Age-related loss of activity of auditory-nerve fibers

Abstract

1. Characteristic frequencies (CF), spontaneous rates (SR), and thresholds were recorded from single fibers in the auditory nerves of gerbils aged for 36 mo in a quiet vivarium. The data from the quiet-aged animals were compared with similar data obtained previously from young controls. Fibers were classified as "low-SR" if their spontaneous rates were < or = 18 spikes/s and "high SR" for higher rates. 2. For CFs > 6 kHz, the percentage of low-SR fibers contacted declined from 57% of the population in young gerbils to 29% in the aged gerbils. This population change is statistically significant (P < 0.01). At CFs < 6 kHz, the population demographics did not change significantly with age, with the low-SR fibers comprising 30 and 39% of the population, respectively, for the young and aged animals. 3. To further test the hypothesis that low-SR fibers with CFs > 6 kHz become less active with age, additional experiments were conducted to examine the recovery of the compound action potential (CAP) response from prior high-level stimuli. Previous work has shown that the CAP recovery curve has two segments: a fast segment associated with the high-SR fibers and a slow segment associated with the low-SR fibers. The curves obtained from quiet aged gerbils show a faster recovery than young controls for probe tones at 8 and 16 kHz, but not at 2 and 4 kHz. Thus these results agree with our single-fiber data indicating that there is a loss of low-SR activity for CFs > 6 kHz in the aged animals. 4. Low-SR fibers typically have larger dynamic ranges than those of high-SR fibers, are better able to preserve information concerning stimulus timing and amplitude modulation, and their responses are more robust in the presence of masking noise. Moreover, low-SR fibers are likely inputs to the crossed-olivocochlear reflex, a reflex that serves an antimasking role in the detection of sounds in a binaural noise field. If true for humans, the loss of the low-SR system could explain many of the hearing deficits often seen in older individuals; e.g., decreased ability to understand speech in noise, changes in masking level differences, and decreased ability to localize sound sources using binaural cues.

Age-Related Changes in the Inhibitory Response Properties of Dorsal Cochlear Nucleus Output Neurons: Role of Inhibitory Inputs

“…Aged rodents exhibit a number of age-related peripheral auditory changes, including a sloping lowfrequency loss of outer-hair cells and a small loss of apical and basal inner-hair cells (for review, see Willott, 1991;Saitoh et al, 1994;Gratton et al, 1996Gratton et al, , 1997Spongr et al, 1997;Ingham et al, 1999; and auditory nerve fiber loss (Keithley et al, 1989(Keithley et al, , 1992Dazert et al, 1996;Schmiedt et al, 1996). As in the present study, decreased acoustic nerve input activity into the DCN resulted in neurochemical evidence of a downregulation of putative glycinergic inhibition (Potashner et Figure 3.…”

Chronic Reduction of Endocochlear Potential Reduces Auditory Nerve Activity: Further Confirmation of an Animal Model of Metabolic Presbyacusis

“…Auditory afferent fibers with spontaneous rates higher than 18 spikes per second have low thresholds to stimuli, whereas fibers with lower rates have higher thresholds. Similar SR-based functional subdivisions of auditory nerve fibers have now been reported in other mammalian species including chinchilla (Salvi et al 1982;Relkin and Pelli 1987;Frisina et al 1996), guinea pig (Winter et al 1990), gerbil (Schmiedt 1989;Schmiedt et al 1996;Ohlemiller and Echteler 1990;Ohlemiller et al 1991;Ohlemiller and Siegel 1994), and mouse (Taberner and Liberman 2005). Older gerbils show a loss of activity in low-SR auditory nerve fibers at high CFs .…”

“…The CAP is an evoked population response that can be free from the sampling errors that may be present in single-fiber recording procedures. The recovery of CAP responses after a conditioning tone is an effective method for measuring the population ratio of active low-and high-SR fibers (Murnane et al 1998;Schmiedt et al 1996;Lang et al 2002;Taberner and Liberman 2005). Low-SR fibers recover from prior stimulation more slowly than high-SR fibers, yielding two components in CAP recovery curves.…”

Primary Neural Degeneration in the Guinea Pig Cochlea After Reversible Noise-Induced Threshold Shift

“…4C), the clear-cut segregation of ribbon morphologies on the pillar vs. modiolar faces of the normal IHC reflects the segregation of high-and low-threshold fiber subtypes, respectively. If the high-threshold subtypes, i.e., those with low-and medium-spontaneous rates that normally comprise 40% of the type I fiber group, were selectively lost, as has been suggested in aging gerbil ears (Schmiedt et al 1996), CAP or ABR thresholds should be unchanged, yet suprathreshold amplitudes should be proportionately decreased. The morphological data obtained in the present study did not reveal a differential pattern of ribbon loss on the two sides of the hair cell; however, ribbon position and ribbon morphologies were also clearly altered by the trauma.…”