Auditory nerve fibers12/9/2023 ![]() However, a typical ANF has a range of ∼40 dB between its threshold and its saturation. Part of the dynamic range problem is no doubt solved by having different classes of nerve fibers with different sensitivity ranges. The remaining ∼20% have high thresholds and low spontaneous firing rates (∼0 Hz). Most IHC have low thresholds (0–20 dB SPL) with high spontaneous firing rates of up to ∼100 Hz. ![]() Essentially, the problem is how to account for a vast range of hearing in which a very sensitive mammalian hearing apparatus is nevertheless able to rate code sound intensity across a gigantic input power range.Įach inner hair cell (IHC) sends ∼20 ANF with different sensitivity thresholds to the cochlear nucleus. This contradiction is known as the dynamic range problem in mammalian hearing. Most fibers are sensitive to very faint sounds, but at the same time still respond to a wide dynamic range of sound inputs. ANF “digitize” the information content of a sound wave into a series of parallel spike trains, with each fiber's output spike range limited to about 300 Hz. There, a graded neurotransmitter signal from an inner hair cell (IHC) is first encoded into a spike train within a small compartment in the dendrite of an ANF. Adaptive processing begins with the hair cells and auditory nerve fibers (ANF) at the periphery. Adaptive processing of sound levels is known to occur throughout the auditory pathway, and there is evidence that it results in drawing auditory attention towards a high probability region of sound intensities. But surprisingly, they are also able to distinguish variations in sound intensity at levels ∼70 dB above this sensory threshold (10 7 fold power increase). Mammals have a powerful cochlear amplifier and so are able to have very low auditory thresholds for detecting sound waves (∼0 dB SPL, corresponding to micro Pascal pressure fluctuations). The ANF spike generator remains very sensitive to threshold currents, but efferent feedback is able to lower its gain in response to noise. This way ANF are able to have a linear frequency to input current (f-I) curve that has a wide dynamic range. Input current induces rapid and proportional leak currents. We model this spike generator compartment as an attenuator that employs fast negative feedback. Inner hair cells activate currents in the unmyelinated distal dendrites of ANF where sound intensity is rate-coded into action potentials. ANF receive efferent feedback, which suggests that the fibers are readjusted according to the background noise in order to maximize the information content of their auditory spike trains. They are also able to discern sounds embedded within background noise. In this way mammals are able to combine sensitivity and wide dynamic range. Then as the sound intensity is increased, they slowly increase their spike rate, with some fibers going up as high as ∼300 Hz. Most of the fibers are very sensitive and raise their quiescent spike rate by a small amount for a faint sound at auditory threshold. Mammalian auditory nerve fibers (ANF) are remarkable for being able to encode a 40 dB, or hundred fold, range of sound pressure levels into their firing rate.
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