Regenerated synaptic terminals on a crayfish slow muscle identify with transplanted phasic or tonic axons

Phasic or tonic nerves transplanted onto a denervated slow superficial flexor muscle in adult crayfish regenerated synaptic connections that displayed large or small excitatory postsynaptic potentials (EPSPs), respectively, suggesting that the neuron specifies the type of synapse that forms (Krause et al., J Neurophysiol 80:994-997, 1998). To test the hypothesis that such neuronal specification would extend to the synaptic structure as well, we examined the regenerated synaptic terminals with thin serial section electron microscopy. There are distinct differences in structure between regenerated phasic and tonic innervation. The phasic nerve provides more profuse innervation because innervation sites occurred more frequently and contained larger numbers of synaptic terminals than the tonic nerve. Preterminal axons of the phasic nerve also had many more sprouts than those of the tonic nerve. Phasic terminals were thinner and had a lower mitochondrial volume than their tonic counterparts. Phasic synapses were half the size of tonic ones, although their active zone-dense bars were similar in length. The density of active zones was higher in the phasic compared with the tonic innervation, based on estimates of the number of dense bars per synapse, per synaptic area, and per nerve terminal volume. Because these differences mirror those seen between phasic and tonic axons in crayfish muscle in situ, we conclude that the structure of the regenerated synaptic terminals identify with their transplanted axons rather than with their target muscle. Therefore, during neuromuscular regeneration in adult crayfish, the motoneuron appears to specify the identity of synaptic connections.