Question

Following is given a randomly arranged list of events that occur at neuromuscular junction to trigger muscle contraction.
(i) Receptor sites on sarcolemma
(ii) Nerve impulse
(iii) Release of $C{a}^{+2}$ from sarcoplasmic reticulum
(iv) The neurotransmitter acetylcholine is released (v) Sarcomere shorten
(vi) Synaptic cleft
(vii) Spread of impulses over sarcolemma on T-tubules
Which of the following gives the correct sequence of these steps ?

• A. (ii) $\to$ (iv) $\to$ (i) $\to$ (vi) $\to$ (vii) $\to$ (v)
• B. (ii) $\to$ (iv) $\to$ (vi) $\to$ (i) $\to$ (vii) $\to$(iii) $\to$ (v)
• C. (i) $\to$ (ii) $\to$ (iii) $\to$ (iv) $\to$ (v) $\to$ (vi) $\to$ (vii)
• D. (vii) $\to$ (vi) $\to$ (v) $\to$ (iv) $\to$ (iii) $\to$ (i)

Answer 1

The correct option is B (ii) $\to$ (iv) $\to$ (vi) $\to$ (i) $\to$ (vii) $\to$(iii) $\to$ (v)

Muscle contraction:

a. As a nerve impulse( action potential) from a motor neuron arrives via its axon at the neuromuscular junction, the vesicles in the end plate release a neurotransmitter, the acetylcholine, into the nerve-muscle gap (synaptic cleft).

b. Acetylcholine depolarises the postsynaptic muscle cell's sarcolemma and set up an action potential in the latter.

c. The action potential spreads deep into the interior of the muscle fibre via T- tubules and Z lines.

d. The incoming action potential changes the permeability of the SR, causing to release calcium ions.

e. The calcium ions bind to troponin molecules.

f. The latter change shape and shift position.

So, the correct answer is '(ii) $\to$(iv)$\to$(vi)$\to$(i)$\to$(vii)$\to$(iii)$\to$(v)'.