors to elicit their effects
Gastrin: stomach
Secretin: duodenal mucosa
Pancreozymin-cholecystokinin: duodenal mucosa
Insulin: pancreas (B-cells)
Explain absorption
-for food to be of use to the body- the nutrients resulting from digestion must be transported across the intestinal epithelium into the blood
-absorption occurs mainly in the small intestine
-absorption of fluid occurs in the small intestine and colon
-the colon absorbs 90% of the water
-disorders of fluid secretion and absorption are important in the pathogenesis of diarrhoea
Describe the motility of the gastric
-the movements of muscular wall allows for storage
-movement from one region to another; mass evacuation
-mechanical degradation
-mixing lumen contents
-transport of nutrients, water, urea’s and electrolytes
-digestion and absorption
What are different excretory ways that drugs leave the body
drugs and some products of normal metabolism may leave the body in:
-saliva
-bile
-faeces
-vomit
-indigestible food residues leave the body in the faeces
Explain the defence of the gastric
-the gut epithelium is exposed to the external environment
-the intestine is the largest mucosal surface in the body- is exposed to the heaviest burden of environmental antigens
What are the defence mechanisms of the gut
1) sight, smell and taste to alert us to harmful food substances
2) vomit reflex
3) acid in stomach kills most harmful bacteria
4) mucus secretions
5) natural bacterial flora prevents colonisation of harmful patterns
6) aggregation of lymphoid tissue able to mount a response to food borne antigens
7) payers patchers: contains mucosal associated lymphatic tissue
What are metabolic functions of the gut
-the liver is a major metabolic organ in the abdominal cavity and weighs 1.3kg in an adult
-it is involved in carbohydrate, nitrogen and lipoprotein metabolism
How does the ANS enable food storage in the stomach
-empty stomach= small volume
-the relaxation of the gastric reservoir is mainly regulated by reflexes.
Three kinds of relaxation can be differentiated:
-the receptive
-adaptive
-feedback-relaxation
-the inhibitory vagal fibres releasing ACH activate inhibitory enteric pathways that release NO, PACAP, VIP and ATP in order to relax the muscle
What can vagotomy lead to
vagotomy can lead to decrease accommodation and gastric compliance by 5%
How does the ANS enable movement of food into the duodenum
1) Ripples of contraction move the food towards the antrum (thicker muscle layer)
2) Pyloric sphincter is often relaxed but closes upon arrival of peristaltic wave
3) Repulsion of chyme causes the opening of pyloric sphincter
4) Small partially digested material is squired through the pyloric sphincter into duodenum
5) Repulsion of natural contents backwards towards the body allows for mixing/grinding
What are different types of control
-endocrine
-paracrine
-neural
-vago-vagal reflex
-enteric or local reflex
-metabolic
Explain endocrine control
-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)
Explain the role of the fundus in the stomach
-pacemaker zone
-spontaneous depolarisation
-BER and peristalsis
How does gastrin stimulate the production of HCl
G cell – makes gastrin (G)
Neuron – releases gastrin-releasing peptide (GRP) and acetylcholine (A)
ECL cell (enterochromaffin-like cell) – releases histamine (H)
Parietal cell – makes hydrochloric acid (HCl)
- Neuron stimulates the G cell
A nerve releases GRP → stimulates G cell to release gastrin (G).
2. Gastrin enters the blood
Gastrin travels through circulation to other stomach cells.
3. Gastrin stimulates ECL cells
Gastrin reaches ECL cells, causing them to release histamine (H).
4. Histamine stimulates parietal cells
Histamine binds to parietal cells → which secrete HCl (stomach acid).
5. Acetylcholine (A) from neurons also helps
Direct stimulation of ECL cells and parietal cells by acetylcholine also boosts acid production.
How does paracrine control helps regulate acid production in the stomach
1) H⁺ (acid) increases in the antral lumen (lower stomach area).
2) This stimulates the D cell.
3) D cell releases somatostatin.
4) Somatostatin acts on G cells, telling them to stop releasing gastrin.
5) Less gastrin = less acid made by the stomach.
What are two nerve fibres intrinsic to the gut
two nerve fibres are intrinsic to the gut:
-myenteric plexus: motor function
-submucosal plexus: intestinal secretions
What does the myenteric and submucosal plexus do
-reflexly regulate GI functions entirely within the wall of the gut
-connected to CNS via parasympathetic and sympathetic fibres but can function autonomously without these connections
-these effects are mediated entirely by the enteric nervous system
-the neurotransmitters are Ach and NO
What are large neuronal connections in the gut wall
there are large neuronal connections in the gut wall:
1) submucosal Meissner plexus: regulates the digestive glands
2) myenteric Auerbach plexus: primarily connected with gut motility
What are intrinsic and extrinsic controls of the GI tract
Intrinsic controls:
-nerve plexuses near the GI tract initiate short reflexes
-short reflexes are mediated by local enteric plexuses
Extrinsic controls:
-long reflexes arising within or outside the GI tract
-involve CNS centres and extrinsic autonomic nerves
-parasympathetic reflexes
Explain Auerbach’s plexus
-most prominent plexus
-cholinergic innervation—> increased gastric motility and secretion
-adrenergic stimulation—> gastric motility and secretion
Explain the vago-vagal reflex
-it describes a type of reflex in which both the afferent and efferent axons are in the vagus nerve trunk
-it is a reflex circuit within the GIT
-pathway is via the brain stem (medulla)
-describes reflex control of responses to gut stimuli via the NTS and DMVN
How does the vagi-vagal reflex cause contraction and relaxation of smooth muscles in the stomach
The vago-vagal reflex is active during the receptive relaxation of the stomach in response to swallowing- the reflex goes from the stomach to the brain then back to the stomach
-this causes active contraction and relaxation of smooth muscles of the stomach
-it also promotes motility and acid secretion
Explain the NTS and DMVN
NTS= Nucleus tractus solitarius- main site of termination of vagal afferents
DMVN- Dorsal motor vagal nucleus- main site of of origin of vagal efferents supplying the gut
What is a vago-vagal reflex pathway
It’s a reflex loop between the brainstem and the gut, using the vagus nerve both to send and receive signals.
It helps coordinate digestion automatically—without you thinking about it.
What is the function of the brain stem vagal motor nucleus, pre ganglionic vagal efferent, post ganglionic and Ach in the vago-vagal reflex pathway
Brain stem-vagal motor nucleus- starts the signal like a control centre
Pre ganglionic vagal efferent- sends the message from the brain to a relay station
Post ganglionic neuron- passes the message to the actual target tissue
Ach- the neurotransmitter that delivers the signal
Explain how different food groups are digested simply
-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
Fatty acids in the duodenum—> decreases in gastric emptying by increasing contractility of pyloric sphincter
Explain peristalsis
-slower in large intestine compared to small intestine
-wave of propulsive contractions moves content of gut towards the anus
-distension initiates contraction
-vagal inhibitory (VIP, NO) and excitatory fibres control movement
