PHYL2002 Study Guide - Final Guide: Myosin Light-Chain Kinase, Voltage-Dependent Calcium Channel, Basal Electrical Rhythm
Smooth Muscle Physiology
• Structure
o Not striated
o Found in blood vessels, respiratory system, alimentary tract,
bladder
o Controlled by autonomic nerves, hormones, local mediators
o Contractions are slow
o Very resistant to fatigue
o Contraction used little energy
o Tone can be maintained indefinably
o Some smooth muscle are phasic → slow rhythmic contraction
• Multi-unit smooth muscle
o Discrete motor units like skeletal muscle
o Units must be activated separately
o Produce smooth tonic contractions when activated
o Not rhythmic
o Found in eye, large blood vessels, airways
• Single unit smooth muscle
o Cells function as a single unit like the heart
o Gap junctions between cells
o Tonic in bladder, small blood vessels
o Phasic (rhythmic) in gut, uterus
o Often show slow waves or basal electrical rhythm
• Phasic smooth muscle
o Interstitial cells of Cajal generate pacemaker potentials slow
waves
o Last seconds and ~10-15 mV
o Pass to muscle through gap junctions
o Action potential due to opening of voltage gated Ca2+ channels (L-
type channels)
o Repolarization from opening Kv channels
o Resting potential from open Kir channels
• Smooth muscle contraction
o Contraction produced by actin and myosin
o Rise in intracellular Ca2+ → trigger for contraction
o Calcium binds calmodulin → activates myosin light chain kinase
(MLCK)
o Myosin is phosphorylated and binds actin → contraction occurs
o Dephosphorylation by myosin light chain phosphatase (MLCP)
o Some smooth muscles use intracellular Ca2+ stores → blood
vessels
o Some smooth muscles rely on extracellular calcium → gut
o Calcium influx can be triggered by action potential or by receptor
binding
• Neural control
o ACh acting on muscarinic (M) receptors → constricts
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