Integration of Synaptic and Intrinsic Conductances Shapes Microcircuits in the Superior Olivary Complex

Abstract

The superior olivary complex is a group of interconnected brainstem nuclei that receive and integrate binaural auditory input. Each nucleus forms part of local microcircuits subserving multiple complimentary roles in auditory processing, including sound localization, detection of signals in noise, and gap detection. The three nuclei of the trapezoid body (medial, lateral, and ventral) provide indirect inhibitory local projections that are integrated with direct excitatory inputs from the cochlear nuclei at the three output nuclei (the medial and lateral superior olivary nuclei and the superior paraolivary nucleus). Each nucleus expresses a different spectrum of ionic conductances that determine the intrinsic excitability of their principal neurons and adapt how the microcircuit integrates the binaural excitatory and inhibitory synaptic inputs. Specialized synapses, such as the calyx of Held, help maintain temporal information and minimize jitter, while the location of synapses on specific dendrites or somatic regions provides further refinement of the microcircuit. This chapter also includes how the principal neurons of each nucleus express differing densities of ionic conductance by which they exhibit a unique threshold, action potential waveform, and characteristic firing properties. A broad perspective will be provided on how each of these functional elements come together to sculpt the local neuronal microcircuit into performing specific physiological roles for interaural timing discrimination, interaural level discrimination, and gap detection.