A MN located in the spinal cord innervates up to several hundred fibers within one muscle, which together form the motor unit. Trains of action potentials within the axon cause the release of acetylcholine at the NMJ, which activates nicotinic receptors on the muscle fibers the MN innervates. This initiates a cascade of signaling events in the muscle fiber that leads to its contraction. A motor pool consists of all the individual MNs that innervate a single muscle. A muscle unit (one muscle and its motor pool) is composed of three different types of functional motor units consisting of alpha (α), beta (β), and gamma (γ) MNs, which are classified according to the contractile activity of the muscle fiber innervated. We will now discuss in more detail the anatomy of those structures involved in movement.