GI Physiology Flashcards
What are the layers of the GI wall?
- Serosa
- Longitudinal smooth muscle layer
- Circular smooth muscle layer
- Submucosa
- Mucosa (bundles of smooth muscle fibers)
What are slow waves in the GIT?
Rhythmic contractions - determine by frequency of slow waves (NOT AP but slow undulating changes in resting membrane potential)
Varies along GIT
What cells are consider the electrical pacemakers for smooth muscle cells?
Interstitial cells of Cajal - Undergo cyclic change in membrane potential
Unique ion channels that periodically open = inward pacemarkers currents
Do the slow waves in the GIT cause muscle contraction?
NO! Except in stomach. Provide electrical background to allow AP when excited by intermittent spike potentials (which excites muscle contractions)
What are spike potentials in GIT?
True AP - Each time peaks of slow waves temporarily more + spike potentials = Peaks
In the nerve fibers how are AP generated?
By rapid entry of Na into nerve through Na channels
How are the action potentials different in the GIT?
Calcium-sodium channels = Much slower to open/close = Longer duration of AP
What are the 3 main actions that can result in depolarization of smooth muscle cells in GIT?
- Stretching of muscle
- Stimulation by acetylcholine release from parasympathetic nn
- Stimulation by several specific GI hormones
What are the effects of norepinephrine and epinephrine on GIT nerves (stimulation of smpathetic nn)?
More negative = Hyperpolarized (less excitable)
What causes Ca to enter the muscle fiber to result in a contraction?
Slow waves do NOT cause Ca to enter (ONLY Na), thus no muscle contraction alone
During a spike potential (generated by slow waves) = Large amount of Ca enters = muscle contraction
What are tonic contractions in the GIT?
Continuous (not associated with slow waves) - can last mins to hours
These are in addition or instead of rhythmical contractions
What is the nervous system that is within the wall of the gut?
Enteric Nervous System
What are the two major plexus and what do they control?
- Myenteric Plexus (Auerbach’s): Outer plexus twn longitudinal and circular muscle layers - mainly control GI movements
- Submucosal Plexus (Meissner’s): Inner plexus - controls mainly GI secretions and local blood flow
Which plexus in GIT controls GI movements?
Myenteric Plexus (Auerbach’s)
Which plexus in GIT controls GI secretions and local blood flow?
Submucosal Plexus (Meissner’s)
What is the main difference btwn the myenteric and submucosal plexuese?
Myenteric Plexus = Linear chain of interconnected neurons that run the length of GIT - Control muscle activity along length of gut
Submucosal Plexus = Functions within the inner wall of each segment of intestine, sensory signal from epithelium integrate at submucosal plexus to help control intestinal secretions, local absorption, location contraction of submucosal muscle
What is the neurotransmitters that results in excitation in GIT?
Acetylcholine
What is the neurotransmitters that results in inhibition in GIT?
Norepinephrine
How does parasympathetic stimulation affect the ENS?
Increases activity of ENS
Mainly vagus nn (some through pelvic nn)
How does sympathetic stimulation affect the ENS?
Inhibits GIT activity = From T5-L2 → sympathetic chains → celiac ganglion → mesenteric ganglia (postganglion neuron bodies) → Postganglic fibers to ENTIRE GIT → Secrete norepinephrine (small amounts of epinephrine)
Inhibits intestinal smooth muscle, blocks/inhibits neurons in ENS
What is the gastrocolic reflex?
Signals from stomach to cause evacuation of colon (reflex from gut to prevertebral sympathetic ganglia back to GIT)
What is the enterogastric reflex?
Signals from colon/small intestines to inhibit stomach motility and secretions (reflex from gut to prevertebral sympathetic ganglia back to GIT)
What is the colonoileal reflex?
Reflexes from colon to inhibit emptying of ileal contents into colon (reflex from gut to prevertebral sympathetic ganglia back to GIT)
What is a reflex that comes from the GIT to spinal cord/brain and then back to GIT?
Defecation reflexes = from colon/rectum to spinal cords and back to produce powerful colon, rectal, and abdominal contractions = Defecation