Final answer:
The primary event happening on the pre-synaptic side of the neuromuscular junction is the release of neurotransmitters(a), specifically acetylcholine (ACh). This binding leads to the opening of ion channels and depolarization of the muscle fiber, which initiates muscle contraction.
Step-by-step explanation:
In the context of the neuromuscular junction, the primary event happening on the pre-synaptic side is the release of neurotransmitters(a). Specifically, the neurotransmitter acetylcholine (ACh) is released from the axon terminal of the motor neuron and diffuses across the synaptic cleft to bind with ACh receptors on the motor end plate of the muscle fiber. This binding leads to the opening of ion channels and depolarization of the muscle fiber, which initiates muscle contraction.