CALCIUM CHANNELS IN THE NEUROMUSCULAR-JUNCTION

This chapter describes the calcium channels in the neuromuscular junction. Calcium channels—forming functional pores in excitable cell membranes—are closed in the normal (resting) state; when opened, Ca flows passively through the channels along the electrochemical gradient. There are two different types of Ca channels—(1) voltage-dependent channels and (2) receptor-dependent channels. Various kinds of voltage-dependent Ca channels (L, T, N, and P), differing in voltage sensitivity, kinetic properties, and pharmacological sensitivies, can be assumed to be involved in presynapsis of the neuromuscular junction. The neuromuscular junction or motor end plate offers excellent possibilities for studying the structure, function, and interrelations of molecular anatomy with neurophysiology, in a model system where all the important ingredients of a neuronal junction are present. Ca channels operate in unison with acetylcholine in the neuromuscular junction. It has been proposed that Ca ions, entering the axon terminal from the extracellular space at preset and ultrastructurally confined sites, termed “calcium gates,” are instrumental in stripping synaptic vesicles from their orderly arranged water coating that normally would prevent their fusion with the presynaptic membrane. © 1993, Elsevier Science Publishers, B.V.