Chemically Mediated Transmission at a Giant Fiber Synapse in the Central Nervous System of a Vertebrate

摘要: The hatchetfish, Gasteropelecus, possesses large pectoral fin adductor muscles whose simultaneous contraction enables the fish to dart upwards at approach of a predator. These can be excited by either Mauthner fiber. In medulla, each fiber forms axo-axonic synapses on four "giant fibers," two side midline. Each pair giant fibers innervates ipsilateral motoneurons controlling muscles. and penetrated simultaneously microelectrodes close between them. Electrophysiological evidence indicates that transmission from is chemically mediated. Under some conditions miniature postsynaptic potentials (PSP's) are observed, suggesting quantal release transmitter. However, relatively high frequency stimulation reduces PSP amplitude below potentials, but causes no complete failures PSP's. Thus quantum size reduced or membrane desensitized. Ramp currents in rise too slowly initiate spikes evoke responses appear asynchronous Probably both ramp act same secretory mechanism. A single spike followed prolonged depression transmission; also little affected current pulses markedly alter presynaptic height. findings suggest even small releases most an immediately available store If so, probability for any transmitter within this store.