Flashcards in Unit 6 - Introduction to Gastrointestinal Physiology Deck (85)
what are the major functional processes of the GI system? how are they initiated?
-motility - movement of food through the system
-secretion - coordinated delivery of appropriate fluids and enzymes
-digestion - hydrolysis of macromolecules in food
-absorption - transport of molecules into circulation
-excretion - elimination of waste products
they are initiated by ingestion of food (sense, smell, sight) through integrated series of endocrine, paracrine, and neural responses
where do absorbed nutrients go before entering systemic circulation?
initially circulate through liver (via portal venous circulation) to be processed, filtered, and detoxified before entering systemic for general distribution to tissues
how many kcal/kg BW do sedentary humans need?
how do GIT sphincters work?
respond to different stimuli and have different resting pressures
-one-way valves with rings of circular muscle, functioning as barriers to flow to maintain positive resting pressures
-activation of inhibitory motor neurons (like VIP) leads to transient relaxation and forward passage of material
upper esophageal sphincter
maintains highest resting pressure of all sphincters, preventing air from entering esophagus
-made of striated muscle, and under control of swallowing center in medulla, relaxing during swallowing to allow food to enter
lower esophageal sphincter
-what happens if it's faulty?
separates esophagus from stomach
-made of smooth muscle that relaxes during swallowing
-coordinates passage of food into stomach after swallowing/deglutition
-prevents reflux of gastric contents (acid) into esophagus
-if incompetent, causes heartburn, acid indigestion
-what happens if it's faulty?
separates stomach from duodenum
-its resting pressure contributes to regulation of gastric emptying to prevent duodenal-gastric reflux
-reflux of bile acids and digestive enzymes can lead to gastritis, ulcers, and perforation
separates ileum and cecum, preventing back flux of colonic contents into ileum
what do some patients with irritable bowel syndrome have?
bloating and pain due to bacterial overgrowth in SI near ileocecal sphincter
what are the internal and external anal sphincters
internal = smooth muscle
external = skeletal muscle
control elimination of waste products
what is necessary to promote digestion, absorption, and detoxification of ingested materials?
substantial fluid shifts and pH changes
what is the comparison between ingested/secreted fluids and absorbed fluids?
both equal each other
-ingested: 2 L, plus 8 L from GIT (SI, pancreas, stomach, saliva for buffers, acid, and enzymes)
-absorbed: back into blood (mostly SI, also colon)
-total excreted in feces = 100-200 mL of fluid
where are and what do parietal (oxyntic) cells do?
found in oxyntic glands of stomach
-acidify contents of stomach to promote digestion and breakdown of ingested bacteria and Ag
pH of stomach VS blood, and how it can be neutralized
highly acidic chyme around 1-2, so [H+] is 3 million times that of arterial blood
-HCO3- secretion from pancreas to duodenum neutralizes acid as it goes into duodenum
what is the enteric nervous system and what is it made of?
"minibrain" branch of ANS with ~100,000,000 neurons (similar to spinal cord)
-primary nerve nets (plexi) in ENS are myenteric (between longitudinal and circular muscle layers) and submucosal (between circular muscle and submjucosal layers)
what is the myenteric plexus?
Auerbach's; between longitudinal and circular muscle layers of GIT, extending from proximal end of esophagus to rectum
-stimulation increases tone (tonic contraction), intensity of rhythmic/phasic contractions, and velocity of conduction of excitatory waves to enhance peristalsis (IOW: MOTILITY)
what is the submucosal plexus?
Meissner's; between circular muscle and submucosa in intestines only
-controls local intestinal secretions, absorption, and contraction of submucosal muscle, affecting local infolding of GI mucosa
layers of GIT, from mucosa to serosa
epithelium - lamina propria - muscularis mucosa - submucosa - submucosal plexus - circular muscle - myenteric plexus - longitudinal muscle - serosa
how does the ENS respond to input from local environment? how is it modulated?
mechanoreceptors, chemoreceptors, and osmoreceptors in epithelial lumen
-can be done in absence of extrinsic innervation, via intrinsic ENS neural network
--sensory neurons, interneurons, and motor neurons intrinsic to ENS are connected synaptically via varicosities for efficient bidirectional flow of info
-activity can be modulated extrinsically by both sympathetic and parasympathetic input from ANS
what can musculomotor and secretomotor neurons alter?
smooth muscle activity, secretion, and absorption of fluid or electrolytes by epithelium, activity of endocrine cells, submucosal blood vessels
what do mechanoreceptors sense?
stretch of smooth muscle; generated signal is transduced through myenteric plexus to stimulate contractions
what do chemoreceptors sense?
chemical composition of chyme and regulate motility and secretion of buffers to control luminal pH during influx of acidic chyme into duodenum
what do osmoreceptors sense? why is this very important? so what does this mean they control?
osmolarity of chyme in small intestine
-important b/c one-cell barrier between chyme (in lumen) and capillaries
--hypertonic chyme exerts osmotic force, pulling fluid out of cells and plasma
-thus they control amount of chyme entering SI and amount of secretions needed to buffer chyme
where do parasympathetic preganglionic fibers (from vagus and pelvic nerves) terminate?
postganglionic cholinergic or peptidergic neurons located in plexi
-PNS activation leads to increased motility in wall of gut, relaxation of sphincters, and enhanced secretions
what are vasovagal reflexes?
long reflexes in which both afferent and efferent impulses are carried by neurons in "mixed" vagus nerve
-prominent in coordinating GI function
how do afferent, interneurons and efferent neurons coordinate?
sensory (afferent) monitor changes in luminal activity, then activate interneurons, which relay signals to activate efferent secretomotor neurons, or neurons that control blood vessels, smooth muscle cells, epithelial cells, and enteric endocrine cells
-ANS can modify these responses
how does the PNS innervate the ENS?
-vagus to proximal 2/3, from pharynx to beginning of distal colon
-pelvic nerves to distal 1/3 of colon
-ACh is major neurotransmitter for pre/post-ganglionic fibers of PNS, but some postganglionic fibers release peptides (substance P, VIP)
-activation promotes digestion and absorption by increasing salivary, pancreatic, and gastric acid secretions; increasing contraction of smooth muscle wall; relaxing sphincters
how does the SNS innervate the ENS?
postganglionic fibers from celiac plexus, hypogastric, and superior/inferior mesenteric ganglia
-NE is major transmitter released by postsynaptic neurons, exerting inhibitory effects on excitatory cholinergic neurons via presynaptic inhibition
-activation will inhibit digestion/absorption by relaxing gut wall, reducing secretions, contracting sphincters, and diverting blood flow from GIT by contracting vasculature
what is the hierarchy of 5 levels for neural organization of digestive tract?
determines moment-to-moment motor behavior
1. ENS - independent integrative nervous system
2. prevertebral sympathetic ganglia
3. central sympathetic fibers
4. central parasympathetic centers
5. higher brain centers