Overview of GI physiology Flashcards
Identify and define the main GI physiological processes.
1) movement of food through the alimentary tract (motility)
2) secretion of digestive juices and digestion of the food;
3) absorption of water, various electrolytes, vitamins, and digestive products;
4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances;
5) control of all these functions by local, nervous, and hormonal systems.
6) Excretion of waste products
Give examples of secretions produced in the GI tract, and the use of each.
Enzymes and fluids which aid in the liberation of nutrients from food, as well as mucus which facilitates transport of food down the alimentary tract
Describe the main role of the stomach, small intestine, and oesophagus.
Stomach: Temporary storage of food
Small intestine: Digestion and absorption
Oesophagus: Passage of food
Identify the main layers of the GI tract.
Mucosa (innermost)
Submucosa
Muscularis Externa
Serosa (outermost)
Describe the main components of the mucosa.
♦ Epithelium (single cell layer, forms lining of GI tract + forms vili and crypts which aid absorption)
♦ Lamina Propria (loose CT made up of elastic and collagen fibers + contains sensory nerve, blood vessels, lymph vessels, secretory glands)
♦ Muscularis mucosa (thin layer of smooth muscle)
How often is the epithelium of the mucosa of the GI tract shed and replaced ?
Every 2-3 days
What do the apical and basolateral sides of the mucosa of the GI tract face ?
Apical side faces GI lumen
Basolateral side faces interstitium and vasculature
What aspects contribute to increasing SA ?
Vili and crypts - Mucosa
Ridges and folds - muscularis mucosa
Describe the main components of the submucosa.
Thicker than mucosa, but similar composition to lamina propria of the mucosa.
Incorporates blood vessels and nerve bundles than form a submucosal plexus (Meissner plexus)
Describe the main components of the muscularis externa.
Circular muscle (inner) Myenteric plexus (Auerbach plexus) Longitudinal muscles (outer)
How is the enteric NS involved in the GI tract structure ?
- The neurons of the ENS are collected into two types of ganglia: myenteric (Auerbach’s) in the mucosa, and submucosal (Meissner’s) plexuses, in the submucosa.
- In the muscularis externa, ENS coordinates contractions to mix and move contents between compartments.
What is the function of circular muscle in the muscularis externa ?
Forms sphincters which regulate flow from one compartment to the next
Describe the main components of the serosa.
Outermost layer of CT + layer of squamous epithelial cells
Do all sections of the GI have a serosa ? Why or why not ?
No (e.g. the esophagus), but connect directly to the adventitia (i.e. CT that blends into pelvic or abdominal wall)
What is the diameter/length of the GI smooth muscle (longitudinal and circular muscle) ?
200-500 um long
2-10 um diameter
Describe the microscopic arrangement of GI smooth muscle. What is the implication of this arrangement ?
- Arranged in bundles with up to 1000 fibers in parallel
- Within bundles, fibers connected via gap junctions
- CT partly separates each bundle, but with many points where bundles fuse to form a branching network
All of this creates a syncytium in each muscle layer (i.e. when an AP is evoked anywhere, it typically travels in all directions)
Characterise the electrical activity of GI smooth muscle.
continuous/intermittent
slow/fast
intrinsic/extrinsic
- (Almost) continuous
- Slow
- Intrinsic
Identify the main kinds of waves of electrical activity in GI smooth muscles.
Slow waves
Spikes
Is the voltage of the resting membrane potential in GI smooth muscle constant or can it change to different levels ?
The voltage of the resting membrane potential in GI smooth muscle can change to different levels.
Define slow waves of electrical activity.
What is their voltage ? What is their frequency ?
They are slow undulating changes in resting membrane potential, they determine rhythmical GI contractions (not APs)
Voltage is 5 to 15 mV
Frequency ranges 3-12 per min
Draw slow waves and spikes.
Refer to “Overview of GI physiology” slide 13
Define spikes of electrical activity.
What is their voltage ? What is their frequency ?
They are true APs, occurring automatically when resting membrane potential > ~40 mV
Explain the control of electrical activity in phasic gastrointestinal muscles, explaining the link between slow waves and spikes ?
1) Pacemaker Interstitial cells of Cajal generate the slow waves (different rhythms depending on location in GI)
2) Slow waves conducted electronically,
3) NS (within varicosities) and hormones then condition the smooth muscle syncytium to generate the desired response
4) Smooth muscle cells respond to slow wave depolarisation with increased Ca+ channel open probability
Slow waves by themselves don’t usually cause muscle contraction (except in stomach) but they influence the
appearance of intermittent spike potentials
which cause muscle contraction
Higher increase in slow wave potential → greater frequency of spike potentials
Distinguish between the spike potential in GI muscle and the APs in nerve fibers.
