When a nerve impulse travels along a nerve fibre and reaches the point where the nerve meets a muscle, the electrical signal alone cannot directly make the muscle contract. Explain why a chemical step is necessary at this junction, and describe in brief what happens as a result of this chemical step that leads to muscle movement.
Generated by claude-sonnet-4-6 · 2026-06-26 01:01 · grounding rag
Model Answer
At the nerve-muscle junction (neuromuscular junction), the electrical impulse alone cannot cross the gap between the nerve ending and the muscle cell. A chemical step is necessary because the gap must be bridged by a chemical messenger.
When the electrical impulse reaches the end of the axon, it triggers the release of chemicals across this junction. These chemicals reach the muscle cell and stimulate it. In response, the special proteins present in the muscle cell change their shape and arrangement, causing the muscle cell to shorten. This shortening of muscle cells results in muscle contraction and movement.
Source: Chapter 6, Section 6.1 — Animals: Nervous System / 6.1.4 How does Nervous Tissue cause Action?
Explanation
- Examiners expect three clear points for 3 marks: (1) why electrical signal alone cannot work — the gap/synapse exists, (2) what the chemical step is — release of chemicals at axon end, (3) what happens next — proteins change shape → muscle shortens → movement.
- The textbook explicitly states: "At the end of the axon, the electrical impulse sets off the release of some chemicals" and "Muscle cells have special proteins that change both their shape and their arrangement… giving the muscle cells a shorter form."
- Use the term neuromuscular junction to score full marks — it signals precise vocabulary to the examiner.
- Do not write about synapses between two neurons in detail; keep focus on the nerve-to-muscle step as the question demands.