The complex sequence of events that triggers a muscle fiber to contract is termed excitation–contraction coupling because it begins with the excitation of a motor nerve and results in contraction of the muscle fibers. The process, depicted in in this video, is initiated by a nerve impulse, or action potential, from the brain or spinal cord to an α-motor neuron. The action potential arrives at the α-motor neuron’s dendrites, specialized receptors on the neuron’s cell body. From here, the action potential travels down the axon to the axon terminals, which are located very close to the plasmalemma. When the action potential arrives at the axon terminals, these nerve endings release a signaling molecule or neurotransmitter called acetylcholine (ACh), which crosses the synaptic cleft and binds to receptors on the plasmalemma. If enough ACh binds to the receptors, the action potential will be transmitted the full length of the muscle fiber as ion gates open in the muscle cell membrane and allow sodium to enter. This process is referred to as depolarization. An action potential must be generated in the muscle cell before the muscle cell can act. Want to teach your course with a top textbook in exercise physiology? Physiology of Sport and Exercise, Seventh Edition includes a web study guide with 66 audio clips, 26 animations, and 27 videos from key leaders in the field. Learn more: