ACTIVITY-RELATED FEATURES OF SYNAPSE MORPHOLOGY - A STUDY OF ENDBULBSOF HELD

The myelinated fibers of the auditory nerve can be divided into two se parate populations on the basis of sensitivity to sound, average level s of spike activity, and central branching patterns. The synaptic endi ngs of these populations were investigated for the presence of structu ral specializations that might correlate with levels of neural activit y. We applied intracellular recording and staining methods in cats to analyze directly the relationship between spike activity and the struc ture of synapses using endbulbs of Held, the large synaptic endings in the anteroventral cochlear nucleus. Endbulbs from fibers having low o r high levels of activity were examined and compared using light and e lectron microscopic methods. All endbulbs exhibited relatively large b ut incomplete coverage by one-to-several lamellae of glial processes. Endbulbs of high activity fibers were large and contained larger mitoc hondria than endbulbs of low activity fibers. Furthermore, the synapse s of high activity endbulbs were on average smaller but more numerous, possessed greater numbers of associated synaptic vesicles, and exhibi ted greater curvature of their postsynaptic densities. These structura l features are hypothesized to reflect specializations that optimize s ynaptic transmission. (C) 1996 Wiley-Liss, Inc.