PHYS Flashcards
major functional processes of the GI system:
motility, secretion, digestion, absorption and excretion which are initiated by the ingestion of food
where are absorbed nutrients circulated before entering the systemic circulation?
the liver via the portal venous circulation
what are sphincters? (list the 5)
one-way valves containing rings of circular muscles that maintain positive resting pressures to prevent backflow and are relaxed by inhibitory motor neurons
- UES: highest resting pressure to prevent entrance of air and is made of striated muscle
- LES: made of a special type of smooth muscle but incompetency leads to heartburn
- pyloric: incompetency leads to acid reflux that causes gastritis :( , ulcers or perforation
- ileocecal: incompetency leads to IBS due to bacterial overgrown in the SI
- anal: controls elimination of waste products
ingested and secreted fluids vs. absorbed fluids
ingested/secreted (10L): diet (2L), saliva, gastric juice (from parietal (oxyntic) cells in the oxyntic glands of the stomach), pancreatic juice and bile, SI
absorbed fluids: colon, SI
submucosal plexus/Meissner’s plexus
part of the enteric nervous system located between the circular muscle and submucosal layers and is involved with controlling secretions, absorption and contraction of the submucosal muscle affecting the local infolding of the small and large intestines
myenteric plexus/Auerbach’s plexus
part of the enteric nervous system located between the longitudinal muscle and circular muscle layers involved with tonic contraction, increasing the velocity of contraction and therefore enhancing peristalsis
*stimulated by mechanoreceptors
purpose of HCO3-
secreted into the lumen of the duodenum by the exocrine pancreas the neutralize the acidic chime delivered from the stomach
which neurotransmitter is found primarily in the bowels?
serotonin (95%)
*more than 30 neurotransmitters are used by the enteric nervous system
intrinsic and extrinsic connections in the ENS
sensory afferent neurons monitor luminal activity changes and activate interneurons which relay signals that activate efferent secretomotor neurons stimulating or inhibiting effector cells all of which is modified by the ANS via the vagus nerve
osmoreceptors
can control the amount of chyme entering the SI and the amount of secretions needed to buffer it by detecting the osmolarity of the chyme
SNS and PNS regulation of the ENS
SNS: postganglionic fibers inhibit digestion and absorption through the use of norepi released by postsynaptic neurons
PNS: vagus and pelvic nerves work to increase motility and secretions through the use of ACh or peptides released by postganglionic fibers to stimulate APs in electrical slow waves (ex: substance P, VIP)
how are the facial (VII) and glossopharyngeal (IX) nerves involved in regulation of GI function?
they initiate salivation during the cephalic phage of salivation upon seeing, smelling and/or tasting food
interstitial cells of Cajal (ICC)
pacemakers in the myenteric plexus that connect the GI musculature with gap junctions in between and generate slow waves (changes in the resting potential that has the greatest frequency in the small intestines, intermediate in the colon and slowest in the stomach) and action potentials (above -40mV) that will generate contraction of the GI muscle wall through Ca2+ entry into L-type VSCCs
what will stimulate an AP/depolarization in electrical slow waves and what will stimulate hyperpolarization?
stimulates depolarization: stretch, ACh, parasympathetics (from neural and hormonal input)
stimulates hyperpolarization: norepi, sympathetics
different type of contractions
segmental/mixing/non-propulsive (approx. 2-3 per min.): contractions in the SI elicited by stretching
peristaltic (approx. 1 cm/min.): contractions enhanced by stretch and the gastroenteric reflex
3 reflexes of the GI tract
- gastroenteric: responds to stretch by enhancing peristaltic contractions
- gastroileal: triggers opening of ileocecal valve to permit chyme passage through relaxation of the sphincter and contraction of the ascending colon when they are distended
- enterogastric: senses an acidic pH in the duodenum releasing gastrin from G-cells to decreases gastric motility and secretions while contracting the pyloric sphincter to inhibit chyme from entering duodenum
what is the point of peristalsis and segmentation?
mixing optimizes contact between ingested food and digestive secretions and circulation of intestinal contents facilitates contact with mucosa (this motility is under local control of the myenteric plexus)
peristaltic rushes
occur when the intestines are irritated causing rapid movement of chyme through the intestines which could be due to infectious agents leading to diarrhea (movement is too quick for proper absorption)
peristaltic propulsion/myenteric reflex
distension/low pH will send signal to the sensory neurons which activate interneurons to stimulate motor neurons to release ACh and NO/VIP
ACh–> will contract the circular muscle while the longitudinal muscle is relaxed (only has excitatory motor neurons) creating a propulsive segment which propels the bolus
NO/VIP–> will contract the longitudinal muscle while inhibiting the circular muscle creating a receiving segment for the bolus (overall relaxation of segment- promotes digestion)
physiological and pathological ileus
physiological ileus: normal state of no movement due to inhibitory neurons
pathological ileus: takes longer for things to move through and could be due to abdominal surgery, anticholinergic or opiate drug treatment
Migrating Motor Complex (MMC)
characterized by three phases modulated by the vagus nerve: quinescence, little activity and strong activity in order to sweep the stomach and small intestines of residue that could build up and cause the production of “Bezoars” obstructing the lumen with the help of Motilin which is synthesized in the duodenal Mo cells
deglutition
food in mouth stimulates swallowing reflexes causing the pharyngeal phase (once sensory neurons project to medulla through vagus and glossopharyngeal nerves and send back efferent impulses) and primary peristaltic wave (second peristaltic wave activated by esophageal distension and will remove any remaining food in the esophagus starting at the point of distension) along with receptive relaxation of the stomach
accommodation
relaxation of the stomach to allow for more food storage without increasing intragastric pressure (mediated by the vagovagal reflex) since the fundus (top portion) is relaxed as well
*gastric emptying is slower after ingestion of a high-fat meal and rapid after ingestion of liquid saline
*achalasia and GERD
achalasia: failure of the LES to relax during swallowing (usually accomplished by NO or VIP) possibly due to damage to the myenteric plexus which cannot transmit the signal for receptive relaxation
GERD: LES tone is not properly maintained (supposed to be high at rest)