Overview of GI Physiology Flashcards
What does the GI tract require in order to successfully function?
- Movement of food
- Secretion of digestive juices
- Absorption of food/electrolytes/vitamins and digestive products
- Circulation of blood through the GI organ to carry away the absorbed substances
Where in the alimentary tract is food passed through?
The oesophagus
Where is food temporarily stored in the alimentary tract?
The stomach
Where does digestion and absorption take place in the alimentary tract?
The small intestine
Name the layers of the GI tract
- Mucosa
- Submucosa
- Muscularis externa
- Serosa (oesophagus does not have this)
Name the layers of the mucosa
- Epithelium
- Lamina propria
- Muscularis mucosa
How often is the epithelium shed away?
2-3 days
What is the lamina propria?
- Loose connective tissue made up of elastin and collagen fibres
- Contain sensory nerves, blood and lymph vessels, secretory glands
What is the muscularis mucosa?
- Thin layer of smooth muscle
- Further increases surface area by creating ridges and folds
What is the submucosa?
- Thicker layer with similar compisition to lamina propria
- Incorporates blood vessels and nerve bundles that form a submucosal plexus - integral part of enteric nervous system
What is the muscularis externa?
- Circular muscle + myenteric plexis + longitudinal muscle
- ENS co-ordinates contractions to mix and move contents between compartments
- Sphincters regulate flow from compartment to next
What is the serosa?
- Outermost layer of CT + layer of squamous epithelial cells
How long and wide are smooth muscle fibres?
- 200-500um long
- 2-10um in diameter
- In bundles of up to 1000 parallel fibres
How do the smooth muscle fibres connect to each other?
- Via gap junctions (they form a syncitium)
- Connective tissue partly seperates each bundle, but with many points where bundles fuse forming a branching network
What does a syncitium allow for?
When an AP is evoked anywhere it typically goes in all direction
What is GI smooth muscle generally excited by?
Slow, intrinsic electrical activity
What are the 2 basic types of electrical waves?
- Slow waves; 2 spikes
What does the intensity of GI muscle smooth waves vary between?
5 - 15 mV
What does the frequency of the GI smooth muscle vaary between?
3 - 12 per min
What are considered true APs?
Spike potentials
When do spike potentials occur naturally?
> ~-40mV
How long does each GI spike last for?
- 10 - 20 ms
- 10-40 X as long in GI muscle as APs in large nerve fibres
What is the main ion responsible for APs in GI SM?
Ca2+
What can cause hyperpolarisaion?
- Norepinephrine
- Sympathetics
What can cuase depolarisation?
- Stretch
- Acetylcholine
- Parasympathetics
Where does the Enteric nervous system lie?
In the wall of the gut, in between the longitudinal and circular muscle layers
What is the enteric plexus?
- Mostly linear chain of interconnecting neurons extending the entire length of GI tract
- Mainly controls muscle activity
What are the effects of the myenteric plexus when stimulated?
- Increased tonic contraction/”tone” of gut wall
- Increased intensity of the rhythmical contractions
- Slightly increased rate of the 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 effects of the submucosal plexus?
- Mainly controls function within small segments of intestine
- Affects secretory and blood flow type activity
What is the role of Vasoactive Intestinal Peptide (VIP)?
- Inhibits sphincter muscles at the ends of different segements
What nerve are almost all of the parasympathetic neurons that affect the GI tract from?
Vagus nerve
Where do almost all sympathetic fibres that affect the GI system originate from?
T5 - L2
What can sensory nerves of the GI tract by stimulated by?
- Irritation of gut mucosa
- Excessive distention of the gut
- The presence of specific chemical substances in the gut
What are the 3 types of GI reflexes?
- Reflexes integrated within the enteric nervous system
- Reflexes from the GI tract to prevertebral sympathetic ganglia
- Reflexes from the GI tract to the brain stem or spinal cord and then back to the GI tract
What are reflexes integrated within the enteric nervous system?
Reflexes controlling secretion, peristalisis, mixing, local inhibitory actions
What are reflexes from the GI tract to prevertebral sympathetic ganglia and then back to 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)
What are reflexes from the GI taract to the brain stem or spinal cord and then back to 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 stimuates gastrin secretion?
- Protein
- Distention
- Nerve
(Acid inhibits release)
What are the 2 types of functional movements in the GI tract?
- Propulsive movements
- Mixing movements
What is the hypothesised reson for how waves of peristalsis move towards the anus?
Due to “polarised” myenteric plexus
How long do local intermittent constrictive contractions last?
5 - 30 s (every few cm in gut wall)
What vein carries blood from the gut, spleen and pancreas to the liver?
The portal vein
What does blood pass through in the liver?
Minute liver sinusoids
What does blood being passed through the liver allow for?
Allows bacteria/particulate matter that might enter the blood from the GI tract to be removed, preventing direct transport of potentially harmful agents into the remainder of the body
What is the blood supply to the intestines called?
The mesenteric web
What are some of the arteries in the mesenteric web?
- Middle colic
- Right colic
- Ileocolic
- Superior mesenteric
- Jejunal
- Ileal
- Inferior mesenteric
What is an increase in GI blood flow likely caused by?
- Vasodilators released from GI mucosa e.g CCK, VIP
- From glands e.g bradykinin
- Via decreased O2 concentration in gut wall due to increased metabolism
- Via subsequent adenosine release
