Q.Describe, in the correct sequence, the events that occur from the arrival of a nerve impulse at the neuromuscular junction up to the relaxation of the muscle fibre.

[!TLDR]

A nerve impulse at the neuromuscular junction triggers calcium release, exposing actin's binding sites for the cross-bridge cycle; the muscle relaxes when calcium is pumped back and tropomyosin re-blocks the site.

[!ANSWER]

Nerve impulse -> ACh release -> action potential -> Ca2+ release -> troponin/tropomyosin shift -> cross-bridge cycling -> relaxation on Ca2+ removal.

Muscle contraction begins with a nerve impulse generated in the central nervous system, which travels along a motor neuron to the neuromuscular junction (motor end plate), the specialised point of contact between the neuron and the muscle fibre. There, the arriving impulse triggers the release of the neurotransmitter acetylcholine into the gap between the neuron and the sarcolemma. Acetylcholine binds receptors on the sarcolemma and generates a new action potential in the muscle fibre itself, which spreads rapidly across the sarcolemma and inward along the T tubules, a system of tubules in close contact with the sarcoplasmic reticulum throughout the fibre. This electrical signal causes the sarcoplasmic reticulum to release its stored calcium ions into the surrounding sarcoplasm. The released calcium binds troponin C, the calcium-binding subunit of the troponin complex sitting on the thin filament, and this binding produces a conformational change that is transmitted through troponin to tropomyosin, physically shifting tropomyosin away from the myosin-binding sites it had been covering on the actin filament. With the sites exposed, the cross-bridge cycle can proceed: an ATP-energised myosin head attaches to the newly available site on actin, forming a cross-bridge; the head then bends in its power stroke, dragging the thin filament a short distance towards the centre of the sarcomere and releasing the ADP and inorganic phosphate left over from the previous cycle; a fresh ATP molecule then binds the myosin head, which causes it to detach from actin; and the hydrolysis of that ATP by the head's own ATPase activity re-cocks the head to its original high-energy position, ready to attach further along the actin filament and repeat the cycle. This cycle continues, drawing the thin filaments progressively further in and shortening the sarcomere, for as long as the nerve keeps stimulating the muscle fibre and calcium remains available in the sarcoplasm. When nervous stimulation ceases, calcium is actively pumped back into the sarcoplasmic reticulum, sarcoplasmic calcium falls, troponin releases its bound calcium and returns to its resting shape, tropomyosin slides back to re-cover the myosin-binding sites, cross-bridges can no longer form, and the muscle fibre relaxes back to its resting length.

[!ANSWER]

Nerve impulse -> ACh release -> action potential -> Ca2+ release -> troponin/tropomyosin shift -> cross-bridge cycling -> relaxation on Ca2+ removal.

Work through the sequence as a chain of triggers, each step causing the next: nerve impulse, acetylcholine, action potential, calcium release, troponin/tropomyosin shift, cross-bridge cycle, and finally reverse the chain (calcium removal) for relaxation.

Omitting the calcium-removal step for relaxation, and simply saying the muscle 'stops receiving a signal' -- relaxation is an active process requiring calcium to be actively pumped back into the sarcoplasmic reticulum, not merely the absence of a new stimulus.