Lecture 19

Question 1

Describe the morphology of smooth muscles, compare with skeletal muscle

Answer 1

sheets in different directions (circular vs longitudinal)
single nucleus
no striations
no t-tubule
actin surrounds myosin filament
thin filaments anchor to dense bodies
no troponin

Question 2

Describe the different types of smooth muscle

Answer 2

1) multi unit = no electrical connection, finer control of contractions
2) smooth unit = electrically connected via gap junction, all cells contract almost simultaneously,
3) phasic contraction = usually relaxed, contracts rhythmically or intermittently
4) tonic contraction = usually activated (sphincters), slow wave modifications of membrane potential triggers contraction

Question 3

Describe antagonistic control of smooth muscle contraction

Answer 3

SNS and PNS have opposite effects via axon varicosities
increase of cAMP (Gs) = relaxation
increase of IP3 (Gq) = contraction
cAMP activates PKA which phosphorylates and decreases activity of MLCK

Question 4

Describe EC coupling in smooth muscle

Answer 4

Ca channels are either voltage-gated, ligand-gated, or stretch-gated, main source of Ca influx from ECF
RyR receptors on SR undergo calcium induced calcium release
Ach may lead to GPCR activation (Gq) creation of IP3 which leads to release of Ca from SR through IP3 channels

Question 5

Describe cross-bridge cycling in smooth muscle

Answer 5

Ca binds to calmodulin, activates MLCK which phosphorylates myosin to increase ATPase activity
same steps of cycling as skeletal muscle
Ca is taken out of cell through SERCA, NCX
MLCP activity (always present) now dominates over MLCK and dephosphorylates myosin
Can have graded contractions depending on amount of Ca depends on how much MLCK is activated