GI physiology Flashcards
components of alimentary canal/GI tract from mouth - anus (6)
- mouth and oropharynx
- oesophagus
- stomach
- small intestine
- large intestine
- rectum and anus
accessory structures of alimentary canal/GI tract (3)
- salivary glands
- pancreas
- liver and gall bladder (hepatobiliary system)
what separates the different compartments of the GI tract
sphincters (circular rings of smooth muscle)
function of mouth and oropharynx (3)
- chop food and lubrication (to ease movement)
- carb (and limited fat) digestion
- propel food to oesophagus
- > alpha amylase breaks down things
function of oesophagus
-delivers food to stomach
function of stomach (3)
- stores food temporarily
- carb, fat AND protein digestion
- regulates delivery of chyme to SI
what type of muscle is oesophagus
- 1/3 skeletal
- 2/3 smooth muscle
function of small intestine (2)
- principle site of digestion
- absorption of nutrients
components of small intestine (3)
- duodenum
- jejunum
- ileum
function of large intestine (2)
- reabsorption of fluids and electrolytes
- stores faecal matter before regulated expulsion
function of rectum and anus
regulated expulsion of faeces
role of pancreas
secretes digestive enzymes
function of liver (2)
- produces bile
- suspends fat droplets
4 basic digestive processes (4)
- motility
- secretion
- digestion
- absorption
3 types of movement of food/motility (3)
- propulsive movements (peristalsis)
- mixing movements (segmentation)
- tonic contractions
what triggers secretion
-hormonal and neural signals
function of secretion
-required for digestion and protection
where does secretion occur
into the lumen of the GI tract from the GI tract itself and accessory structures
what do digestive secretions contain (3 plus eg’s)
- water (large vol. borrowed from plasma)
- electrolytes (Na, K, HCO3, Cl)
- organic compounds (enzymes, bile salts, mucus etc)
what occurs during digestion
- complex foodstuffs broken down into smaller more absorbable units by enzymatic hydrolysis
- mediated by diff. enzymes
what are carbs broken down to during digestion
-carbs (mostly polysaccharides) and some disaccharides (sucrose) -> monosaccharides (glucose, galactose and fructose)
what are proteins broken down to during digestion
constituent amino acids, dipeptides and tripeptides
what enzymes mediate carbohydrate digestion (2)
amylases and disaccharides
what enzymes mediate protein digestion (3)
proteases, dipeptidases and tripeptidases
what are fats broken down to during digestion
fats (mostly triglycerides) -> monoglycerides and free fatty acids
what enzymes mediate fat digestion
lipases
what are absorbed from the digestive tract to the blood, or lymph
-absorbable products of digestion (along with water, electrolytes and vitamins)
what mediates absorption
numerous transport mechanisms
what are the layers of the digestive tract wall (from inside out) (4)
- mucosa
- submucosa
- muscularis externa
- serosa
what controls motility
- mostly smooth muscle (circular, longitudinal layers and the muscular mucosae)
- skeletal muscle in the mouth, pharynx, upper oesophagus and external anal sphincter
what happens when circular muscle contracts during motility
lumen becomes narrower and longer
what happens when longitudinal muscle contracts during motility
intestine becomes shorter and fatter
what occurs when muscularis mucosae contracts during motility
- change occurs in the absorptive and secretory area of mucosa (folding)
- mixing activity occurs
where are ICC’s located in relation to the muscles of the GI tract (2)
- in between the longitudinal and circular muscle layers
- within the submucosal layer
what controls the spontaneous electrical and contractile activity of the GI tract (3)
- intrinsic (enteric) and extrinsic (autonomic) nerves
- various hormones
where in the GI tract does the spontaneous electrical activity occur as slow waves (3)
- stomach
- small intestine
- large intestine
what are slow waves
-‘rhythmic patterns of membrane depolarisation and repolarisation that spread cell- cell via gap junctions aka ICC’s’
what drives/controls slow waves
ICC’s (interstitial cells of cajal) = pacemaker cells
why are ICC’s described as gap junctions
electrically couple to each other and adjacent smooth muscle cells to allow the spread of electrical current from cell-cell and allow hundreds of cells to depolarise and contract at once (functional synctium)
what do slow waves determine (3)
max. frequency, direction and velocity of rhythmic contractions
when does muscle contraction occur
-only if slow wave amplitude reaches threshold/signif to trigger action potential
