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

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

Definitions

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

Macroscopic

Motor unit = nerve ending + muscle fibre

10-100mm diam fibre

Striated, ordered sarcomeres (functional unit)

Microscopic

Multiple parallel myofibrils

Multi nucleated

Mitochondria (aerobic & anaerobic metabolism)

SR (Ca++ storage)

Glycogen (E storage)
Myoglobin (O2 storage)

T-tubules

Molecular

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

E-C-C

Skeletal Muscle RMP

-70mV

Source of Activation

NMJ

Spread

Via T tubules

AP

Opens fast Na ch

Ca Release

AP propagates along sarcolemma & down T-tubules

Activates L-type Ca++ ch of

T-tubles

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

SKELETAL

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

Relaxation

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