Smooth muscle function and mobility Flashcards
Organizations of layers of GIT
-Serosa
-Muscular Layer
-Submucosa
-Mucosa
3 layers of muscle
1.submucosa muscularis mucosae
2.circular smooth muscle (inner)
3.longitudinal smooth muscle (outer)
Submucosal musclaris mucosae
-muscle moves villi in the small intestine
Circular smooth muscle
-layer closest to the lumen
-fibers run at right angles to the long axis of intestine
-helps with peristalsis
Longitudinal smooth muscle
-outer layer
-fibers run parallel to the long axis of the intestine
-help with peristalsis
Types of contractions in GI smooth muscle
1.segmental contraction
2. Peristalsis
Segmental contraction
-mixing reaction that is carried out by the circular smooth muscle
Peristalsis
-propels bolus down the GI tract aborally
-coordinated contraction of the outer longitudinal muscle and the inner circular muscle behind food bolus to push it along
»process requires entry of calcium from extracellular fluid (making this process sensitive to hypocalcemia)
Smooth muscle contraction during peristalsis
-requires Ca from extracellular fluid
-contraction of cells is syncytium (possible due to gap junctions), therefore no need for nerve fibers to innervate every muscle fiber
Smooth vs. striated muscle
-smooth contract slowly compared to striated
-smooth have longer actin filaments and can therefore contract 3-4 x the distance of striated muscle
-smooth muscle contraction requires entry of Ca into the cell from extracellular fluid
-smooth muscle contract as syncytium. Individual neurons not required to stimulate each cell
Mechanotransduction in GI tract
-process by which cells convert mechanical forces into electrical and chemical signals that result in cellular responses
>smooth muscle cells have a wide array of membrane associated molecules to sense and process the mechanical stimuli
»mechanical stimuli then converted into short (changes in ion concentrations and voltage) and long (changes in gene expression) effects
Membrane associated molecules used in mechanotransduction
-Surface receptors (ion channels, G protein coupled receptors, kinases
-specialized intracellular cytoskeletal proteins
-extracellular cell-cell connections
Gi smooth muscle activity
-show spontaneous (myogenic) activity, conduction of electrical impulses from fiber to fiber, and sensitivity to stretch
-modulated by autonomic nerves
Interstitial cells of Cajal
-smooth muscle cells within the GI tract that undergo rhythmic depolarization caused by variations in conductance of Na, Ca, Cl across the cell membrane at regular intervals (pacemakers of the gut)
like pacemaker cells/purkinje fibers of the heart
Function of interstitial cells of cajal
-responsible for the basal or basic electrical rhythm (BER)
>the spontaneous depolarization and repolarization of these pacemaker cells in the smooth muscle of the stomach, small intestine, and large intestine
Cajal resting membrane potential `
Negative 50 mV
Depolarization of Cajal cells
1.Slow waves
2.Spikes
Slow waves
-spontaneous, slow, transient depolarization, conducting varying distances along the tract (basic electrical rhythm)
Spikes
-faster transient depolarization that can occur in burst at the periods of maximal depolarization of a slow wave
>proceed and initiate contraction of smooth muscle
Parasympathetic stimulation of cajal cells
-increases the rate at which these potentials depolarize the membrane, thus increasing the rate of the spike discharge and contraction
Sympathetic stimulation of cajal cells
-prolongs (slows) the rate at which the generator potential depolarizes the membrane and thus slows the rate of the spike discharge
Slow Wave membrane potential
-resting membrane potential (-50 to -60 mV). Membrane potential fluctuations are spontaneous slow waves (moving from 5-15mV) in smooth muscle
>fluctuations appear to be an intrinsic property of smooth muscle, and does not depend on nervous stimuli
Are slow waves action potentials?
-No, they are not action potentials and do not elicit contractions
**sometimes stomach may be considered an exception
Slow Wave frequency in Small intestine vs. stomach and large intestine
-Small intestine: 10-20x per minute
-Large intestine and stomach: 3-8x per minute
Spikes
-occur on top of a slow wave. AP’s caused by rapid entry of Ca ions
-occur when stretched by a bolus of food OR receptors activated by parasympathetic stimulation
Smooth muscle contraction steps
1.L-type channels open, Ca enters
2. Ca binds to calmodulin, forming complex
3.Complex combines with Myosin light chain kinase which can act on myosin and actin and results in either contraction (phosphorylation) or relaxation
Speed of contractions (slow waves and spikes)
-Slow waves take a few seconds
-Spikes are very quick, within a second
What causes spikes/depolarizations?
1.stretch
2.acetylcholine
3.parasympathetics
What causes hyperpolarizations?
1.NE
2. Sympathetics
Contractile Activity patterns
-Tonic
-Phasic
Tonic smooth muscle contractions
-normally contracted and generate a variable steady-state force
-Ex. sphincters, blood vessels, airways
Phasic Smooth muscle contraction
-rhythmic contractions (Ex. peristalsis in GI tract) but may also contract intermittently during physiological activities under voluntary control (Eg. Voiding of urine or swallowing)
Swallowing and relaxation
-swallowing through pharyngeal stimulation results in neural reflex relaxation of the lower esophageal sphincter and the proximal part of the stomach to allow entry of food
Deoxynivalenol (DON)
-vomitoxin
-produced by Fusarium graminearum (fungi)
-commonly found in corn, barley, wheat
-develops in cool, damp weather
-negative effects begin at ~4 ppm, but inconsistent
Fusarium Head Blight (FHB)
-disease caused by infection with Fusarium fungal species
-bleached heads and shriveled chalky/pinkish kernels
-during infection, a variety of mycotoxins can be produced (including DON)
DON impact on weanling pigs
-growth effects greater than feed intake, proving that growth was not related just to feed intake
-thickness of smooth muscle in the jejunum and ileum decreased, which affects animals growth
DON impact on mice
-small intestine shorter
-motility affected because mice given charcoal after DON and moved less compared to control
**direct effect on proteins involved in motility of small intestine. Which effects intake and therefore growth
