Melih Yayli, Ian C. Bruce
Department of Electrical and Computer Engineering, McMaster University, Hamilton, Canada
Background: Auditory nerve fibers (ANFs) tend to synchronize to low-frequency stimuli, and this synchrony is increased in bushy cells of the ventral cochlear nucleus (VCN) (Joris & Smith 2008). Synchrony enhancement in globular bushy cells (GBCs), receiving many ANF synaptic inputs, can be explained by a coincidence-detection mechanism. However, the possible mechanisms behind spherical bushy cell (SBC) synchrony enhancement remain unclear, since they receive very few excitatory inputs. Gomez-Nieto & Rubio (2011) found that bushy cells are also connected to each other via gap junctions (i.e., electrical synapses), which are known to influence synchrony in other neural systems.
Methods: We have developed biophysically detailed models of GBC and SBC microcircuits in the VCN based on Manis & Campagnola (2018), with inputs provided by the ANF model of Bruce et al (2018). The effects of broadband and narrowband inhibition, coming from D-stellate (DS) and tuberculoventral (TV) cells respectively, on synchronization enhancement are investigated, as well as the effects of gap junction conductance strength between a pair of bushy cells.
Results: Inhibition from model DS and TV cells appears to fill the gaps between peaks in firing of model ANFs, which tends to eliminate some of the spontaneous firing in SBCs caused by ANFs. This enhances synchronization in model SBCs but not GBCs. Similarly, gap junctions increase synchronization for a pair of model SBCs but not for a pair of model GBCs.
Conclusion: The results with a pair of model bushy cells connected via gap junctions suggest that both the inhibition and the gap junctions affect synchrony enhancement for SBCs but not GBCs, indicating that coincidence detection fully explains synchrony enhancement in GBCs. In ongoing work, we are increasing the number of model SBCs connected with gap junctions and are exploring effects on synchrony of different cluster structures and of the gap junction strength between these clusters.
VCN microcircuit structure of a pair of bushy cells. Green lines show the excitatory inputs originating from ANFs. Blue lines represent inhibitory inputs from DS cells while red lines are inhibitory inputs from TV cells. Yellow lines represent the gap junction connections between the bushy cells.