A toxin that is the same shape as a neurotransmitter prevents muscle contraction. Which statement explains why the toxin prevents muscle contraction?
A) The toxin blocks the binding of neurotransmitters to receptors on motor neurones.
B) The toxin increases the release of neurotransmitters from relay neurones.
C) The toxin increases the release of neurotransmitters from sensory neurones.
D) The toxin stimulates impulses in motor neurones.
▶️ Answer/Explanation
Ans: A
The toxin prevents muscle contraction because:
- It mimics the shape of neurotransmitters, allowing it to bind to muscle receptor sites.
- By occupying these receptors, it blocks the actual neurotransmitter (e.g., acetylcholine) from binding.
- This prevents depolarization of the muscle membrane, inhibiting contraction.
Options B–D are incorrect because:
- B/C: Relay/sensory neurones are unrelated to direct muscle stimulation.
- D: The toxin inhibits (not stimulates) motor signals.
Thus, A is the only plausible explanation.
The diagram shows a synapse in a reflex arc.
What are the identities of the two neurones and in which direction does the neurotransmitter pass?
| neurone P | neurone Q | direction of passage of the neurotransmitter | |
|---|---|---|---|
| A | motor | relay | P → Q |
| B | motor | sensory | P → Q |
| C | relay | motor | Q → P |
| D | relay | sensory | Q → P |
▶️ Answer/Explanation
Ans: D
In a reflex arc, the neurotransmitter passes from the relay neurone (P) to the sensory neurone (Q). The direction is always from the presynaptic neurone to the postsynaptic neurone, which in this case is Q → P. Therefore, the correct answer is D.