The electrotonic structure of regular-spiking neurons in the ventral cochlear nucleus may determine their response properties

1. Intracellular recordings were obtained from neurons in parasagittal brain slices of the guinea pig ventral cochlear nucleus (VCN). The principal neurons of the VCN can be parceled into two categories. Regular-spiking (Type I) neurons have a linear current-voltage (I-V) relationship over a large range of intracellularly injected currents and fire tonically in response to suprathreshold depolarizing currents. Phasically spiking (Type II) neurons have a nonlinear I-V relationship and fire only phasically at the onset of a depolarizing current or offset of a hyperpolarizing current. Regular-spiking neurons have been shown to be of the stellate morphological type, whereas phasically spiking neurons have been shown to be bushy cells. 2. The electrotonic structure of regular-spiking neurons was studied by applying previously developed modeling techniques based on the somatic shunt model. In these techniques, physiological data are used to determine the set of parameters best describing the neuron. As predicted from previous theoretical investigations, the use of an anatomic constraint (somatic surface area) reduces the variance in estimates of model parameters, especially for the dendritic membrane time constant τ(D). 3. Model representations of regular- spiking cells fall into two categories: those with (passive) somatic membrane properties that are nearly identical to those of the dendrite (8/15 cases), and those with a significant amount of somatic shunt (7/15). Estimates of τ(D) (mean = 7.7 ms) are lower than those often described in the literature. We argue that this low value of τ(D) may be related to the need of neurons in the auditory brainstem to operate at high firing rates and/or to encode audio-frequency temporal fluctuations. 4. Dendritic transfer functions were calculated as functions of synaptic location using somatic shunt representations of regular-spiking neurons. These transfer functions allow us to predict that mid-range auditory frequencies (~1 kHz) are greatly attenuated, even for synapses near the soma. Thus it is suggested that the electrotonic architecture of regular-spiking cells creates sufficient low- pass filtering of synaptic inputs to reduce the synchronization of firing of these neurons to mid-frequency auditory stimuli.