When an action potential reaches the presynaptic terminal, it causes 〖Ca〗^(2+) channels to open. 〖Ca〗^(2+) enter the presynaptic terminal and cause several synaptic vesicles to release acetylcholine into the synaptic cleft by exocytosis (figure 7.6). The acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptor sites on the 〖Na〗^+ channels in the muscle fiber cell membrane. The combination of acetylcholine with its receptor opens 〖Na〗^+ channels and therefore causes an increase in the permeability of the cell membrane to 〖Na〗^+ . The resulting movement of 〖Na〗^+ into the muscle fiber initiates an action potential in the muscle fiber, which travels along the length of the muscle fiber and causes it to contract. The acetylcholine released into the synaptic cleft between the neuron and muscle fiber is rapidly broken down by an enzyme, acetylcholinesteraes. This enzymetic breakdown ensures that one action potential in the neuron yields only one action potential in the skeletal muscle fibers of that motor unit, and only one contraction of each muscle fiber.