Question

Given below is a randomly arranged list of events that occur at the neuromuscular junction to trigger muscle contraction.Which of the following gives the correct sequence of these steps?

(i) Acetylcholine binds to the receptor sites on sarcolemma

(ii) Nerve impulse

(iii) Release of Ca+2 from sarcoplasmic reticulum

(iv) The neurotransmitter acetylcholine is released

(v) Sarcomere shortens

(vi) Acetylcholine diffuses across the synaptic cleft

(vii) Spread of impulses over sarcolemma on T-tubules

• A. (ii), (iv), (i), (vi), (vii), (iii), (v)
• B. (ii), (iv), (vi), (i), (vii), (iii), (v)
• C. (i), (ii), (iii), (iv), (v), (vi), (vii)
• D. (vii), (vi), (v), (iv), (iii), (ii), (i)

Answer 1

The correct option is B

(ii), (iv), (vi), (i), (vii), (iii), (v)

(ii), (iv), (vi), (i), (vii), (iii), (v) is the correct sequence of muscle contraction.
A nerve signal arrives at the synaptic knob and stimulates voltage-gated calcium channels to open, calcium ions enter the synaptic knob.
Calcium stimulates exocytosis of the synaptic vesicles, which release acetylcholine into the synaptic cleft.
Acetylcholine diffuses across the synaptic cleft and binds to receptor proteins on the sarcolemma.
A wave of action potentials spreads from the end plate in all directions.When this wave of excitation reaches the T- tubules, it continues down into sarcoplasm.
Action potentials open voltage-gated ion channels in the T- tubules. These are physically linked to calcium channels in the terminal cisternae of the sarcoplasmic reticulum. Channels in the SR open and calcium diffuse out. Calcium binds to the troponin of the thin filaments. a series of reaction follows and the sarcomere shortens.