March 19 - Gastrointestinal Physiology Flashcards
What are the five functions of the GI system?
- Propulsion
- Secretion
- Digestion
- Absorption
- Barrier function
Describe propulsion
Movement from mouth to anus
Deglutition
Peristalsis
Mass movements (elimination)
Describe secretion
It is equally important (we would not be able to digest or absorb without secretion)
Mucous production for protection
Water
Enzymes
Describe digestion
Mechanical
Chemical
Describe absorption
Transcytosis
Describe the barrier function
We need to keep the bacteria of our stomach from getting into our body
What is MALT?
Mucosa associated lymphoid tissue
What is GALT?
Gastrointestinal associated lymphoid tissue
What is the importance of GALT?
There are more lymphocytes in the intestines than in the spleen; they determine that if anything should cross if it is either benign or if it requires the mounting of an immune response. Intestines are inflamed at all times; partly due to GALT
What are the organs of the alimentary canal?
Mouth (addition of saliva, beginning of digestion)
Esophagus (upper two thirds is skeletal muscle, the lower two thirds is smooth muscle; swallowing begins as voluntary and transitions to involuntary)
Stomach
Small intestine (proximal section is the duodenum (first 10-15%), the middle bulk section is the jejunum and the distal section (last 10%) is the ileum)
Caecum (appendix: has a little digestion activity and is important for immunity)
Large intestine (formation of feces)
Rectum
Anus
What are the accessory organs of the alimentary canal?
Liver (production of bile)
Gall bladder (storage of bile)
Pancreas (enzymatic role, buffering role)
What are the seven sphincters of the GI tract?
Upper esophageal Lower esophageal Pyloric Ileocecal Colorectal Internal anal External anal
What are the four tissue layers of the GI tract?
The mucosa
The submucosa
The muscularis
The serosa
What makes up the mucosa?
A single columnar epithelium (with the exception of the esophagus - it has a stratified squamous epithelium) Lamina propria (connective tissue) Muscularis - thin layer of smooth muscle (anatomical landmark - it separates the mucosa from the submucosa)
Describe the submucosa?
Complex tissue Lots of blood vessels Extensive lymphatic system Lots of fatty tissue Submucosal plexus
Describe the submucosal plexus
Nervous tissue - a neuron network that is highly organized but not a in a tract (like in the spinal cord); it exists more as a network around the entire structure and it controls the functions of the submucosa and mucosa
What makes up the muscularis?
Circular smooth muscle - when it constricts the diameter of the lumen decreases Myenteric plexus (similar to the submucosal plexus; it control the muscularis) Longitudinal smooth muscle - when it constricts, that segment of gut gets shorter
How is the gut controlled?
The gut has more neurons than the spinal cord. There are many chemoreceptors, baroreceptors, interneurons to integrate signals, motor neurons. We have whole reflex arcs that occur entirely within the gut, so the gut can control itself using these neurons (however it is still influenced by the CNS)
Describe the serosa
Basically just the visceral peritoneum
Describe the small intestine ultrastructure
The epithelium of the small intestine is folded over on itself. It has a lot of finger-like projections call villi, which increase surface area by about 20x. The apical membrane of each epithelial cell has bristles created by the extending of the cytoskeleton called microvilli. This further increase the surface area by about 50x. This is where transport proteins are found and there are lots of enzymes here.
Compare smooth muscle to skeletal muscle
There is no actin myosin striations There is more actin There is less mysoin It is organized very loosely There are no sarcomeres; dense bodies, attached together by intermediate filaments, organize the actin
What causes contraction of smooth muscle?
When stimulated (neuronal signal, hormonal signal, gap junction communication, etc.) a smooth muscle cell membrane’s voltage-gated ion channels open, calcium enters the cell (in smooth muscle the SR is underdeveloped, so the bulk of the Ca comes from the ECF). The concentration of calcium increases. Calcium binds to calmodulin creating the calcium-calmodulin complex. This complex activates the enzyme myosin light chain kinase (MLCK). MLCK phosphorylates myosin. The phosphorylated form of myosin is active and can bind to actin, which causes contraction
What causes relaxation of smooth muscle?
The smooth muscle actively pump out the calcium. The concentration of calcium decreases so the calcium-calmodulin complex dissociates. This causes the inactivation of MLCK. Also myosine phosphatase inhibits MLCK by dephosphorylation. All this ends cross bridging, which causes relaxation
Name three different GI movements
Segmentation
Peristaltic waves
Migrating motor complexes
Describe segmentation
Parts of the tissue contracts and the other parts relax (chopping motion). When the muscle contracts, the contents are forced away in opposite directions (homogenization of contents)
Describe peristaltic waves
Allows movement from one organ to the next.
Very short, weak waves (one wave will travel about 1 cm. However, hundreds of thousands of them cause efficient movement of contents down the GI tract. We don’t want to move things too fast; we want time for digestion and absorption
Describe migrating motor complexes
These are very strong, long waves that move like peristaltic waves. They go long distances (one wave can travel the stomach and intestines). They occur between meals. They keep things moving. They prevent infection. They rid the body of waste and accumulating toxins
What are gastric movements?