Spike potentials in GI muscle lasts 10-40x as long as APs in large nerve fibers.
How long does each GI spike last ?
10 to 20 ms
What is the resting membrane potential in GI smooth muscle, in normal conditions ?
-55 mV
Identify factors which can reduce resting membrane potential. What is the impact of this on the excitability of the muscle ?
-Norepinephrine
-Sympathetics
Renders smooth muscle less excitable (hyperpolarises)
Identify factors which can reduce resting membrane potential. What is the impact of this on the excitability of the muscle ?
-Stretch
-Acetylcholine
-Parasympathetics
Renders smooth muscle more excitable (greater frequency of spike potentials)
Identify the nervous systems responsible for control of GI function.
♦ Enteric NS, which can work independently but also has inputs form PSNS and SNS.
Responsible for GI movements and secretions
♦ Two plexuses as part of enteric NS:
1) Myenteric (controls GI movement)
2) Submucosal (controls GI secretions and blood flow)
♦ PSNS and SNS each connected to both myenteric and submucosal plexus
♦ Sensory neurones also present, from GI epithelium to both plexuses, as well as to pre-vertebral ganglia/spinal cord/brainstem
Where is the enteric NS located ?
Enteric NS lies entirely in wall of gut, from oesophagus to anus
Identify the following for myenteric plexus.
- What does it control ?
- What are its main effects when stimulated ?
- Where found in the GI tract ?
- Additional info
-What does it control: muscle activity
- What are its main effects when stimulated ?
1) Increased tonic contraction/tone of gut wall
2) Increased intensity of the rhythmical contractions
3) Slightly increased rate of the rhythm of contraction
4) Increased velocity of conduction of excitatory waves along the gut wall, causing more rapid movement of the gut peristaltic waves
-Where found in the GI tract ?
Linear chain of interconnecting neurones extending entire length of GI tract
-Additional info: some neurones are inhibitory, secreting an inhibitors transmitter (VIP)
Explain the effect of inhibitors effects in the myenteric plexus.
Inhibit some of the intestinal sphincter muscles that impeded movement of food along successive segments of the GI tract (e.g; pyloric sphincter controlling emptying of the stomach into the duodenum + sphincter of the ileocecal valve, controlling emptying from small intestine into caecum
Identify the following for submucosal plexus.
-Where found in the GI tract ? /what does it control
- Where found in the GI tract: mainly controls function within small segments of intestine
e. g. sensory signals from the GI epithelium, integrated in the submucosal plexus, help control local intestinal secretion, absorption, and contraction of the submucosal muscle that causes infolding of the GI mucosa
Identify neurotransmitters secreted by enteric neurones and state whether they are excitatory or inhibitory.
ACh- often excitatory NA- inhibitory ATP Serotonin Dopamine Cholecystokinin
Describe autonomic control of the GI tract, including where the fibers originate.
PSNS:
- Increases activity of enteric NS
- Almost all fibres are in vagus nerve
SNS:
- Inhibit GI tract activity
- Almost all fibers originate between T5 and L2
Where are the cell bodies of the sensory nerve fibers of the GI ?
- Some have their cell bodies in the ENS
- Others have them in dorsal root ganglia of the spinal cord
Identify possible stimulations of the sensory nerves of the GI.
- Irritation of the gut mucosa
- Excessive distention of the gut
- Presence of specific chemical substances in the gut
What are possible results of the signals transmitted via the sensory fibers of the GI ?
Excitation or inhibition of intestinal movements/secretions
True or False: Some sensory signals from gut go to multiple areas of the spinal cord or the brain stem
True
What are the main types of GI reflexes ?
1) Reflexes integrated within the enteric NS
2) Reflexes from the GI tract to the pre-vertebral sympathetic ganglia and then back to the GI tract
3) Reflexes from the GI tract to the brainstem or spinal cord and then back to the GI tract
Give examples of reflexes integrated within the enteric NS.
Reflexes controlling secretion, peristalsis, mixing, local inhibitory actions
Give examples of reflexes from the GI tract to the pre-vertebral sympathetic ganglia and then back to the GI tract.
Reflexes that transmit signals to other areas of the GI tract:
- Gastrocolic reflexes (stomach-colon)
- Enterogastric reflexes (stomach and colon inhibiting gastric motility)
- Colonoileal reflexes (inhibition of ileal emptying)
Give examples of reflexes from the GI tract to the brainstem or spinal cord and then back to the GI tract.
1) Reflexes from stomach and duodenum to brainstem and back to stomach to control gastric motor and secretory activity
2) Pain reflexes causing general inhibition of entire GI tract
3) Defecation reflexes from the colon and rectum that travel via spinal cord back to produce powerful colonic, rectal, and abdominal contractions
Identify hormones involved with GI motility.