what determines the force of muscle contractions
the number of action potentials discharged
what determines the upstroke of slow wave action potentials
voltage activated calcium channels
what determines the downstroke of slow wave action potentials
voltage activated potassium channels
what does the basic/ basal electrical rhythm determine
the frequency of contractions in the GI tract
what determines the basic/basal electrical rhythm
- slow waves
- differs along length of GI tract as not all slow waves trigger action potentials as threshold must be reached
what determines whether slow waves amplitude reaches threshold (3)
- neuronal stimuli
- mechanical stimuli
- hormonal stimuli
BER frequency in the stomach
3 slow waves per min
BER frequency in the small intestine (2)
- approx 10-12 slow waves per min in duodenum
- 8 slow waves in terminal ileum (tends to drive luminal contents to aboral direction)
BER frequency in the large intestine (2)
- approx 8 slow waves per min in proximal colon
- approx 16 slow waves per min in distal colon (favours retention of luminal contents facilitating absorption of of water and electrolytes)
intrinsic supply of GI tract
enteric nervous system (ENS), modulated by hormones and extrinsic nerve output
where are ganglia of enteric nervous system located
- within myenteric (auerbach’s) plexus and submucous (meissner’s) plexus
- ganglia are connected by the interganglionic fibre tracts
where is the myenteric plexus located
in the muscularis external layer of GI tract
what does the enteric nervous system coordinate muscular, secretive and absorptive activities via (3)
- sensory neurones
- interneurones (coordinate reflexes and motor programs)
- effector neurones (excitatory and inhibitory motor neurones)
what do effector neurones (excitatory and inhibitory motor neurones) of GI tract supply (4)
- longitudinal and circular smooth muscle layers (for modifying motility)
- secretory epithelium
- endocrine cells (for varying secretion of GI hormones)
- blood cells
extrinsic innervation of GI tract
-autonomic: parasympathetic and sympathetic
parasympathetic supply of GI tract (2)
- via vagal nerves signalling to the dorsal vagal complex of medulla oblongata
- pelvic nerves from the sacral region of spinal cord (S2-S4)
sympathetic supply of GI tract
-spinal nerves from the thoracolumbar region
in which ganglia do the preganglionic sympathetic nerves synapse before supplying GI tract (4)
- superior cervical ganglion
- celiac ganglion
- superior mesenteric ganglion
- inferior mesenteric ganglion
neurotransmitter of parasympathetic system supplying GI tract
-acetylcholine at synapse between preganglionic fibres and ganglion cells (in essence post ganglionic neurones) within the ENS
neurotransmitter of sympathetic system supplying GI tract
- acetylcholine at synapse between preganglionic fibres and prevertebral ganglion
- noradrenaline between postganglionic fibres and enteric structures/other structures
excitatory influences of parasympathetic nervous system on GI tract (3)
- increased gastric (stomach), pancreatic and small intestinal secretion
- increased blood flow
- smooth muscle contraction
inhibitory influences of parasympathetic nervous system on GI tract (2)
- relaxation of some sphincters
- receptive relaxation of stomach
inhibitory influences of sympathetic nervous system on GI tract (3)
- decreased motility
- decreased secretion
- decreased blood flow
what has greater functional effect on GI tract: the parasympathetic or sympathetic nervous system
parasympathetic
nerve reflexes that occur due to receptor activation and modulate the motor and secretory function of the GI tract (3)
- local reflex (intrinsic, all elements of reflex located within GI tract itself)
- short reflex (extrinsic involves autonomic nervous system)
- long reflex (extrinsic, known as long reflex as autonomic reflexes involve long pathways between the central nervous system and digestive system)
eg. of local reflex of GI tract
-peristalsis
eg. of short reflex of GI tract
-eg. intestino-intestinal inhibitory reflex (local distension/enlargement activates sensory neurones exciting sympathetic preganglionic fibres that cause inhibition of muscle activity in adjacent areas)
sensory receptors of the GI tract
- mechanoreceptors (pressure receptors sensitive to stretch or tension within the wall)
- chemoreceptors (sensitive to chemical components)
- thermoreceptors
eg. of long reflex of GI tract
-eg. gastroileal reflex (increase in gastric activity causes increased propulsive activity in the terminal ileum)
major patterns of motility in the GI tract (3)
- peristalsis
- segmentation
- tonic contractions