There are three layers smooth muscle specific to the stomach; when they contract, they twist. There are mixing or churning waves, which produce liquid chyme. Throughout the first wave, the pyloric sphincter is closed Peristaltic waves, a second, stronger wave, which increases pressure, which overcomes the tonicity of the pyloric sphincter and it will open up. The pyloric sphincter is under precise control. The amount of chyme that is released into the duodenum at one time is very small
Describe the control of the myenteric plexus
It is under local control via the enteric nervous system (a division of the ANS). In this plexus there are sensory, motor and interneurons. They have excitatory functions; they increase muscle tone (mainly in the sphincters). We automatically control the intensity and the frequency of these contraction, and thus we automatically control the speed of peristalsis. There are inhibitory functions as well, so when material needs to move past the sphincter, it needs to relax. In most cases we want to inhibit the back flow (exception: there’s a little backflow in the stomach and occasionally we need to vomit)
Describe the intrinsic control of GI function
Smooth muscle membrane potential fluctuates. When they are at rest, the values are below threshold. The reason is due largely to Na/K pumps. If the waves become depolarized and the peaks reach threshold, we get these spike potentials. This is stimulated by a number of functions (e.g., stretch when food is present). The greater the depolarization, the higher the frequency of spike potential and the stronger the contraction. The are also inhibitory functions; any stimulus that causes hyperpolarization
Describe the extrinsic control of GI function
The initiation and the development of motility is controlled internally by the ENS but there are external influences (extrinsic coordination). Acetylcholine and the parasympathetics promote digestion. Norepinephrine and the sypmathetics control fight, flight and reproduction so they will redirect blood away from the GI tract. Other controls include hormones, as well as cognitive and emotional control
Describe the cognitive control
Modulation of gastric functions (anger and stress)
Direct neurological pathways (acid production, motility, blood flow)
Describe GI reflexes
There is communication between the different parts of the GI tract (intragut). Various stimuli send signals (distention, chemicals, irritants). When your stomach is stretched, this will have an excitatory effect on the distal areas of the GI. Proximal to distal tend to be excitatory. In contrast, distal to proximal tend to be inhibitory (slow motility); if the colon is full, there is not room to accept new contents (distal to proximal will override proximal to distal signals; the material needs to be handled properly).
What happens if motility is increased too much?
This leads to diarrhea (increased migrating motor complexes). This can be a protective mechanism (if there is a parasite, etc.)
What happens if motility is decreased too much?
This leads to constipation. It allows more time to take the water out of feces, which makes it harder to move. Consequences include: distention, perforation, death. This can be caused by Chagas disease (caused by a parasite that destroys the myenteric plexus)
How does codeine lead to constipation?
Codeine binds to opioid receptors to alleviate pain and increase sympathetic activity (norepinephrine). In the myenteric plexus, codeine binds to u-Opioid receptors, which causes serotonin and norepinephrine release. Serotonin has antimotility effects, as does norepinephrine (via decreasing the release of ACh). In addition, norepinephrine is an antisecretagogue, so less water gets into the stools
What are prokinetic agents?
Used to treat constipation. They stimulate the secretion of ACh at the gut. Serotonin agonists turn off inhibitory signals and turn on excitatory signals. Dopamine-R antagonists turn off inhibitory signals
What is retropulsion?
Aka vomiting. It is a very special type of GI movement. The emetic centre controls vomiting. The process is incredibly complex. Peristalsis needs to change direction, the epiglottis needs to be closed, contraction of stomach muscles need to be controlled and everything needs to be done in the right order at the right time. There are many paths that lead to the emetic centre. Vomiting is a good thing in small doses (to get rid of toxins). Too much can cause the body to lose to many hydrogen ions (alkalosis)
How is excess vomiting treated?
With antiemetics. They are receptor antagonists (serotonin-R, histamine-R, muscarinic-R, dopamine-R)
How much is secreted throughout the GI tract daily?
8.5L
What are the various functions of fluid secretions of the GI tract?
Liquefaction (converting solid or semi solid food into liquid chyme)
Lubrication (mucous to help keep things moving smoothly so that we don’t damage the epithelium)
Digestion (enzymes)
What controls secretion?
Local control by the submucosal plexus. Secretion is typically due to the presence of luminal contents (biggest stimulus)
Describe the secretion of saliva
Multiple glands secrete 1.5 L of saliva daily. Its functions include: moistening, lubrication (mucin), defence (lysozyme, which breaks down the cell walls of bacteria, IgA)
What controls salivation?
It is controlled locally or centrally. Stimulations include: tactile, parasympathetic, cognitive. The autonomic nervous system have a big effect over salivation
What cells are responsible for gastric secretions?
Gastric pits, which lead to gastric glands. Here the epithelial cells include surface mucous cells, mucous neck cells, parietal cells, chief cells and endocrine cells (Enterochromaffin-like cells, G cells and D cells)
