14A08: Exam Report

Describe the physiology of skeletal muscle cell contraction.

34% of candidates passed this question.

This question required a description of excitation B contraction coupling. Marks were gained for a brief outline of the structure of a sarcomere and how it facilitates shortening. An explanation of membrane processes, receptor interactions and the contraction processes was required.  Mention of the role of ATP was also required and marks were gained for commenting on the mechanism of return to the relaxed state.

Most candidates wrote extensively on the nerve action potential and neuromuscular junction transmission, with minimal reference to events occurring within the skeletal muscle cell membrane. They could not gain marks for this.  Few candidates demonstrated knowledge of the ATP dependent walk along processes of myosin heads during contraction.

L1iii / 14A08: Describe the physiology of skeletal muscle contraction


Skeletal muscle:  Striated muscle tissue which has voluntary control by the somatic nervous system

Sarcomere:  The structural unit of a myofibril in striated muscle

Excitation-Contraction-Coupling (E-C-C):  the rapid events occurring converting an electrical stimulus in the plasma membrane into a mechanical response of muscle contraction

Sarcomere Structure


Motor unit = nerve ending + muscle fibre

10-100mm diam fibre

Striated, ordered sarcomeres (functional unit)


Multiple parallel myofibrils

Multi nucleated

Mitochondria (aerobic & anaerobic metabolism)

SR (Ca++ storage)

Glycogen (E storage)
Myoglobin (O2 storage)



Contractile proteins = actin & myosin  → Striated

Regulatory proteins = Tropomyosin & troponin

  • Troponin: protein w 3 subunits

I – inhibits myosin ATPase

C- binds Ca++

T- binds Tropomysin

Tropomysin – blocks actin/myosin interaction

Labelled Diagram of a Sarcomere


Skeletal Muscle RMP


Source of Activation



Via T tubules


Opens fast Na ch

Ca Release

AP propagates along sarcolemma & down T-tubules

Activates L-type Ca++ ch of


Flux of Ca++ intracellularly↑↑­↑­­

Ryanodine Rec of SR open = mass x60↑­ Ca++

Cross Bridge

Ca++ binds TnC

Removes inhibition of Troponin-Tropomyosin Complex

Exposes Actin binding site to myosin → commencement of cross-bridging

Myosin head pulls actin via ATP hydrolysis (ATPase) = contraction


Pi release allows myosin head to move towards M line, pulling actin along with it.

Each movement pulls filaments approx. 10nm toward M line – this movement KA Powerstroke

As the sarcomere shortens, the muscle contracts


  • Continues until Ca++ ↓
  • SERCA pumps Ca++ back into SR → relaxation