Final answer:
Acetylcholine (ACh) is the neurotransmitter released into the synaptic cleft between motor neurons and muscle cells, leading to muscle depolarization and contraction.
Step-by-step explanation:
The neurotransmitter that is released into the gap between motor neurons and muscle cells is acetylcholine (ACh). This neurotransmitter binds to receptors in the motor end plate, initiating depolarization of the muscle cell. When an action potential reaches the synaptic terminal of a motor neuron, it triggers the influx of Ca2+ ions, which in turn allows the synaptic vesicles containing ACh to release the neurotransmitter into the synaptic cleft. Once across the cleft, ACh binds to ACh receptors on the muscle cell, producing an end-plate potential that leads to an action potential spreading along the sarcolemma and ultimately causing muscle contraction.