To activate skeletal muscle, the central nervous system initiates an action potential that travels down the spinal cord to the motor neurons. As the nerve fiber branches, the action potential travels down each branch. Each nerve fiber branches many times and stimulates several skeletal muscle fibers. The union of the axon and muscle fiber is called the neuromuscular junction. Zooming into the microscopic level, each branch of the neuron has a terminal that invaginates the muscle fiber, while remaining outside the muscle fiber plasma membrane. The action potential arrives at the axon terminal. In the terminal the action potential causes the release of acetylcholine from the synaptic vesicles into space between the axon terminal and the muscle fiber called the synaptic cleft. In the synaptic cleft, the acetylcholine binds with a receptor site on the fiber membrane, which opens a chemically-gated ion channel. Sodium then rushes through the ion channel into the muscle fiber, causing an action potential to form on the fiber membrane. The action potential spreads along the muscle fiber. As more nerve branches activate additional fibers, the action potential spreads over the entire muscle. Upon activation, the muscle contracts.