(Transcribed from Dr. Nardell lecture, 15-17 May 2000 by Brian Buschman)
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The role of the GI tract is to:
1) Move food
2) Secrete digestive juices
3) Absorb nutrients
4) Excrete indigestible stuff
Past of the control of the GI tract is by hormonal regulation such as with colecystokinin (CCK). CCK is a hormone which is stimulated by the presence of fatty acids and causes the contraction of the gal bladed and relaxation of the sphincter or Oddi to allow the release of pancreatic enzymes into the duodenum.
The other major source of GI control is of the nervous system. The GI tract is innervated by the parasympathetic NS, which is usually stimulatory in the GI, and the sympathetic, which is usually inhibitatory in the GI. The sympathetics come from the thoracic and lumbar splanchnic nerves and the parasympathetics from the vagus nerve and the pelvic splanchnics.
The parasympathetic fibers synapse in plexuses close to the organs. One such location is in the submucosal plexus which controls secretion and local activities. The other location is the myenteric plexus which controls peristalsis. ACh is the NT for parasympathetic stimulation and stimulate GI function. The usual sympathetic NT is NE but Epi often functions as a hormone to inhibit GI action.
Histamine is a NT in the gut that causes parietal cells to secrete HCl It can be blocked by sematadine (tagament) which is an H2 receptor antagonist. Somatostatin is an inhibitory hormone/NT used in the brain and pancreases. It blocks pancreatic secretion and pituitary secretion.
In the gut the sympathetic nerves run all over the place while the parasympathetics seem to be more ordered. This accounts for the presence of sympathetic pharmacological interactions all over the GI tract.
There are two types of smooth muscle control in the stomach:
1) Phasic control is when the muscle contracts in sinusoidal phases. This is the contraction that is characteristic of peristalsis.
2) Tonic contraction is when the muscle is either completely contracted or completely relaxed. This is how the sphincters of the gut operate.
The smooth muscle cells function together as a syncisium because they are well connected by gap junctions.
The peristaltic motility of the gut is stimulated by slow waves of electrical activity. The activity slowly depolarizes to a level around threshold. When it reaches threshold it stimulates a spontaneous AP to fire and causing a peristaltic wave. This mechanism will cause one wave after another to come. If the slow wave has a greater depolarization it will allow stimulation during the relative refractory period and cause a higher frequency for the peristaltic waves.
Chewing obviously mixes food, lubricates it, decreases the particle size and inserts oral digestive enzymes such as a-amylase.
Swallowing is a process that is broken into three phases:
1) The oral phase is where it is voluntarily pushed backwards by the tongue until it triggers the reflexes to start the next phase.
2) Pharyngeal phase is when:
a. The soft palate rises to keep food out of the nasopharynx.
b. The vocal folds adduct and epiglottis lowers to keep food out of the trachea.
c. Upper esophageal sphincter (UES) relaxes.
d. A peristaltic wave moves food down into the esophagus.
3) Esophageal phase is when the swallowing reflex sends a peristaltic wave down the esophagus to more the food to and open the lower esophageal sphincter (LES). In an upright position water can actually move ahead of the wave and therefore must wait for the wave to come and open the LES to allow it into the stomach. If food gets caught up the esophageal distention will cause another peristaltic wave to come along to move it.
The esophagus must open the UES to allow food in, close it to keep air out, move the food, open the LES to pass the food ad close it to prevent gastric reflux.
Gastric motility can be divided into three main functions:
1) Relax the orad stomach (fundus and superior part of the body) to receive the food.
2) Contract to mix the food with gastric enzymes.
3) Contract to propel the food into the duodenum.
The stomach receives autonomic innervation from the vagus (parasympathetic) and greater splachnic nerve (T5-T9) via the celiac ganglia (sympathetic). It effects the movement of the three muscle layers. (Inner oblique, middle circular and outer longitudinal.)
Distension of the low esophagus causes two reflex activities:
1) Opening of the LES.
2) Relaxation of the orad stomach.
The reflex is a vasovagal reflex meaning that both the afferent and efferent limbs are carried by the vagus.
Peristaltic waves begin in the middle of the body and move distally. It mixed and makes pieces smaller. It may even propel some through the pyloric valve but in general this reflex induces forceful closure of the pyloric valve. Retropulsion is the forcing of food in a retrograde direction when it is pushed against the closed valve.
During fasting there is a wave that occurs about once every 90 minutes, called the myoelectric complex, who’s job is to help clear residual chyme from the stomach. This wave begins in the stomach and continues throughout the small intestine. Slow waves occur in the stomach at about 3-5 per minute.
The stomach will want to empty when either there is an elevated level of gastrin or when the stomach is really full. Foods will be released from a full stomach across a three hour period. Liquids, especially isotonic solutions, go first and solids are held longer. It must be held long enough to break the food down to 1mm3 particles and to make the mixture isotonic with the duodenum.
A few situations require the slow release f food. It must go slow enough to allow both the Bruner glands of the duodenum to neutralize the acidity of the chyme from the stomach. Slow is also important so the intestines have time to absorb all the good stuff in the food. The presence of both fat and CCK will slow the process of release from the stomach. Fates take extra time to break down and CCK is a sign of lots of fat.
Like in the stomach the motility of the small intestine is controlled by the slow wave cycle. It also has the migrating myoelectric complex every 90 minutes or so to help clear residual chyme. The stomach receives parasympathetic innervation from the vagus nerve and the greater (T5-T9) and lesser (T10-T11) splanchnic nerves via the celiac and superior mesenteric ganglia.
The motility of the small intestine can be divided into two types:
1) Segmentation contractions which divide a bolus of food and then allow it to migrate back together. This helps in mixing and absorption.
2) Peristaltic contractions which begin orad to the chyme and move caudally. This will unction to move the bolus.
The flow of food through the small intestine is mainly by five reflexes:
1) Sympathetic control usually regulates the pace of movement by slowing down the action.
2) Gastroileal reflex increases ileal activity in response to gastric activity.
3) Ileogastric reflex ileal distension decreases gastric activity.
4) Interstitialintestinal reflex relaxes one part of the intestine in response to distention of another part.
5) The ileocecal valve control is related to distention. Ileal distention opens the sphincter while colic distention closes it.
The colon has two major types of movement:
1) Segmentation contractions move the feces between the individual haustra for optimum water absorption.
2) Mass movements occur one to three times a day to move feces a major distance, such as from the transverse to descending colon.
Remember that the innervation is different in the colon:
1) Sympathetic innervation is from both the lesser (T10-T11) and lease (T12) splanchnic nerves and joined by the lumbar splanchnics.
2) The parasympathetic innervation is from the vagus nerve up to the left colic flexure but past that point it comes from pelvic splanchnic nerves.
When the rectum becomes partly filled the rectosphincteric reflex causes relaxation of the internal anal sphincter. The reflex requires the spinal cord and pelvic nerves. When the rectum is bout ¼ full a person will have the urge to defecate. Since the external sphincter is striated muscle it is under voluntary control a person will not go until they chose to relax the external sphincter (so long as they don’t wait too long).
The gastrocolic reflex is the increase of mass movements in response to increased gastric activity. (i.e. the way you have to poop after a good meal.)
Vomiting is caused by two methods, the first being reflex behavior controlled by the medulla. The other by stimulation of receptors in the stomach and duodenum by bad things that it needs to get out of there.
The entry of vomit into the trachea is prevented by the adduction of vocal folds and such as we saw in the reflex stage of the swallowing reflex.
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