Introduction to the function and control of the alimentary tract Flashcards
How does the ANS enable the storage of food in the stomach?
- ANS allows for accommodation of food
- Does this through receptive relaxation of the fundus and body, which is coordinated by the ANS, not voluntarily controlled
What is the fundus and what does it do?
- Bulbous dome-shaped gastric region, stores undigested food + gases released during chemical digestion process
Describe receptive relaxation of the fundus and body
- Refers to muscle relaxation in stomach that occurs before entry of food through oesophagus
- Gastric motility is first movement of digestive system, activated when food present or absent in stomach
- With entry of food as liquid or bolus, stomach wall receptors are stimulated to relax to take up and store food that is swallowed.
Outline the process of digestion
- Fundus and body of stomach (thinner muscle tone) relaxes, allowing larger volume (1.5L) of food storage
- Vagal reflex inhibits smooth muscle tone - mechanoreceptors → fundic relaxation
- Vasoactive intestinal peptide (VIP) and NO
- Antral region mixes/grinds food with gastric secretions - digestion
- Colon/rectum- storage of indigestive residues and faecal matter
Why is receptive relaxation considered a vagovagal reflex?
As it is both excited and inhibited by the vagus nerve
What happens after receptive relaxation?
- Peristaltic movements that mix the food, and various changes convert the food into chyme
- After chyme is formed, the contents of the stomach empty into the duodenum, in a process called gastric emptying
What branch of the ANS enables accommodation?
Peripheral - vagus inhibitory fibres
How much can a full stomach accommodate?
1.5 L
What branch of the nervous system allows the movement of food into the duodenum?
Autonomic
Explain the mechanism of the peritalsic wave and how food is mixed and transported into duodenum
- Ripples of contraction move food to antrum and pyloric sphincter closed upon arrival of peristaltic wave
- Repulsion of chyme causes pyloric sphincter to open and small partially digested material is squired through pyloric sphincter into duodenum
- Repulsion of astral contents back into body allows mixing/grinding
- Sieving effect - viscous and solid matter are retained in the stomach
What are paracrine secretions and name an example and its function?
- Secreted from cells in mucosa
- Act on adjacent cells via interstitial fluid
- Somatostatin- inhibits gastrin release in stomach
Where are exocrine secretions secreted from?
Secreted from numerous glands with ducts
What are the generic functions of exocrines?
- Digestion
- Lubrication
- Protection
Name salivary exocrine secretions and their function
Mucus for lubrication for mastication and speech
Name gastric gland exocrine secretions
Secrete HCL, pepsin, and mucus
Name pancreas exocrine secretions
Bicarbonate ion and enzymes- amylase, lipase, carboxypeptidase
Name liver exocrine secretions and their function
Bile acids - emulsification of fats
Name the different types of gastric secretions and describe their functions
What cells are they secreted from?
- Mucus -(secreted by goblet cells + mucus neck cells) - acts as lubricant by acting as a barrier that prevents the stomach and colon especially from gastric acid (prevents trauma)
- Lipase - Converts triglycerides to fatty acids + glycerol
- Pepsin - Secreted by chief cells or peptic cells as pespinogen. Protein digestion
- Intrinsic factor - Secreted by parietal cells for vitamin B12 absorption
- HCL - Secreted by parietal cells. Important in defence
What are endocrine secretions?
Hormone secretions - synthesised by ductless glands, enter blood stream, travel to target and bind to specific receptor
Name examples of endocrine secretions, where are they found?
- Gastrin - released by G cells in antrum
- Secretin - duodenal mucosa
- Pancreoxymin-cholecystokinin- duodenal mucosa
- Insulin - secreted from Beta cells of pancreas
What are G cells?
