Overview of GI Physiology Flashcards
What organ of the digestive tract is responsible for passage of food?
oesophagus
What organ of the digestive tract is responsible for temporary storage of food
stomach
What organ of the digestive tract is responsible for digestion and absorption
small intestine
What does the alimentary tract provide the body with a continoual supply of?
- water
- electrolytes
- vitamins
- nutrients
What are the 5 main functions of the digestive tract?
- Movement of food through the alimentary tract
- secretion of digestive juices and digestion of food
- absoprtion of water, various electrolytes, vitamins and digestive products
- Circulation of blood through the GI organs to carry away the absorbed substances
- Control all these functions by local, nervous and hormonal systems
Mucosa =
epithelium + lamina propria + muscularis mucosa
Describe the features of the epithelium in the mucosa
- single cell layer forming continous lining of GI tract
- shed and replaced every 2-3 days
- apical side faces GI lumen; basolateral side faces interstitium and vasculature
- extent of villi and crypts vary with GI secretion function e.g. absoptive vs motility
Describe the features of the lamina propria
- Loose CT made up of elastin and collagen fibres
- contain sensory nerves, blood and lymph vessels, secretory glands
Describe the the muscularis mucosa
- thin layer of smooth muscle
- increases SA by creating ridges and folds
Describe the submucosa
- thicker layer with similar composition to lamina propria
- incorporates blood vessels and nerve bundles that form a submucosal plexus (Meisnner plexus) - integral part of enteric nervous system
Muscularis externa =
circular muscle + myenteric plexus + longitudnal plexus (Auerbach plexus)
Describe the muscularis externa
- muscle layers named based on orientation
- ENS co-ordinates contractions to mix and move contents between compartments
- Sphincters regulate flow from one compartment to the next
Describe the serosa
- Outermost later of CT and squamous epithelial cells
- some GI tract secretions do not have a serosal layer (e.g. oesophagus), but connect directly with the adventitia (CT that blends into the abdominal or pelvic wall
Give the layers of the GI wall in a typical cross section from the outer surface
- serosa
- longitudnal smooth muscle layer
- circular smooth muscle layer
- mucosa
AND
- sparse bundles of smooth muscle fibres lie in deeper layers of muscoas
Describe how a syncytium is generated in GI smooth muscle
- fibres connected via gap junctions
- CT partly separates each bundle but at many points bundles fuse forming a branching network
- Syncytium is created in each muscle layer (AP is evoked = it travels in all directions)
Give the length and breadth of a typical GI smooth muscle
- 200-500μm long
- 2-10μm diameter
- in bundles of up to 1000 parallel fibres
What is GI smooth muscle excited by?
almost continual slow, intrinsic electrical activity
What are the two basic types of electrical waves in GI SM?
- slow waves
- spikes
How can the voltage of the resting membrane potential change?
change to different levels
What determines the rhythmical GI contractions?
slow waves
- are not APS; are slow undulating changes in resting membrane potential
Between what values does the intensity of slow waves vary?
5 and 15mV
What is the range of frequency of slow waves?
3-12 per minute
Where are slow waves the fastest?
small intestine and duodenum
Are spike potentials true action potentials?
Yes
When do spike potentials occurs?
when resting membrane potential >~40mV
What causes a greater frequency of spike potentials?
higher increase in slow wave potential
How long does each spike potential last?
10-20ms
10-40x as long as APs in large nerve fibres
What can spike potentials be stimulated by?
- stretch
- acetylcholine
- parasympathetics
Draw a diagram showing the changes in voltage of the resting potential of GI smooth muscle
What is hyperpolarisation of GI smooth muscle stimulated by?
- Noradrenaline
- Sympathetics
What is the neural control of GI function?
enteric mervous system
Where is the enteric nervous system located?
in wall of gut from oesphagus to anus
important role in GI movements and secretion
What are the main features of the myenteric plexus
- linear chain of interconnecting neurons extending the entire length of the GI tract
- controls muscle activity
- some neurons are inhibitory;s ecrete an inhibitory transmitter (VIP)
What are the main effectd og the myenteric plexus when stimulated?
