Hearing research
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A surgical approach to the cat's VIIIth nerve has been developed which allows recordings to be made from efferent fibers of the olivocochlear bundle (OCB) as well as primary afferent fibers, without compromising the acoustic responsiveness of either ear. The designation of OCB fibers as those with regular interspike intervals was confirmed in five cases by intracellular labeling with horseradish peroxidase. Labeled fibers could be traced centrally to somata in the superior olivary complex and peripherally to large endings on outer hair cells. ⋯ Binaural units (none of which were labeled) often had spontaneous discharge and were generally restricted to low-CF regions. Differences between low- and high-CF units, which cut across the monaural/binaural distinction, were seen in the dynamic range and minimum latency. Interanimal differences seen in the responses of the efferent neurons may be related to differences in the level of anesthesia.
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The current distribution of bipolar electrodes implanted into the scala tympani of the cat cochlea was investigated using a two-electrode masking technique. Two electrode masking is a non-invasive technique which requires two electrically independent electrodes and relies upon the forward masking of the electrically evoked brainstem response to a probe stimulus by that of a preceding test stimulus. ⋯ Model results have shown good agreement with electrophysiological results. Application of the model confirmed the scala tympani length constant within the basal turn of the cochlea to lie between 3 and 4 mm.
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The responses of single auditory nerve (AN) fibers in the cat were recorded in response to 25 ms tone pips. Peristimulus time histograms (PSTH) of discharge patterns recorded from fibers with high spontaneous rates (high SRs), show that the discharge rate rapidly adapts to a much lower steady-state level over a 15 ms period with shorter times for units with best frequencies (CFs) greater than 5 kHz. The PSTHs of auditory nerve fibers with low SRs do not show this pattern of rapid adaptation. ⋯ The high SR population shows a number of responses to transients which are weak or absent in the low SR population. Increasing the recovery time shortened the latency of both high and low SR AN fibers by as much as 1 ms. A number of other response properties of AN fibers are also reported that are important when interpreting the responses of cochlear nucleus neurons to tone pips.
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Single afferent and efferent neurones in the guinea pig spiral ganglion were injected with horseradish peroxidase. They could be recovered in subsequent histological processing and traced from the injection site in the ganglion to their final termination in the organ of Corti. ⋯ This cell was non-spiking and unresponsive to acoustic stimulation. Neurones having properties previously attributed to cochlear efferents, terminated on the outer hair cells in regions of the cochlea consistent with their characteristic frequencies.
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Physiological properties of the electrically stimulated auditory nerve. II. Single fiber recordings.
Single fiber recordings from the electrically stimulated auditory nerve yield post-stimulus time (PST) histograms demonstrating several response patterns. With pulsatile stimulation of the cochlea, the PST histogram for most fibers at threshold consists of a long-latency (500-800 microseconds), broad response peak with significant latency variability. At increased stimulus intensities, the response pattern changes to a short-latency (300-500 microseconds), high-synchrony peak. ⋯ It is postulated that threshold excitation of normal auditory neurons occurs on the dendritic processes. At higher stimulus intensities, the site of excitation appears to shift to the axonal region of the cells. Two additional response patterns to electrical stimulation which are attributed to synaptic excitation of the auditory neurons via the hair cells are described.