Answer:
acetylcholine
Explanation:
In the pre-synaptic knob there are vesicles which contain a transmitter substance called acetylcholine. When an impulse reaches the synaptic knob, it stimulates the vesicles to move towards the pre-synaptic membrane releasing the acetylcholine.
The transmitter substance makes the membrane permeable. The neuromuscular junction contains the sensory neurone(afferent), motor neurones(efferent) and the relay neurones(connector).
