intro to function and control of alimentary tract Flashcards
what are the digestive functions of the stomach ?
- Accommodation & storage
- Mechanical and enzymatic breakdown
- Slow delivery of chyme to duodenum
describe storage as a stage in digestion
- Stomach: Food stored 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
- Antral region mixes/grinds food with gastric secretions → Digestion
- Colon/rectum: storage of indigestive residues and faecal matter
describe storage of gastric secretions
- Stomach stores 2-3 litres of gastric juice/24hr (mucus, pepsinogen, 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 ( pepsinogen converted to active form pepsin under acidic conditions)
- HCl (secreted by parietal cells) – important in defence
- Intrinsic factor (secreted by parietal cells) – for vitamin B12 absorption
what are paracrine secretions ?
- Often called “local hormones”
- 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
what are exocrine secretions ?
- 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
- Secretions from numerous glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection
what are endocrine secretions ?
- 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: stomach (G-cells in antrum)
- Secretin: duodenal mucosa -( influences pancreases to release bicarbonate rich secretions - CCK in response to fat)
- Pancreozymin-cholecystokinin: duodenal mucosa
- Insulin: pancreas (-cells)
- Exocrine, endocrine and paracrine secretions allow active digestion and control of digestion, gastric motility (and energy homeostasis)
describe absorption as a stage in digestion
- 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
describe motility
- The movements of the muscular wall (mostly smooth muscle except extreme ends of the upper oesophagus/rectum) allows:
- Storage, e.g. proximal stomach, descending colon
- 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 and of urea and electrolytes
- Digestion and absorption
describe excretion
- Drugs and some products of normal metabolism may leave the body in:
Saliva
Bile, bilirubin
Faeces
(Vomit) - Indigestible food residues (e.g. tomato skin) leave the body in the faeces
describe defense in terms of GI function
- Like the skin and airways, the gut epithelium is an interface with the “contaminated” outside world
- That is it is exposed to the 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 breache in the barrier, “toxins” will enter the blood
- It is also the largest lymphoepithelial organ
give some defense mechanisms to protect the gut
- The gut is unsterile as it is open to external environment and the following help to protect the gut:
- Sight, smell and taste alerts us to harmful food substances
- Vomit reflex
- Acid in stomach (HCl) kills most harmful bacteria
- Mucus secretions
- Natural bacterial flora prevents colonisation of harmful bacteria
- 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 extend into the submucosa of the ileum; contain mucosal-associated lymphatic tissue- WBCs and lymphocytes (protects against infection)
how does the ANS enable movement of food into the duodenum ?
- 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 duodenum
- Repulsion of antral contents backwards towards the body allows mixing/grinding
what is 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)
describe gastrin mediated effects
- Vago-vagal reflex can cause the release of gastrin.
- The stimulation of G cells by gastrin releasing peptide (GRP) causes the release of gastrin.
- Gastrin is released into circulation and able to elicit the following effects:
- Stimulate enterochromaffin-like (ECL) cells to cause histamine release. The released histamine is able to cause acid (HCl) secretion by binding to H2 receptors on parietal cells;
- Gastrin can directly stimulate the parietal cells by binding to its receptors and cause acid (HCl) secretion;
Other effects of gastrin:
* Gastrin and motilin upregulate the force of gastric contraction
* Gastrin may stimulate the emptying of the ileum
Histamine: an endogenous local hormone which mediates a range of physiological responses. The important effects of histamine here is that involving acid secretion
Acetylcholine (ACh): a neurotransmitter; vago-vagal reflex causes the release of ACh. As in the case of gastrin, ACh can mediate its effects via:
* Stimulating ECL cells to cause acid (HCl) secretion;
* Stimulating muscarinic receptors (M3 receptors) on parietral cells to cause acid (HCl) secretion
describe the enteric nervous system
- 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 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
describe intrinsic neuronal plexus of the gut
- The gut has an intrinsic, enteric nervous system
- There are large neuronal connections in the gut wall:
- Submucosal Meissner plexus: regulates the digestive glands
- Myenteric Auerbach plexus:primarily connected with gut motility
describe the vago-vagal reflex
- 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 Gut
- Pathway is via the brain stem (medulla)
- Describes 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 of the stomach in response to swallowing – and the reflex goes from stomach to brain and then back to stomach → active contraction and relaxation of smooth muscles of the stomach
- It also promotes motility and acid secretion
describe the mechanism of the vago-vagal reflex
Afferent (Sensory) Pathway:
The reflex begins when sensory neurons in the gastrointestinal tract detect changes such as the presence of food, distension (stretching of the stomach or intestines), or chemical signals (like acidity).
These sensory signals are transmitted via afferent fibers of the vagus nerve to the medulla oblongata in the brainstem.
Processing in the Brainstem:
The sensory information is processed in the nucleus of the solitary tract (NST) in the brainstem, which integrates the signals and determines the appropriate response based on the condition of the gut.
Efferent (Motor) Pathway:
The processed information is sent back via efferent fibers of the vagus nerve to the gastrointestinal tract.
This results in a motor response that includes actions such as increased gastric motility, secretion of digestive enzymes, or relaxation of the pyloric sphincter (to allow the passage of food into the small intestine) DMVN = Dorsal motor vagal nucleus-main site of origin of vagal efferents (motor) supplying the gut
describe peristalsis
- Slower in large intestine compared to small intestine
- Wave of propulsive contractions moves contents of gut towards the anus
- Distension initiates contraction
- vagal inhibitory (VIP, NO) and excitatory fibres (ACh, SP) control movement
Name 2 areas of the digestive tract that have storage functions
Fundus; colon/rectum
Name 1 paracrine secretion of the digestive tract and its function
Somatostatin – inhibits gastrin-mediated acid secretion
Name 3 endocrine secretions of the digestive tract
Gastrin; secretin; CCK
Name 1 lymphoid tissue within the digestive tract
Peyer’s patches
Name 2 processes by which food is moved along the digestive tract
Peristalsis; segmentation; mass movement; migrating motor complex (Phase III)
Name 1 process important in the receptive relaxation of the stomach and name 2 substances important in this relaxation
Neural control/vago-vagal reflex/parasympathetic stimulation; NO & VIP
List 2 effects of acetylcholine on the gastrointestinal tract
Acid secretion (a histamine-mediated effect); gut motility
Name the cell type that mediates gastrin release from the antrum
G cells
What is the effect of gastrin on parietal cells?
Acid secretion (histamine-mediated effects, direct effects on parietal cells and also effects on chief cells)
Name 2 parts of the stomach
Fundus; antrum (and body or corpus)
Two nerve fibres are important in the local control of the functions of the gastrointestinal tract. Name these and state their functions
Submucosal plexus (Meissner’s plexus): Intestinal secretion and local movement; Myenteric plexus (Auerbach’s plexus): motor function, tone and velocity of movement of GIT
State how fast/slow the following foods are emptied from the stomach: Rice
Fast
State how fast/slow the following foods are emptied from the stomach: Some cubes of rump steak
Slow
State how fast/slow the following foods are emptied from the stomach: Fatty bacon cooked in pure lard
Slower than the rate of emptying of rump steak
What is the name of the gastrointestinal tract’s ‘little brain’ that can regulate its functions without input from the higher centres?
Enteric nervous system
What neurotransmitter is primarily involved in the neural mechanism for relaxation?
Acetylcholine
Fill in the blank: The primary neurotransmitter involved in the relaxation of the gastrointestinal tract is _______.
Acetylcholine
True or False: The enteric nervous system requires input from the brain to function.
False
