What is the primary excitatory neurotransmitter at the insect neuromuscular junction?

• A. GABA
• B. Glutamate
• C. Acetylcholine
• D. Dopamine

Answer: (B)

At the insect neuromuscular junction, the signal from the motor neuron to the muscle is carried by acetylcholine, the primary excitatory transmitter. When the motor neuron fires, it releases acetylcholine into the synaptic cleft, which binds to nicotinic acetylcholine receptors on the muscle cell. This opens ion channels, causing a depolarizing end-plate potential that triggers a muscle action potential and contraction. Acetylcholine is then rapidly broken down by acetylcholinesterase, terminating the signal.

GABA is inhibitory in the insect nervous system, not the main transmitter at the NMJ. Glutamate is a major excitatory transmitter in many CNS synapses and in some invertebrate systems, but not the principal transmitter at the insect NMJ. Dopamine acts as a neuromodulator rather than the primary transmitter at the neuromuscular junction.

So the correct concept is that acetylcholine mediates rapid, direct excitation of insect muscles at their NMJ.