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.