SYNAPTIC DIFFERENTIATION OF A SINGLE MOTOR-NEURON - CONJOINT DEFINITION OF TRANSMITTER RELEASE, PRESYNAPTIC CALCIUM SIGNALS, AND ULTRASTRUCTURE

The opener muscle in the walking legs of the crayfish (Procambarus cla rkii) is innervated by only one excitatory motor neuron, yet excitator y postsynaptic potentials (EPSPs) of proximal fibers are eightfold lar ger than those of central muscle fibers at low frequencies of activati on, due in large measure to differences in presynaptic properties, We investigated quantal release properties, calcium signals, and ultrastr ucture of presynaptic terminals to elucidate factors that could accoun t for the physiological differences, Focal macropatch electrodes were placed over individual visualized terminal varicosities to obtain reco rds of quantal contributions to the excitatory junctional current (EJC ), At low frequencies of activation, mean quantal content is greater f or proximal than for central varicosities, This difference is due to a higher probability of release per synapse, and not to a larger number of active synapses, Redorded varicosities were labeled with fluoresce nt beads deposited by the electrode, These beads adhered to the muscle fibers, outlining the recorded site for subsequent serial thin sectio ning and reconstruction from electron micrographs, Comparisons of stru cture and function were made for individual varicosities, The number o f active zones per terminal surface area and the number of synapses wi th multiple active zones (complex synapses) were greater in high-outpu t varicosities, Calcium indicators were loaded into proximal and centr al nerve terminals by axonal injection to compare the relative differe nces in calcium buildup during stimulation, Presynaptic calcium signal s were larger for proximal varicosities than for central varicosities, Since the number of synapses per varicosity is about the same, the di fference could arise from a larger number of responsive calcium channe ls per synapse in proximal varicosities, from possible differences in calcium buffering mechanisms, or from more pronounced depolarization o f proximal terminals.