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 flex or muscle in adult crayfish regenerated synaptic connections that displayed large or small excitatory postsynaptic potentials (EP-SPs), 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 electr on microscopy, There are distinct differences in structure between regenera ted phasic and tonic innervation. The phasic nerve provides more profuse in nervation because innervation sites occurred more frequently and contained larger numbers of synaptic terminals than the tonic nerve. Preterminal axon s of the phasic nerve also had many more sprouts than those of the tonic ne rve. Phasic terminals were thinner and had a lower mitochondrial volume tha n their tonic counterparts. Phasic synapses were half the size of tonic one s, although their active zone-dense bars were similar in length. The densit y of active zones was higher in the phasic compared with the tonic innervat ion, based on estimates of the number of dense bars per synapse, per synapt ic area, and per nerve terminal volume. Because these differences mirror th ose seen between phasic and tonic axons in crayfish muscle in situ, we conc lude 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. (C) 2000 John Wiley & Son s, Inc.