STRONG AND WEAK SYNAPTIC DIFFERENTIATION IN THE CRAYFISH OPENER MUSCLE - STRUCTURAL CORRELATES

The single exciter motoneuron to the limb opener muscle in the crayfis h Procambarus clarkii provides multiterminal innervation to individual muscle fibers. At low impulse frequencies, these neuromuscular synaps es generate a threefold larger junctional potential in fibers of the p roximal region of the muscle compared to those in the central region. Focal extracellular recording from synapse-bearing ''boutons'' showed more quantal release at low frequencies in the proximal region. Struct ural correlates for the physiological differences were sought. Fluores cence microscopy of surface innervation stained with a vital fluoresce nt dye, 4-Di-2-Asp, showed that density of innervation was not greater in the proximal region and thus could not account for the overall dif ferences in synaptic strength. Freeze fracture studies showed that the intramembrane organization of excitatory synapses and their active zo nes was qualitatively similar in proximal and central sites. Serial se ction electron microscopy of several innervation sites in proximal and central regions showed homogeneity in number and size of synapses. Ho wever, presynaptic dense bars (at release sites, or active zones) were longer and occurred at a higher density in proximal than in central s ynapses. The differences in number and length of presynaptic dense bar s correlate positively with the differences in synaptic strength repre sented by junctional potential amplitudes and quantal contents of indi vidual surface recording sites. Since many individual proximal synapse s have multiple dense bars, co-operativity among these may serve to en hance transmitter output. It is concluded that occurrence of dense bar s is a significant presynaptic correlate of synaptic strength in this neuron. (C) 1994 Wiley-Liss, Inc.