IMAT 2020 Q35 [Acetylcholine & Contraction]

What is the correct order of the following events that occur immediately after acetylcholine binds to postsynaptic receptors of a muscle fibre, causing the muscle to contract?

  1. Ca2+ ions diffuse into the sarcoplasm.
  2. The muscle fibre membrane is depolarized.
  3. The myosin binding site on the actin filament is uncovered.

A. 2, 1, 3
B. 2, 3, 1
C. 3, 2, 1
D. 1, 2, 3
E. 1, 3, 2

The correct answer is A.

First, the action potential arrived at the synapse and triggers the opening of calcium channels.

Acetylcholine is then synthesized from a molecule of Acetyl-coA and choline and then are excreted through exocytosis into the synaptic cleft to reach the targeted cell.

They bind to the Acetylcholine receptors on the targeted cell’s membrane, which triggers the depolarization of the membrane and opening of sodium channels.

This, in turn, triggers the excretion of calcium by the sarcoplasmic reticulum through the ryanodine receptors.

The ions diffuse into the sarcoplasm (cytoplasm of muscular fibers) which raises the concentration of calcium inside the cytoplasm.

The myosin binding site on the actin filament is uncovered (troponin and tropomyosin are removed) by the binding of calcium to the myosin heads.

The myosin heads are then able to form cross-bridges with actin filaments, and through a sliding mechanism, ultimately
induces muscle contraction.