- increased tonic contraction/”tone” of gut wall
- increased intensity of rhythmical contractions
- slightly increased rhythm of contraction
- increased velocity of conduction of excitatory waves along the gut wall, causing more rapid movement of the gut peristaltic waves
What are the main features of the submucosal plexus?
mainly controls function within small segments of the intestine
**control local intertisital secretion, absoprtion and contractions of submucosal muscle that causing infolding of GI mucosa
Effects of PSNS on control of GI Tract
- increase activity of enteric NS
- almost all fibres are in the vagus nerve
Effects of SNS on the GI Tract
- Inhibits GI tract activity
- almost all fibres originate between T5 and T12
What are the possible locations of afferent sensory nerve fibres of the gut
- cell bodies in the ENS
- dorsal root ganglia of the spinal cord
What can sensory nerves be stimulated by?
- irritation of the gut mucosa
- excessive distention of the gut
- presence of specific chemical substances in the gut
What are the 3 types of gastrointestinal reflex?
- Reflexes integrated within the enteric nervous system
- Reflexes from the GI tract to prevertebral sympathetic ganglia and then back to the GI tract
- Reflexes from the GI tract to the brain stem or spinal cord and then back to the GI tract
Describe :
Reflexes integrated within the enteric nervous system
Reflexes controlling secretion, peristalsis, mixing, local inhibitory actions
Describe :
Reflexes from the GI tract to prevertebral sympathetic ganglia and then back to the GI tract
Reflexes that transmit signals to other areas of the GI tract – gastrocolic reflex (stomach – colon); enterogastric reflexes ( stomach and colon inhibiting gastric motility) ; colonoileal reflexes (inhibition of ileal emptying)
Describe:
Reflexes from the GI tract to the brain stem or spinal cord and then back to the GI tract
- Reflexes from stomach and duodenum to brain stem and back to stomach to control gastric motor and secretory activity
- Pain reflexes causing general inhibition of entire GI tract
- Defecation reflexes from the colon and rectum that travel via the spinal cord back to produce powerful colonic, rectal and abdominal contractions
What is the stimuli for the secretion of:
Gastrin
- protein
- distention
- Nerve
- (acid inhibits release)
What is the stimuli for the secretion of:
Cholecytokinin
- Protein
- Acid
- Fat
What is the stimuli for the secretion of:
Secretin
- Acid
- Fat
What is the stimuli for the secretion of:
Gastric (inhibitory and peptide)
- protein
- fat
- carbohydrate
What is the stimuli for the secretion of:
Motilin
- Fat
- Acid
- Nerve
What is the site of secretion of:
Gastrin
G cells of the antrum, duodenum and jejunum
What is the site of secretion of:
Cholecystokinin
I cells of the duodenum, jejunum and ileum
What is the site of secretion of:
secretin
S cells of the duodenum, jejunum and ileum
What is the site of secretion of:
Gastric (inhibitory and peptide)
K cells of the duodenum and jejunum
What is the site of secretion of:
Motilin
M cells of the duodenum and jejunum
What is the action of:
Gastrin
Stimulates
- Gastric acid secretion
- Mucosal growth
What is the action of:
Cholecystokinin
Stimulates
- Pancreatic enzyme secretion
- Pancreatic bicarbonate secretion
- Gallbladder contraction
- Growth of exocrine pancreas
Inhibits
- Gastric emptying
What is the action of secretin
Stimulates
- Pepsin secretion
- Pancreatic bicarbonate secretion
- Biliary bicarbonate secretion
- Growth of exocrine pancreas
Inhibits
- Gastric acid secretion
What is the action of gastric (inhibitory and peptide)
Stimulates
- Insulin release
Inhibits
- Gastric and secretion
What is the action of motilin?
Stimulates
- Gastric motility
- Intestinal motility
What are the functional movements of the GI tract?
- propulsive movements - peristalsis
- mixing movements
What is an increase in blood flow to the GI tract likely to be caused by?
- vasodilators released from GI mucosa (CCK<vip>
</vip><li>from GI glands - bradykinin</li><li>via decreased oxygen conc in gut wall due to increased metabolism</li><li>adenosine release</li>
</vip>
