An investigation of dendritic delay in octopus cells of the mammalian cochlear nucleus

Abstract

Octopus cells, located in the mammalian auditory brainstem, receive their excitatory synaptic input exclusively from auditory nerve fibers (ANFs). They respond with accurately timed spikes but are broadly tuned for sound frequency. Since the representation of information in the auditory nerve is well understood, it is possible to pose a number of questions about the relationship between the intrinsic electrophysiology, dendritic morphology, synaptic connectivity, and the ultimate functional role of octopus cells in the brainstem. This study employed a multi-compartmental Hodgkin-Huxley model to determine whether dendritic delay in octopus cells improves synaptic input coincidence detection in octopus cells by compensating for the cochlear traveling wave delay. The propagation time of post-synaptic potentials from synapse to soma was investigated. We found that the total dendritic delay was approximately 0.275 ms. It was observed that low-threshold potassium channels in the dendrites reduce the amplitude dependence of the dendritic delay of post-synaptic potentials. As our hypothesis predicted, the model was most sensitive to acoustic onset events, such as the glottal pulses in speech when the synaptic inputs were arranged such that the model's dendritic delay compensated for the cochlear traveling wave delay across the ANFs. The range of sound frequency input from ANFs was also investigated. The results suggested that input to octopus cells is dominated by high frequency ANFs.

Broadband Onset Inhibition Can Suppress Spectral Splatter in the Auditory Brainstem

“…The rate of change of the membrane voltage is calculated over one time step (10 μs ), and a spike is initiated when this value exceeds a threshold. This is a simplification of previous detailed modelling work [ 23 ].…”

General neural mechanisms can account for rising slope preference in localization of ambiguous sounds

“…We model onset behaviour abstractly as the weighted difference of two low-pass filtered signals (with different time constants). This can be interpreted either as two precisely timed excitatory/inhibitory pathways, or in a similar way to the octopus cell model of Spencer et al (2012Spencer et al ( , 2018 and Ferragamo and Oertel (2002). Their model uses a spike threshold on the rate of change of the membrane potential Brette, 2010, 2011), which we reformulated in terms of firing rates (see Methods and Appendix).…”

“…The compression and gain mechanisms did not contribute substantially ( Figure 3A). We investigated the octopus cell model discussed earlier (Ferragamo and Oertel, 2002;Spencer et al, 2012Spencer et al, , 2018 and found that it could provide a good fit only if adaptation was present alongside the onset mechanism ( Figure 3AD). For model variants including the onset mechanism, the positive or excitatory time constant τ e must be small (Figures 2A and 4B), and the negative or inhibitory time constant τ i must be larger than τ e ( Figure 3CD, τ e versus τ i ).…”

“…However, if β = 1 and τ e τ i the same equations can also be interpreted as a single onset neuron. The equations are approximately equal to a rate-based version of the octopus cell model of Spencer et al (2018), which is itself a simplification of the models of Spencer et al (2012) and Ferragamo and Oertel (2002). In their spiking model,…”

“…Proposed models of peripheral adaptation range from simulating the ringing of the basilar membrane (Tollin, 1998) to adaptation occurring at the synapse between the inner hair cells and the auditory nerve fibers (Hartung and Trahiotis, 2001;Xia and Shinn-Cunningham, 2011). Similarly, onset cells located in the cochlear nucleus (Rothman and Manis, 2003;Spencer et al, 2012Spencer et al, , 2018 are likely to play a role in discarding counfounding localization cues. On the other hand, models of the auditory system including lateral inhibitory circuits in the medial superior olive (MSO) or inferior colliculus (IC) were also able to account for some aspects of the precedence effect.…”