The neuromuscular junction between motor nerve fibres and skeletal muscles is a good place at which to begin a study of the ways in which drugs can influence the processes of synaptic transmission. Other synapses may be considerably more complex. At the neuromuscular junction the system is both anatomically and functionally relatively simple and a functional system can be isolated from the whole organism.

The majority of mammalian skeletal (striated) muscle fibres are focally innervated by motor axon terminals, each one forming a single junction on any one muscle fibre at the motor end plate. A motoneurone in the ventral horn of the spinal cord gives rise to a single axon which branches within the muscle to innervate from one to six muscle fibres, these groups of muscle fibres together comprising a single motor unit. This motor unit therefore behaves as a single functional entity (Fig. 3.1). The large, extrafusal muscle fibres which generate the major component of the force developed in muscle contraction are innervated by large α-motoneurones through myelinated A-fibres. The small intrafusal fibres of the muscle spindles are innervated by the small γ-fibres of motor nerves.

Some of the muscle fibres in amphibia and in birds, e.g. the rectus abdominis muscle in the frog and the biventer cervicis muscle in the chick respectively, have multiple endings upon them and this results in important differences in their physiological responses to nerve stimulation and in their pharmacological responses to drugs.