At the neuromuscular junction, acetylcholine (ACh) is the primary neurotransmitter responsible for transmitting the nerve impulse from the motor neuron to the muscle fiber. Upon arrival of an action potential, ACh is released into the synaptic cleft and binds to nicotinic cholinergic receptors on the muscle cell membrane. This binding causes depolarization of the muscle membrane, leading to muscle contraction. The unique role of ACh in this synapse makes it essential for voluntary muscle movement initiation. Defects in ACh release or receptor function result in clinical conditions like myasthenia gravis.

Reference: Guyton and Hall, Textbook of Medical Physiology, 14th Edition.