Intro Flashcards
what occurs in GI blood vessels after meal ingestion?
there is increased bloodflow via local release of vasodilators
where do nutrients go after they are absorbed in the GI tract? Why?
to the liver to be processed, filtered and detoxified
how many Cal does a sedentary adult human need per day?
30 Cal/kg/day
what are the five areas with sphincters and which one has two?
upper esophagus, lower esophagus, pylorus, ileocecal junction and the anus (has internal and external)
what type of pressure is maintained by the sphincters?
positive resting pressure
activation of what neurons cause sphincter relaxation?
inhibitory motor neurons
which sphincter maintains the highest resting pressure?
the upper esophageal sphincter
which sphincters are made up of skeletal muscle?
smooth muscle?
the upper esophageal and external anal sphincters
lower esophageal, pyloric, ileocecal and internal anal
what controls the upper esophageal sphincter?
the swallowing center in the medulla
relaxes during swallowing
when does the lower esophageal sphincter relax?
during swallowing
what is the purpose of the lower esophageal sphincter? what occurs when it goes wrong?
allows food entry into stomach and prevents reflux
heartburn/acid indigestion
what occurs with reflux of the pyloric sphincter?
reflux of bile acids and digestive enzymes–gastritis, ulcer formation and perforation risk
what is a risk of patent ileocecal sphincter?
bacterial overgrowth in the small intestine
what is the ratio of ingested fluid to fluid added to ingested material? how much of this fluid is excreted/day?
2:8 L
100-200 mL excreted
where are most of the GI fluids absorbed?
in the small intestine
which cells in the stomach acidify contents? what pH does it reach?
parietal cells
reaches pH 1-2
what neutralizes stomach contents and what secretes it?
bicarbonate
secreted by the pancreas into the duodenum
what are the two nerve plexi in the enteric nervous system?
the myenteric (between longitudinal and circular muscle layers) and submucosal (between submucosa and circular muscle layer)
what is the myenteric plexus responsible for?
increases tone of the gut, intensity of contractions and enhances peristalsis
what is the submucosal plexus responsible for?
controls intestinal secretions, absorption and contraction of the muscularis mucosae (affects local infolding of mucosa)
what is contained in the submucosa of the GI tract?
collagen, elastin, glands and blood vessels
how does the ENS function independently?
what does this require?
it responds to input from mechanoreceptors, chemoreceptors and osmoreceptors even without extrinsic innervation
need intrinsic network of receptors, interneurons and effector neurons
what modulates the ENS?
the sympathetic and parasympathetic nervous system
what do the efferent neurons of the ENS control?
blood vessels, smooth muscle cells, epithelial cells and enteric endocrine cells
where else does the ENS sensory system relay to? what reflexes does this trigger
the CNS via the vagus nerve
triggers vasovagal reflexes to coordinate GI function
what do mechanoreceptors stimulate?
signal is transduced to the myenteric plexus and it stimulates contractions
what do chemoreceptors influence?
gut motility and secretion of buffers
why are osmoreceptors in the GI tract important? what do they control?
because a hyperosmotic chyme will pull fluid from enterocytes
control amt of chyme entering small intestine and amount of secretions needed to buffer the chyme
what autonomic nerves contribute to the ENS plexi? what does their activation cause?
vagus and pelvic PNS neurons
activation increases motility, sphincter relaxation and enhanced secretions
In the GI system, where do the vagus and pelvic nerves innervate?
vagus innervates proximal 2/3
pelvic nerves distal 1/3
what do the pre and postganglionic neurons of the PNS release in the gut?
pre- ACh
post- ACh and some peptides
where do neurons from the SNS that innervate the gut come from?
the celiac plexus, hypogastric and superior and inferior mesenteric ganglia
what is the major transmitter released by postsynaptic SNS neurons and what do they do?
norepinephrine
inhibit excitatory cholinergic neurons presynaptically
what is the overall effect of the SNS on the GI tract?
inhibits digestion and absorption
relaxes gut wall, reduces secretions, contracts sphincters and diverts blood flow
what nerves initiate salivation?
gastric acid secretion?
pancreatic secretion?
CN VII and IX
vagus and CN X
vagus
which nerves stimulate primary peristalsis?
relaxation of sphincter of oddi
relaxation of stomach and duodenum to accommodate?
swallowing center of medulla and vagus
vagus
vagus
stimulating bile synthesis
stimulating intestinal motility
vagus
vagus to upper colon and pelvic nerves to lower colon
what are the pace makers in the GI tract? what do they do?
interstitial cells of cajal
generate electrical slow waves (5-15 mV in intensity)
what causes contraction of GI muscle?
when action potentials are generated at the peaks of the slow pace maker waves
what are spike potentials?