what is peristalsis
-wave of contraction that normally proceeds along the gut in aboral direction triggered by distension of gut wall
what happens to the muscles at the propulsive segment of GI tract during peristalsis (2)
- longitudinal muscles relax (due to release of neurotransmitters VIP/vasoactive intestinal peptide and NO/nitric oxide from inhibitory motoneurone)
- circular muscle contracts (due to release of neurotransmitter acetylcholine and substance P/peptide neurotransmitter from excitatory motoneurone)
what happens to the muscles at the receiving segment of GI tract during peristalsis (2)
- longitudinal muscles contract (due to the release of Ach and substance P/peptide neurotransmitter from excitatory motoneuron)
- circular muscle relaxes due to release of VIP/vasoactive intestinal peptide and NO/nitric oxide from inhibitory motoneurone)
describe segmentation
- mixing or churning movements
- rhythmic contractions of the circular muscle layer that mix and divide luminal contents
where does segmentation occur (2)
- in small intestine (in fed state)
- in large intestine
what is haustration
segmentation in the large intestine
what are tonic contractions
sustained contractions found in the sphincters of the GI tract
what are the sphincters of GI tract composed of
specialised circular, generally smooth muscle
-skeletal = UES and external sphincter
name the sphincters of the GI tract (6)
- upper oesophageal sphincter (UES)
- lower oesophageal sphincter (LOS)
- pyloric sphincter
- ileocecal valve
- internal (smooth muscle) and external (skeletal muscle) sphincters
- sphincter of oddi (where pancreatic contents enter the duodenum)
function of GI sphincters (2)
- act as one way valves by maintaining positive resting pressure
- open and close due to stimuli/pressure proximal and distal to sphincter (opens due to +ve pressure above, closes due to +ve pressure below)
function of upper oesophageal sphincter (UES) (2)
- relaxes to allow swallowing
- closes during inspiration
function of lower oesophageal sphincter (LOS) (2)
- relaxes to permit entry of food to the stomach
- closes to prevent the reflux of gastric contents into the oesophagus
function of pyloric sphincter (2)
- regulates gastric emptying
- usually prevents duodenal gastric reflux
function of ileocecal valve
- regulates flow from ileum to colon
- > distension of ileum closes valve
- > distension of proximal colon closes valve
what regulates the internal and external sphincters
the defecation reflex
3 pairs of major salivary glands (3)
- parotid
- submandibular
- sublingual
where are minor salivary glands present (5)
- lips
- cheeks
- palate
- oropharynx
- tongue
type of salivary secretion from parotid gland
-serous/watery
type of salivary secretion from submandibular gland
-mixed (mucous/serous)
type of salivary secretion from sublingual gland
-mucous (thick viscous saliva with more mucoprotiens)
what cells secrete saliva into the duct system
acinar cells
how is primary saliva altered as it passes through duct
- duct reabsorbs ions (sodium chloride)
- potassium is secreted into saliva from duct and bicarbonate enters glands to help neutralise saliva and raise pH (to prevent decay)
osmolarity of primary saliva
=isotonic (contains water and electrolytes, sodium and chloride)
osmolarity of secondary/ final saliva entering the mouth
=hypotonic (contains some proteins, electrolytes, potassium and bicarbonate ions)
function of resting secretion (3)
= very small amount of saliva at constant rate
-keeps mouth and oropharynx moist, lubricated and protected
what neurotransmitters causes contraction of muscles of GI tract (2)
- acetylcholine
- substance P/peptide
what neurotransmitters causes relaxation of muscles of GI tract (2)
- Nitric Oxide
- vasoactive intestinal peptide (VIP)
how many times do you swallow per minute during resting salivary secretion
2/3
what sensory receptors are activated during eating and stimulate secretion of saliva (5)
- gustatory receptors (taste receptors on tongue)
- mechanoreceptors (pressure/pushing/vibration)
- olfactory receptors (sense of smell)
- nociceptors (pain will trigger response)
- higher centres in the brain trigger reflex activation of secretory pathways
varying effect of stimulus on single gland secretions
-volume and composition varies depending on the type and duration of stimulus
components of whole mouth saliva (3)
- mixed secretions of all glands (variable volumes and contents)
- debris
- other oral fluids (e.g. gingival crevicular fluid)
contributions to unstimulated whole saliva (4)
- parotid = 20%
- submandibular = 65%
- sublingual = 7-8%
- minor glands = 7-8%
