ABSTRACT
The neuromuscular junction (NMJ) is the remarkable structure at the interface of the motor axon and its innervated muscle fiber that is responsible for neuromuscular transmission. It is a synapse, but a highly specialized synapse because of its both critical and unique physiologic task. The NMJ is designed to transfer the motor axon potential to a muscle fiber action potential with 100% fidelity. In this respect, it is unlike all other synapses in the brain where various excitatory and inhibitory influences engage in a competition with one another to influence postsynaptic firing. That the NMJ normally functions without failure is remarkable given the size difference: the terminal motor axon within the synapse is tiny and the innervated muscle fiber is massive. The infusion current required in order to bring the muscle cell membrane to its depolarization threshold is correspondingly large. Neuromuscular transmission is critical to viability: it is not an accident that the various steps in the process of neuromuscular transmission are the biologic target of choice for evolved toxins injected by many different predators, or that a wide array of rare genetic and acquired disorders of the neuromuscular junction manifest with obvious and often life-threatening symptoms. The complexity of neuromuscular transmission, and the early and obvious manifestation of its dysfunction, is expressed in the array of disorders that affect children. Fortunately, the sophistication of diagnosis rivals that in any other area of molecular, genetic, immunologic, or physiologic branch of neuroscience, and many of the various disorders of neuromuscular transmission are associated with specific and successful treatment.
