Final answer:
The gap between axon terminals and the plasma membrane of a neighboring muscle cell is called the synaptic cleft, which plays a crucial role in neurotransmission.
Step-by-step explanation:
The gap between the axon terminals and the plasma membrane of a neighboring muscle cell is known as the synaptic cleft. This small gap is essential for the process of neurotransmission, where signaling molecules known as neurotransmitters are released from the presynaptic neuron's axon terminal into the synaptic cleft. These molecules then travel across the synaptic cleft to bind to receptors on the postsynaptic cell, such as a muscle cell, to transmit the signal and possibly generate an action potential in the postsynaptic cell.