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).

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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.


  • Excitation-contraction coupling is the series of events that link the sarcolemma action potential to muscle contraction and relaxation
  • Smooth muscle action potentials:
    • Duration and magnitude is extremely variable
    • Originate from innervation, or from smooth muscle pacemakers (capable of automaticity)
    • Multi-unit smooth muscle may be connected into an electrical syncytium and action potentials may propagate from cell to cell via gap junctions
    • Contraction can also occur without an action potential, eg. hormonally mediated
  • Voltage-gated L-type calcium channels are activated by action potentials
    • They allow calcium into the cell
    • This activates the ryanodine receptor using calcium as a second messenger 
    • The ryanodine receptor then acts as a calcium channel, releasing stored calcium from the sarcoplasmic reticulum.
  • Calcium release from the sarcoplasmic reticulum occurs
    • Cytoplasmic free calcium concentration increases to 20 μmol/L
  • Intracellular calcium acts on calmodulin
    • Calmodulin is a regulatory protein which activates myosin light chain kinase
    • Myosin light chain kinase then phosphorylates the light chain of myosin, greatly increasing its ATPase activity
  • Crossbridge cycling occurs
    • Myosin binds ATP, dissociates from actin, and "cocks" its head to a 90º angle.
    • Then its head binds actin again, which is the cross-bridge
    • It then releases the inorganic phosphate and returns its head to its original position, which results in the movement of the myosin molecule about 11 nm along the actin filament.
  • Calcium buffering by proteins removes some calcium from the cytosol
    • Free calcium concentration decreases when it binds to troponin, ATP and parvalbumin
  • Calcium removal from the cytosol is required for relaxation
    • Calcium is removed mainly by the SERCA ATPase pump which removes it from the cytosol and returns it to the sarcoplasmic reticulum.
    • Smooth muscle myosin can still continue to function as an ATPase until it is dephosphorylated by myosin light chain phosphatase
  • Regulation of smooth muscle relaxation
    • Myosin light chain phosphatase activity is regulated endocrine and paractrine systems, eg. by nitric oxide:
      • Nitric oxide activates guanylate cyclase
      • Guanylate cyclase produces cyclic GMP
      • cGMP activates protein kinase G
      • Protein kinase G  phosphorylates myosin light chain phosphatase, activating it and bringing about smooth muscle relaxation. 


Sweeney, H. Lee, and David W. Hammers. "Muscle contraction." Cold Spring Harbor Perspectives in Biology 10.2 (2018): a023200.

Endo, Makoto. "Excitation-contraction coupling in smooth muscle." Japanese Journal of Pharmacology 33 (1983): 4.

Somlyo, Andrew P. "Excitation-contraction coupling and the ultrastructure of smooth muscle." Circulation Research 57.4 (1985): 497-507.