Question

Put the following phrases in proper order to describe what occurs at the neuromuscular junction to trigger muscle contraction.

(i) Receptor site on the sarcolemma.

(ii) Nerve impulse

(iii) Release of Ca+2 from the sarcoplasmic reticulum.

(iv) The neurotransmitter Acetylcholine is released.

(v) Sarcomere shortened.

(vi) 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

Hint:
During the muscle contraction, the thin filaments slide over the thick filaments. This is known as the sliding filament theory. The thin filaments are called actin whereas thick filaments are called myosin.

Complete step by step answer:
Following are the steps that take place to initiate and trigger muscle contraction:
1. The central nervous system sends a nerve impulse to the muscle fibre via a motor neuron.
2. A motor unit is formed by a motor neuron along with the muscle fibres.
3. The neural signal that reaches the neuromuscular junction (junction between a motor neuron and the sarcolemma of the muscle fibre) releases a neurotransmitter called Acetylcholine. This generates an action potential in the sarcolemma.
4. This action potential gets spread throughout the muscle fibre over the sarcolemma on T-tubules.
5. Calcium ion from the sarcoplasmic reticulum is released.
6. The calcium ions then bind with the subunit of troponin on actin filaments and thus remove the masking of active sites for myosin. The energy released through ATP hydrolysis causes the sliding of the thin filaments over the thick filaments.
7. And thereby, causes the shortening of the sarcomere.
Thus, the correct answer is:- (ii), (iv), (vi), (i), (vii), (iii), (v)

Option ‘B’ is correct

Note:
At the neuromuscular junction, the neurotransmitter Acetylcholine is broken down by acetylcholinesterase. The shortening of the sarcomere results in the reduction of the I band and the A band retains its length. The myosin releases ADP and P1 and goes back to a relaxed state.