AP when there is a further depolarization on top of the electrical slwo waves- trigger phasic contractions
what are spike potentials mediated by?
Ca influx through L type channels (prolong AP)
what are tonic contractions?
maintain constant gut tone without periods of relaxation
how does the ENS control the distance and direction of muscle contraction?
uses both excitatory (ACh and substance P) neurons and inhibitory (VIP) neurons
how do hormones, paracrine factors and the ANS influence gut contraction?
by changing the slow wave motor patterns
how does slow wave frequency change in the GI tract?
small intestine > colon > stomach
how does contraction frequency relate to the slow waves?
slow wave frequency is maximum contractile frequency but it can be less if not all slow waves coordinate with AP
describe segmental contractions.
non propulsive contractions that mix chyme (at about 12 contractions per minute)
what do peristaltic contractions do? what intensifies them?
move material from mouth to colon at 1 cm/min
intensified by duodenal stretch and gastroenteric reflex
what does the gastroileal reflex do?
triggers opening of the ileocecal valve by ileal distension
closing of the spoincter by distension of the ascending colon
describe the enterogastric reflex
decreases gastric motilityand secretion and contracts pyloric sphincter- inhibits chyme from entering duodenum
what does irritation of the intestinal mucosa do?
causes peristaltic rush–diarrhea
what is another name for the myenteric reflex? describe it.
peristalsis contractile ring (circular muscle contraction) on oral side of distended segment and relaxation downstream (longitudinal contraction)
what causes the opposing muscle contraction in paristalsis?
excitatory motor neurons in circular portion propelling the bolus and inhibitory motor neurons in front of the bolus
what is a physiological ileus?
absence of motility in small and large intestines
normal state with inhibition active to suppress activity
describe a pathological ileus
when normal periods of inactivity are much longer
passage of stool and gas are impaired
common with surgery and anticholinergic or opioid drugs
what is the migrating motor complex?
sweep the stomach and small intestine of residue during fasting
pyloric sphincter is relaxed
when does the MMC occur? where?
3 hours after a meal at cyclic intervals of 90 min
sweeps from distal 1/3 of stomach to terminal ileum
what are bezoars?
indigestible material that accumulates without MMC
what causes MCCs?
secretion of Motilin from the duodenum
vagus also plays a role
what is deglutition?
swallowing
what are the voluntary stages of swallowing?
shaping food into bolus, collection on tongue and raising tongue against hard palate to push the bolus into the pharynx
what afferents activate the involuntary events of swallowing?
vagus and glossopharyngeal nerves> swallowing center in medulla
what is the pharyngeal phase of swallowing?
pulling the soft palate upwards and movement of epiglottis and pharynx while relaxing the upper esophageal sphincter
what is the primary peristaltic wave? when does it occur?
it propels food through the upper esophageal sphincter
during the esophageal phase of swallowing
what happens if there is still food in the esophagus after the primary peristaltic wave?
a second wave results from the esophageal distension
which part of the esophagus is smooth muscle and which skeletal?
upper 1/3 is skeletal and lower 2/3 is smooth
what dauses disorders of swallowing?
CN V, IX or X damage
diseases of the swallowing center
paralysis of swallowing muscles-deep anesthesia
what occurs in the stomach upon swallowing?
decrease in pressure with receptive relaxation (VIP) by vasovagal reflex
what is accommodation?
gradual active relaxation of stomach as food accumulates mediated by a vasovagal reflex
how is gastric emptying regulated?
chemoreceptors and mechanoreceptors in the duodenum sense the pH, calorie content, lipid and AA content of the chyme (also osmolarity)
presence of any of these slows emptying
what signals cause modification of gastric emptying?
vagal nerve, secretin, CCK and GIP released from duodenum
what is achalasia and GERD?
achalasia- failure of LES to relax during swallowing
GERD- LES tone is low and there is reflux
what causes achalasia?
damage to myenteric plexus of lower 2/3 or esophagus
describe the two components of the stomach.
the proximal gastric reservoir (fundus and 1/3 of body) and distal antral pump (caudal 2/3 of body, antrum and pylorus
describe activity in the gastric reservoir
maintain continuous contractile tone- not phasic contractions. accommodate for meals
describe activity in the antral pump
phasic contraction to propel chyme and trap it in the antrum where it is ground into smaller particles
retropulsion returns gastric contents to the body of the stomach