contributions to stimulated whole saliva (4)
- parotid = 50%
- submandibular = 30%
- sublingual = 10%
- minor glands = 10%
average resting salivary flow rate
0.3 +/- 0.22ml per min
average stimulated salivary flow rate
1.7 +/- 2.1 ml per min
total daily salivary flow rate
between 500-700ml per day
inorganic compounds of saliva and their functions (7)
- sodium
- potassium
- chloride
- fluoride (helps to reminilarise enamel)
- calcium (enamel protection)
- phosphate (enamel protection/calcium phosphate)
- bicarbonate (buffer)
organic compounds of saliva (9)
- carb
- blood group substances
- glucose
- lipids
- cortisol
- amino acids
- urea
- ammonia
- proteins (all synthesised in the gland)
functions of saliva (10)
- protects tissues
- enhances taste
- lubricates food
- speeds oral clearance of food
- facilitates removal of carbohydrates
- neutralises organic acids
- inhibits demineralisation/enhances reminerilisation
- recycles ingested fluoride to the mouth
- discourages bacterial growth
- proteins sustain enamel surface
components of saliva involved in digestion (2)
- amylase (hydrolyses starch to maltose/maltose triose)
- lipase (fat digestion)
protein components of saliva with microbial characteristics (3)
- lysosome
- sialoperoxidase
- lactoferrin
function of stratherins (protein component of saliva) (2)
- manage calcium
- also have microbial functions
what is salivary secretion dependant upon
reflex activity (very little is spontaneous)
salivary secretion stimuli (8)
- gustatory
- masticatory
- olfactory
- psychic (contributes very little to salivary secretion)
- visual
- thermoreceptive
- possibly nociceptive
- combinations of the above
gustatory salivary reflex
- stimuli delivered to gustatory receptors in taste buds
- basic tastes cause salivary secretion
effect of one sided chewing on salivary secretion (2)
- higher rate of secretion on side of chewing
- preferred side of chewing has larger parotid gland
which salivary gland does the olfactory salivary reflex involve (2)
either:
- parotid (true olfactory parotid gland reflex does not exist in humans)
- submandibular gland
function of the stomach (5)
- starting point for digestion of proteins
- mixes food with gastric secretions to produce chyme
- limited amount of absorption
- stores food before passing it into the small intestine as chyme for further digestion/absorption
- secretes gastric juice from gastric pits in the gastric mucosa
anatomy of stomach (3 components)
- fundus
- body
- antrum
sphincter between stomach and duodenum (SI)
pylorus
defecation
the discharge of faeces from the body
what is the funds (anatomy of stomach) (3)
- thin smooth muscle layer next to oesophagus
- receives food but little mixing
- usually pocket of gas
describe the body of the stomach (3)
- thin smooth muscle of middle section
- little mixing occurs
- food stored here
describe the antrum of the stomach (3)
- thicker smooth muscle next to duodenum
- highly contractile
- much mixing occurs here
what breaks down proteins in stomach (2)
- pepsin
- HCl
what is the most potent stimulus for the gustatory salivary reflex
- SOUR/ACID stimulus is the most potent
- followed by salt then bitter and sweet
what neurotransmitters stimulate contraction of the GI tract muscles (2)
-Ach
-substance P/peptide neurotransmitter
(from excitatory motoneuron)
what neurotransmitters stimulate relaxation of the GI tract muscles (2)
-VIP/vasoactive intestinal peptide
-NO/nitric oxide
(from inhibitory motoneuron)
what determines whether chyme escapes through the pyloric sphincter
the strength of antral wave
what factors determine the strength of the antral wave (2)
- gastric
- duodenal
gastric factors that determine the strength of the antral wave (2)
- rate of emptying is proportional to the vol. of chyme in the stomach
- consistency of chyme
how does an increase in the vol of chyme in stomach/distension increase motility (4)
- stretch of smooth muscle
- stimulation of intrinsic nerve plexuses
- increased vagal nerve activity
- gastrin increases
what is gastrin
- hormone which stimulates secretion of gastric juice
- secreted into the bloodstream by the stomach wall in response to the presence of food
which consistency of chyme favours stomach emptying
finely divided, thick liquid chyme
gastric factors that can decrease strength of antral wave/delay gastric emptying
- neuronal response
- hormonal response
which neuronal reflex decreases antral peristaltic activity/gastric emptying
-enterogastric reflex through signals from intrinsic nerve plexuses and autonomic nervous system
which hormone released from the duodenum inhibits stomach contraction?
