22A12: Exam Report

Describe the process of excitation-contraction coupling and relaxation in smooth muscle

36% of candidates passed this question.

This is a straightforward fact-based and process related question. It is important that candidates take the time to read the entire question prior to starting to write an answer.

Unfortunately, many candidates wrote about skeletal muscle contraction which scored no marks.

A good answer template included a description of the contractile elements of smooth muscle, the regulatory proteins and the role of calcium, and highlighted how these elements interact.

They included descriptions of both the contractile and relaxation processes.

L1iii / 22A12: Describe the process of excitation-contraction coupling and relaxation in smooth muscle

Excitation-contraction coupling

  • Is the process linking depolarisation of muscle cell membrane to initiation of myocyte contraction

Smooth muscle

  • Is a non-striated muscle tissue under involuntary control
  • Contractile elements (actin & myosin) disordered → no striations
  • RMP: -50 to -60 mV

Excitation by

  • Autonomic nerve (can stimulate multi-unit SM)
    • Sympathetic (Noradrenaline)
    • Parasympathetic (ACh)
  • Hormones/circulating molecules
    • O2, CO2, NO
    • Adrenaline, noradrenaline, prostaglandins, serotonin, histamine
  • Mechanical stretch
  • Pacemaker activity (migrating motor complex in GIT)
  • Action potentials propagate rapidly through gap junctions, caveolae increase surface area

Depolarisation opens Ca2+ channels (voltage-gates, ligand-gates, mechanical-gated) → Ca2+ enters cell

  • Ca2+ binds to calmodulin → Ca2+-calmodulin complex activates smooth muscle contraction
    • Myosin light chain kinase (MLCK): phosphorylates MLC allowing cross-bridge formation
    • Caldesmon: covers myosin binding site, Ca2+-calmodulin uncovers
    • Calponin inhibits ATPase activity; inhibited by Ca2+ and Ca2+-calmodulin complex → activation of ATPase

Contraction: cross-bridge cycling

  • Myosin head binds ATP → hydrolysis transfers energy to myosin head → binds to neighbouring actin (crossbridge formation) → powerstroke of myosin head → contraction → ADP & Pi dissociate → new ATP binds → actin and myosin dissociate


  • Ca2+ removed from cell by Ca2+ATPase and Na+/Ca2+ exchanger on cell membrane
    • ↓[Ca2+] → Ca2+ dissociates from calmodulin → return to resting configurations (myosin binding site blocked)
  • Myosin light chain phosphatase dephosphorylates MLC → inhibits ATPase

Author: Emily Little