Gastrin Cholecystokinin Secretin Gastric inhibitory peptide Motilin
Identify the stimuli for secretion, site of secretion, and actions of Gastrin.
- Stimuli for secretion: Protein, distention, nerve, (acid inhibits release)
- Site of secretion: G cells of antrum, duodenum, jejunum
- Actions: Stimulates gastric acid secretion, mucosal growth
Identify the stimuli for secretion, site of secretion, and actions of Cholecystokinin.
- Stimuli for secretion: Protein, fat, acid
- Site of secretion: I cells of duodenum, jejenum, ileum
- Actions:
a) Stimulates pancreatic enzyme secretion, pancreatic bicarbonate secretion, gallbladder contraction, growth of exocrine pancreas.
b) Inhibits gastric emptying
Identify the stimuli for secretion, site of secretion, and actions of Secretin.
-Stimuli for secretion: Acid, fat
-Site of secretion: S cells of duodenum, jejunum, ileum
-Actions:
Stimulates pepsin secretion, pancreatic bicarbonate secretion, biliary bicarbonate secretion, growth of exocrine secretion
b) Inhibits gastric acid secretion
Identify the stimuli for secretion, site of secretion, and actions of Gastric inhibitory peptide.
- Stimuli for secretion: Protein, fat, carbs
- Site of secretion: K cells of the duodenum and jejunum
- Actions:
a) Stimulates insulin release
b) Inhibits gastric acid secretion
Identify the stimuli for secretion, site of secretion, and actions of Motilin.
- Stimuli for secretion: Fat, acid, nerve
- Site of secretion: M cells of the duodenum and jejunum
- Actions: Stimulates gastric motility, intestinal motility
Identify the main functional types of movements in the GI tract.
- Propulsive movements (i.e. peristalsis)
- Mixing movements
What is the usual stimulus for peristalsis ?
Distention of the gut, leading to wave (squeezing contents of tube forward) moving towards anus (probably due to polarised myenteric plexus which stops wave moving backward)
What are the requirement for an effectual peristalsis ?
Active myenteric plexus
Define mixing movements as one of the functional types of movement in the GI tract.
- Local intermittent constrictive contractions
- Every few cm in guy wall, 5-30 s duration
- Results in chopping and shearing of contents
- In some areas, peristalsis causes most mixing
Identify and describe the GI blood flow.
♦ Splanchnic circulation (provided by coeliac, super mesenteric and inferior mesenteric arteries), blood flow through gut + spleen + pancreas + liver
♦ a) Blood from gut, spleen and pancreas flows into liver via portal vein
b) There, blood passes through minute liver sinusoids then leaves via hepatic veins to the vena cava of the general circulation
What is the significance of the blood from the gut, spleen and pancreas passing through minute liver sinusoids before leaving via hepatic veins to the vena cava of the general circulation ?
This allows bacteria/particulate matter that might enter the blood from the GI tract to be removed, preventing direct transport of potential harmful agents into the remainder of the body
Identify the blood supply specific to the intestines.
Mesenteric web (superior and inferior mesenteric) supply walls of the small and large intestines.
Describe the blood supply to the vili.
Countercurrent arrangement of blood flow in the arterioles and venules in villi, meaning arterial and venous flow are in opposite directions, and the vessels are in close apposition with each other, meaning much of the blood O2 diffuses out of arterioles directly into adjacent venules without ever being carried in the blood to the tip of the villus.
What is the role of vili in the GI ?
Increases SA to increase absorption
Describe an important feature of the structure of the wall of arterioles and the implication this has on the role of arterioles.
Walls of arterioles very muscular so very active in terms of controlling blood flow to specific area (e.g. depending on if need for nutrients to be absorbed)
What factors affect specific area GI and gut wall layer blood flow ?
♦ Usually, blood flow to specific GI area and guy wall layer is directly related to the level of local activity.
♦ Increases in flow may be caused by:
- Vasodilators released from GI mucosa (e.g. CKK, VIP)
- Vasodilators released from glands (e.g. bradykinin)
- Via decreased O2 concentration in gut wall due to the increased metabolism
- Via subsequent adenosine release
Describe nervous control of blood glow in the GI.
PSNS stimulation of nerves to stomach and lower colon:
-Increase local blood flow (likely indirectly due to the effects of increased glandular secretion)
SNS stimulation:
-Intense vasoconstriction and decreased blood flow (direct effect on most GI tract)
Give an example of an instance where nervous reduction in GI blood flow is important.
When great increases in blood supply are needed elsewhere (e.g. during exercise)