-enterogastrones (eg.secretin and cholecystokinin CCK)
which stimuli within duodenum drives the neuronal and hormonal response which delay stomach emptying (4)
- fat
- acid
- hypertonicity
- distension
why does the presence of fat cause/require a delay in gastric emptying
for digestion and absorption in the small intestine
why does the presence of acid cause/require a delay in gastric emptying
for neutralisation (by bicarbonate secreted from the pancreas)
why does hypertonicity cause delay in gastric emptying
- products of carb and protein digestion draw water into small intestine
- danger of reduced plasma vol and circulatory disturbances
secretions from the gastric glands of oxyntic mucosa (5)
- HCl
- pepsinogen
- intrinsic factor
- histamine
- mucus
function of HCl secreted in stomach (3)
- activates pepsinogen to pepsin
- denatures protein
- kills most micro-organisms ingested with food
what is pepsinogen secreted in stomach
- inactive precursor of the peptidase, pepsin
- pepsin once formed activates pepsinogen
function of intrinsic factor secreted in stomach
binds vitamin B12 allowing absorption in terminal ileum
function of mucus secreted in stomach
protection
what hormone stimulates HCl secretion in stomach
gastrin
what hormone inhibits HCl secretion in the stomach
somatostatin
gastric secretions in the pyloric gland area (3)
- gastrin
- somatostatin
- mucus
what cells secrete somatostatin
D cells present in the pyloric gland area (PGA)
what cells secrete gastrin
G cells present in the pyloric gland area (PGA)
what cells secrete pepsinogen
chief cells present in gastric glands of the oxyntic mucosa
what is secreted from parietal cells present in gastric glands of the oxyntic mucosa (2)
- HCl
- intrinsic factor
what are the 2 diff gastric glands and where are they (2)
- glands in the pyloric gland area (PGA) at area of antrum
- gastric glands of oxyntic mucosa (OM) at area of fundus and body
describes the steps involved in HCl secretion from gastric parietal cell (present in gastric glands of oxyntic mucosa) (8)
- CO2 + H2O -> H2CO3 (CA)
- H2CO3 -> (H+) + (HCO3-)
- HCO3- exits via Cl-/HCO3- antiporter
- Cl- builds up in cell and eventually leaves
- H+ exits via H+/K+ ATPase
- (H+) + (Cl-) -> HCl
- H+ exchanges for K+
- K+ returns to lumen (K+ channel) or to interstitial fluid (Na+/K+ ATPase)
3 molecules that stimulate HCl secretion from parietal cell (and where they come from)
- ACh (enteric neurones)
- gastrin (G cells in PGA)
- histamine (ECL cells)
how does gastrin cause an increase in HCl secretion from parietal cell
stimulates histamine release from ECL cells
hormone that inhibits HCl secretion from parietal cell
somatostatin (from D cells in PGA)
where is HCl secreted in stomach
into canaliculi (deep invaginations of plasma membrane) of the oxyntic mucosa
effect of secretagogues (substances which promote secretion) on H+/K+ ATPase in gastric parietal cells
- in parietal resting state = within cytoplasmic tubulovesicles
- parietal stimulated state = traffics to apical membrane into extended microvilli
phases of gastric secretion (3)
- cephalic (in head phase, prepares stomach for food)
- gastric (when food is in stomach)
- intestinal (after food has left stomach)
stimuli of cephalic phase of gastric secretion (6)
- sight
- smell
- taste
- conditioned reflexes
- chewing
- swallowing
what happens in cephalic phase of gastric secretion (5)
- activation of enteric neurons (via parasympathetic preganglionic neurons traveling in the vagus nerve)
- > release of ACh
- > activation of ECL cell to release histamine
- > activation of G cell to release gastrin which stimulates ECL cell to release histamine
- > activation of D cell to release ss to inhibit G cell releasing gastrin
factors that stimulate gastric secretion in the gastric phase (3)
- distension (via mechanoreceptors)
- protein digested products stimulate G cells to release gastrin
- food acts as buffer raising pH, somatostatin no longer needed
factors that reduce gastric secretion in intestinal phase (3)
- SI factors switch off secretion
- factors that reduce gastric motility reduce secretion
- secretion of somatostatin (low pH during meals)