- Neuroendocrine cells responsible for the synthesis and secretion of gastrin
What is gastric juice stored in the stomach composed of
- Mucus
- Lipase
- Pepsin
- HCL
- Intrinsic factor
Describe the role of mucus in the GI system
Mucus (secreted by goblet cells + mucus neck cells) - acts as lubricant by acting as a barrier that prevents the stomach and colon especially from gastric acid (prevents trauma)
Describe the role of lipase in the GI system
Converts triglycerides to fatty acids + glycerol
Describe the role of pepsin in the GI system
Secreted by chief cells or peptic cells as pespinogen
Protein digestion
Describe the role of HCL in the GI system
Secreted by parietal cells
Important in defence
Describe the role of intrinsic factors in the GI system
Secreted by parietal cells
For vitamin B12 absorption
What are the digestive functions of the stomach?
- Accommodation & storage
- Mechanical and enzymatic breakdown
- Slow delivery of chyme to duodenum
Describe the role of absorption in the alimentary tract
- Nutrients from digestion transported across intestinal epithelium into blood or lymph (via lacteals - fats/lipids)
- Absorption occurs mainly in small intestine
- Absorption of fluid occurs in small intestine + colon
- Colon absorbs 90% of water, reducing volume to 200ml semi-solid faecal matter
- Disorders of fluid secretion + absorption important (together with motility) in pathogenesis of diarrhoea
Describe the role of motility in the alimentary tract
- Motility = movements of muscular wall
- Allows:
- Movement from one region to another part of gut; mass evacuation
- Mechanical degradation, e.g. gastric antrum
- Mixing lumen contents, e.g. small intestine
- Transport of nutrients, water and of urea and electrolytes
- Digestion
- Absorption
Describe the role of excretion in the alimentary tract
- Drugs + some products of normal metabolism may leave body in:
- Saliva
- Bile
- Faeces
- Vomit
- Indigestible food residues (e.g. tomato skin) leave body in faeces
Describe the role of defence in the alimentary tract
- Gut epithelium = interface that is exposed to external environment
- Intestine is largest mucosal surface in body, all the largest lymphoepithelial organ
- If there’s a breach in the barrier, toxins enter the blood to destroy microorganisms
What are the defence mechanisms of the alimentary tract?
- Sight, smell, taste alerts us to harmful food substances
- Vomit reflex
- Acid in stomach (HCL) kills most harmful bacteria
- Muscus secretions
- Natural bacterial flora, prevents colonisation of harmful bacteria
- Aggregation of lymphoid tissue (e.g. Peyer’s patches) able to most a response to food-borne antigens - analyse and respond to pathogenic microbes
- Peyer’s patches- Located in laminate propria layer of mucosa and extending into submucosa of ileum
How is mucosal repair and integrity maintained?
Growth factors (e.g. epidermal growth factor, insulin-like growth factor I) and prostaglandins
What 2 nerve fibres are intrinsic to the gut and what do they control?
- Myenteric/ Auerbach;s plexus - control motor functions
- Submucosal/ Meissner’s plexus - control intestinal secretions
What is unique about the enteric nervous system?
- These fibres regulate GI function in wall of gut
- Connected to CNS but can function on own and mediated entirely by ENS
What are some neurotransmitters of the ENS?
- Noradrenaline
- Ach
- 5-HT
- GABA
- ATP
- NO
Where are the two nerve fibres that are part of the ENS found?
- Auerbach’s - most prominent between longitudinal and circular muscle
- Meissner’s - surrounds the submucosa under circular
Describe the innervations of the gut
- Brain/ spinal cord send efferent signals to ENS, acts on gut muscle + mucosa
- Afferent vagus nerve sends gut signals to brain
- Efferent vagus nerve can mediate signal between brain/ ENS
- Afferent splanchnic nerves sends gut signals to spinal cord
- Efferent splanchnic nerve sends signals to ENS
What is the colo-colonic reflex?
- Plexuses found in wall of colon innervated by ANS
- Parasympathetic nervous system increases colon contraction and sympathetic nervous system decreases colon contraction = colo-colonic reflex
What is the role of cholinergic transmission in the GI tract?
Increases motility + secretions
What is the role of adrenergic transmission in the GI tract?
