The nerve-muscle synapse of the garter snake

Snake nerve-muscle preparations are well-suited for study of both motor innervation patterns at the systems level and NMJ function at the cellular level. Their small size ( approximately 100 myofibers) and thinness (one fiber) allows access to all NMJs in one muscle. Snake NMJs are of three types, two twitch subtypes and a single tonic type. Properties of the NMJs supplied by a particular motor neuron, and of the motor unit fibers they innervate, are precisely regulated by the motor neuron in a manner consistent with the Henneman Size Principle. Unlike its amphibian or mammalian cousins, the snake NMJ comprises approximately 50 (twitch) or approximately 20 (tonic) individual one-bouton synapses, similar to synapses found in the central nervous system. Each bouton releases a few quanta per stimulus. Larger fibers, which require more synaptic current to initiate contraction, receive nerve terminals that contain more boutons and express receptor patches with higher sensitivity to transmitter. Quantal analysis suggests that transmitter release sites in one bouton do not behave independently; rather, they may cooperate to reduce fluctuations and enhance reliability. After release, two mechanisms coexist for retrieval and reprocessing of spent vesicles-one involving clathrin-mediated endocytosis, the other macropinocytosis. Unanswered questions include how each mechanism is regulated in a use-dependent manner.