Question 18

Describe the processes of excitation and contraction within a smooth muscle cell (60% 
of marks). Briefly outline the mechanism by which nitric oxide affects smooth muscle 
cell activity (40% of marks).

[Click here to toggle visibility of the answers]

College Answer

Insufficient breadth and depth of knowledge limited candidates’ performance to this 
question. Candidates were expected to mention mechanisms of muscle cell membrane 
activation (e.g. Ca2+ channel mediated action potential, Pacemaker potential, etc), sources 
of rise in intracellular Ca2+, intracellular Ca2+ binding to calmodulin in cytoplasm
Ca2+calmodulin complex binding to, and activation of myosin light chain kinase, energy 
dependent myosin cross bridges and cycling and mechanism of smooth muscle relaxation. Nitric Oxide (NO) activates guanylyl cyclise which in turn catalyzes the dephosphorylation of 
GTP to cyclicGMP which in turn induces smooth muscle relaxation, and the various 
mechanisms by which this occurs..