Smooth Muscle Flashcards
What are the two types of smooth muscle?
Unitary/Visceral - Electrically connected through gap junctions, contract as a unit
Multi-unit - cells not connected, each cell needs to be electrically stimulated
Describe the structure (actin and myosin, SR) of smooth muscle?
Filaments longer than skeletal and arranged in the cell periphery, less myosin
No roponin, just tropomyosin
Myosin isoform has slower ATPase activity
SR but no T-tubules
What ways does intracellular calcium increase to initiate contraction?
From ECF via VG and LG Ca channels
Release from the SR via an IP3 channel
Sustained contraction requires the extracellular calcium
What is the function of calcium in smooth muscle contraction?
Binds to calmodulin, which activates myosin light chain kinase
What is the function of myosin light chain kinase?
Phosphorylates the regulatory light chains of myosin, which increases myosin ATPase activity and binding to actin
Is smooth muscle thick- or thin-filament regulated?
Smooth muscle is thick-filament regulated
How is calcium removed from the cell?
SERCA = back into SR
Ca pump in the plasma membrane
Na-Ca exchanger in the plasma membrane
What is the function of myosin light chain phosphatase?
Dephosphorylates MLC, removing the ability of myosin to bind to actin
What determines the level of tension in muscle?
Levels of Calcium
What is the difference between phasic and tonic contractions?
Phasic - contracts rhythmically or intermittently
Tonic - contracts continuously
What are the three types of potentials that regulate smooth muscle contraction?
Slow waves - rhythmic changes in MP that only cause contraction if they reach threshold
Pacemaker - Regular depolarizations that cause a regular rhythm of contraction
Pharmomechanical - Change in tension without a change in potential
What are three ways to cause smooth muscle contraction?
Increase activity of Ca channels
Increase release of Ca from SR
Inhibit MLC Phosphatase
What are two ways to cause relaxation of smooth muscle?
Block Ca channels
Open K channels
What is the result of increased levels of cGMP?
Reduction of intracellular Ca and therefore relaxation through the activation of MLCP
What is the result of increased levels of cAMP?
cAMP-dependent kinase phosphorylates MLCK, leading to a reduction in Ca-dependent increase in MLCK activity
Also reduces intracellular Ca by phosphorylating ion channels
