Synaptic structure and transmitter release in crustacean phasic and tonic motor neurons

The paired phasic and tonic motor neurons supplying the extensor muscle in the crayfish leg were investigated to establish whether differences in synaptic structure could account for large differences in transmitter release at the neuromuscular junctions. Nerve terminals with transmitter release that had been assessed from recordings made with a focal "macro-patch" electrode were subsequently labeled, processed for electron microscopy, and reconstructed from serial sections. At a frequency of 1 Hz, quantal contents of phasic terminals were 90-1300 times greater than those of tonic terminals when both were recorded at the same location. At higher frequencies, facilitation was pronounced at tonic, but not phasic, terminals. Reconstructions of recording sites showed that both phasic and tonic terminals possessed many small synapses, usually with one or more structurally defined active zones. Mean synaptic contact area was larger for tonic terminals, and the number of individual synapses per length of nerve terminal was also larger. Active zones were not different in size for the two terminals. At low frequencies, quantal emission per synapse is much greater for phasic terminals. The higher quantal content of phasic terminals and their synapses cannot reasonably be accounted for by more or larger synapses or active zones at the recording sites. Because structural features alone are not likely to produce the very large differences in quantal content of phasic and tonic terminals observed at low stimulation frequencies, it is likely that other properties of the nerve terminal are largely responsible for these differences.