Recall from Chap. 7 that each skeletal muscle cell is connected to a nerve fiber branch coming from a nerve cell. These nerve cells are called motor neu- rons, and they extend outward from the spinal cord. The motor neuron and all the muscle fibers it in- nervates are called a motor unit. Stimulation from mooror neurons initiates the contraction process.the site where the motor neuron and muscle cell meet is called the neuromuscular junction. At this junc tion the sarcolemma forms a pocket that is called the motor end plate (see Fig. 8 4. The end of the motor neuron does not physically make contact with the muscle fiber, but is separated by a short gap called the synaptic cleft (also called the neuromuscular cleft). When a nerve impulse reaches the end of the motor nerve, the neurotransmitter ace tylcholine is released and diffuses across the synaptic cleft to bind with receptor sites on the motor end plate. This causes an increase in the permeability of the to sodium, resulting in a depolariza tion called the end-plate potential (EPP). The EPP always large enough to exceed threshold and is the is signal to begin the contractile process.