Decreases motility + secretions
What is the role of adrenergic transmission in the GI tract?
Decreases motility + secretions
What are hormones produced by the gut called?
Peptides
What hormones controls gastric HCL secretion?
- Gastrin - peptide hormone primarily responsible for enhancing gastric mucosal growth, gastric motility, and secretion of HCL into the stomach
Where does gastrin primarily act?
- Body of stomach
Describe how gastrin mediated effects are stimulated
- G cells stimulated by neurone, releasing GRP (gastrin releasing peptide) causing gastrin release
- Gastrin travels in blood and bind to enterochromaffin- like cells (ECL) in gastric glands of gastric mucosa
- ECL cells release histamine acting on parietal cells
- This causes HCL release, which can also occur through:
- Gastrin also directly acting on parietal cells
- ECL also acted upon by Ach, causes release of histamine which acts on parietal cells then causing HCL release by parietal cells
- Ach also directly acting on parietal cells
What are the roles of paracrine factors in the regulation of gastric HCL secretion?
H+ from HCL bind to D cells which release somatostatin which binds to G cells inhibiting gastrin release = less gastrin = less acid secreted by parietal cells
What do D cells in the GI do?
- Endocrine cell that releases hormone somatostatin, which binds to G cells inhibiting the synthesis and secretion of gastrin, and therefore preventing the secretion of HCL
What are the roles of nerves in regulation of gastric HCL secretion?
- Vagus nerve afferents innervate lower oesophagus, fundic region and duodenal region
- Vago-vagal reflex:
- Used to relax stomach allowing swallowed food to enter + be stored
- Vagal afferents from gut enter into medulla and synapse/terminate at nucleus tracts solitaries (NTS)
- Info processed and vagal efferents originate and leave medulla at dorsal motor vagal nucleus (DMVN)
- Vagal efferents then release Ach at target tissue of gut = affects secretion and motility of gut
How long does food stay stored in the stomach?
- 1 hr
How does the stomach accommodate for larger volumes?
- Fundus and body of stomach allows for larger volume of storage
What is the mechanism which allows for the stomach to relax and increase its volume?
Vagal reflex inhibits smooth muscle - allows fundic relaxation (via mechanoreceptors in lower oesophagus and fundic area)
VIP (vasoactive intestinal peptide) and NO involved in this.
What is the role of the astral region of the stomach?
Antral region of stomach mixes/grinds food with gastric secretions and expels food down the pyloric sphincter - a sieve allowing food of certain site through
What is the role of the colon/rectum?
Storage of digestive residues and faeces
What does motility allow for?
- Allows:
- Movement from one region to another part of gut; mass evacuation
- Mechanical degradation, e.g. gastric antrum
- Mixing lumen contents, e.g. small intestine
- Transport of nutrients, water and of urea and electrolytes
- Digestion
- Absorption
Describe the parasympathetic innervation of the GI tract
- Vagus nerve
- Pelvic splanchnic nerves
- 2 nerves provide parasympathetic innervation of GI tract
- Ganglia are in myenteric and submucosal plexuses - regulate GI motility and secretion respectively
Describe the sympathetic innervation of the GI tract
- Celiac ganglion - This is where the greater splanchnic nerve terminates
- Superior mesenteric ganglion - This is where the lesser splanchnic nerve terminates
- Inferior mesenteric ganglion - This is where the lumbar splanchnic nerve terminates
- Hypogastric ganglion
- The preganglionic neurones synapse outside the GI tract in the 4 ganglia
Describe the metabolic control of the GI system?
- Rate of gastric emptying dependent on ability of material to be absorbed:
- Carbohydrates emptied quickly into duodenum
- Proteins - slow emptying
- Fatty food - even slower emptying
- Fatty acids in dudodenum - decrease in gastric emptying by increasing the contractility of the pyloric sphincter
What parasympathetic nerve innervates the hindgut?
Pelvic sphlanchnic nerves
