Crustacean phasic and tonic motor neurons.

Crustacean motor neurons subserving locomotion are specialized for the type of activity in which they normally participate. Neurons responsible for maintained activity ('tonic' neurons) support moderate to high frequencies of nerve impulses intermittently or continuously during locomotion, while those recruited for short-lasting rapid responses ('phasic' neurons) generally fire a few impulses in a rapid burst during rapid locomotion and are otherwise silent. The synaptic responses of the two types, recorded at their respective neuromuscular junctions, differ enormously: phasic neurons exhibit much higher quantal release per synapse and per muscle fibre, along with more rapid synaptic depression and less short-term facilitation. We have analyzed the factors that are responsible for the large difference in initial release of neurotransmitter. Several possibilities, including synapse and active zone size differences, entry of calcium at active zones, and immediately releasable vesicle pools, could not account for the large phasic-tonic difference in initial transmitter output. The most likely feature that differentiates synaptic release is the sensitivity of the exocytotic machinery to intracellular calcium. Molecular features of the phasic and tonic presynaptic nerve terminals are currently under investigation.