Functional morphology of the nerve terminal at the frog neuromuscular junction: recent insights using immunocytochemistry

This chapter describes the functional morphology of the nerve terminal in a frog neuromuscular junction by using immunocytochemistry. Acetylcholine (ACh) is secreted from motor nerve terminals by two means: by release of multimolecular packages or quanta and by molecular leakage across the presynaptic membrane. The spontaneous or neurally evoked release of quanta generates the discrete, transient miniature end-plate potentials and the endplate potentials that mediate neuromuscular transmission. As far as molecular leakage is concerned, this process can be the predominant way in which neurotransmitter is released under resting conditions. The identification within the nerve terminals of a rich population of synaptic vesicles filled with neurotransmitter lead to the interpretation that synaptic vesicles are the anatomical correlates of quanta. The most widely accepted hypothesis to explain the quantal nature of transmitter release holds that the molecular packets are preformed in the nerve terminal, each quantum being confined within one synaptic vesicle and released by exocytosis. The study of synapsin I and of the regulation of its state of phosphorylation at the neuromuscular junction may provide important information on its role in synaptic function