As a result of an action potential invading the motor neuron terminal, acetylcholine is released into the synaptic cleft. The acetylcholine binds to receptor sites at the end plate of the muscle. This causes an increase in permeability to Na and K ions. Na enters the muscle cell and depolarizes it to threshold, thus initiating an action potential. The action potential then enters the inside of the muscle cell by way of the t-tubules. The esterase is at the synaptic cleft to inactivate the acetylcholine after it has successfully depolarized the muscle cell. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong's Review of Medical Physiology, 24e.