Final answer:
The gap that separates a muscle cell membrane from the neuron cell membrane in a neuromuscular junction is called the synaptic cleft. Acetylcholine diffuses across the synaptic cleft to bind to receptors on the muscle cell membrane, initiating muscle contraction.
Step-by-step explanation:
The gap that separates a muscle cell membrane from the neuron cell membrane in a neuromuscular junction is called the synaptic cleft.
The synaptic cleft is a small gap between cells in a chemical synapse where neurotransmitters diffuse from the presynaptic element to the postsynaptic element.
In the case of the neuromuscular junction, the neurotransmitter acetylcholine diffuses across the synaptic cleft to bind to receptors on the muscle cell membrane, initiating muscle contraction.