![]() ![]() At the same time the fiber’s dynamic range is about 40 dB, or about a hundred fold pressure range. Most of these fibers are very sensitive, with their thresholds near 0 dB spl that correspond to sound waves which make only 20 micro Pascal pressure fluctuations. We show that when the SNIC bifurcation adds a dynamical variable that provides fast voltage-dependent negative feedback, it becomes well-suited for linear rate-coding across a wide dynamic range.Įach inner hair cell (IHC) originates about 20 ANF. We will show that this along with other evidence argues that its spike generator is based on a modified version of dynamical instability called a saddle node on invariant circle (SNIC) bifurcation. The ANF has an extremely wide output spiking range from 0 to about 300 Hz. It is this same sort of negative feedback that linearizes an op amp’s output. It is a generic property of strongly nonlinear spike generators that negative feedback is able to linearize their firing frequency as a function of input current ( f - I curve), provided that their no feedback f - I curve is sufficiently nonlinear. This result has been mathematically generalized. Previously, negative feedback has been investigated as one likely means for slowing down a spike generator’s initial rate of increase (specifically in the case of cortex pyramidal neurons ). The apparent contradiction of the ANF behaving as a sensitive but also wide range linear filter is referred to as the dynamic range problem in mammalian hearing. But in general, spike generator output is known to be intrinsically nonlinear, with spiking typically turning on abruptly when an input current threshold is passed its rate rising steeply with increasing input current –. When they use the transient frequency of their spike rate to encode the intensity of a tone, ANF function as linear filters. ANF are able to adjust to noisy environments, enabling them to more accurately rate-code loud tones. Mammalian auditory nerve fibers (ANF) respond to faint sounds by increasing the frequency of their action potentials by a small amount, but they are also able to respond to a wide dynamic range of sound inputs by making large increases in their spike rate. In a quiet environment by increasing H the olive would be able to make spike trains similar to those caused by synaptic input. We show that an olive able to decrease H would be able to shift the spike generator’s dynamic range to higher sound intensities. Also, the spike generator compartment has a cholinergic feedback connection from the olive and experiments show that such feedback is able to alter the amount of H conductance inside the generator compartment. A Poisson random source simulates an inner hair cell, outputting a series of noisy periodic current pulses to the model ANF whose spikes phase lock to these pulses and have a linear frequency to current relation with a wide dynamic range. We add negative feedback in the form of a low voltage-threshold potassium conductance that slows down the generator’s rate of increase of its spike rate. The generic 2d SNIC increases its spike rate as the square root of the input current above its spiking threshold. Here we model the spike generator as a 3 dimensional version of a saddle node on invariant circle (SNIC) bifurcation. Its spike generator “digitizes” CA output into trains of action potentials and behaves as a linear filter, rate-coding sound intensity across a wide dynamic range. While the outer hair cells of the CA employ positive feedback, poising on Andronov-Hopf type instabilities which make them extremely sensitive to faint sounds and make CA output strongly nonlinear, the ANF appears to be based on different principles and a different type of dynamical instability. These synaptic channels then charge its dendritic spike generator. Mammalian inner hair cells transduce the sound waves amplified by the cochlear amplifier (CA) into a graded neurotransmitter release that activates channels on auditory nerve fibers (ANF). ![]()
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