Outer- and Middle-Ear Contributions to Presbycusis in the Brown Norway Rat

Abstract

This paper examines the contribution of the outer and middle ears to the hearing loss associated with presbycusis in Brown Norway rats. Animals were formed into two groups; young adults (2–3 months old) and aged animals (approximately 34 months old). Auditory brainstem response (ABR) thresholds were obtained with the outer ear intact or surgically removed. Tympanic membrane (TM) velocity transfer functions were measured from the umbo with the outer ear removed. The length of the auditory meatus, TM surface area, and TM thickness were quantified. The ABR thresholds were 17–26 dB less sensitive in the aged animals between 8.0 and 40.0 kHz when the outer ear was intact. A significant and reliable reduction in the aged rat velocity transfer function of 5–8 dB occurred between 10.0 and 32.0 kHz, while the low frequency velocity response was only a few decibels greater in the younger animals. The ABR threshold differences between young adult and aged ears were compensated by removing the outer/middle ear effects of aging to reveal a purely sensorineural component of presbycusis. The outer and middle ear effects were calculated directly when the ABR and TM velocity data were obtained with the outer ear removed. The outer ear intact condition was modeled in order to compare the ABR data obtained with the outer ear intact with the TM velocity data obtained with the outer removed. With either procedure, removal of the age-related contributions of the outer and middle ear to the ABR threshold resulted in similar age-related ABR threshold shifts between the two age groups. The pure sensorineural threshold shift component of the ABR response was restricted to frequencies between 5.0 and 20.0 kHz and reached a maximum of approximately 15 dB. These results support the conclusion that there is an outer- and middle-ear contribution to the threshold loss defining presbycusis.

Recovery of functional and structural age-related changes in the rat primary auditory cortex with operant training

“…For very short interneuronal distances (0-0.4 mm), overlap values in AT were even slightly lower than Y values (P < 0.04). Modest increases in hearing thresholds predominantly for low and high frequencies as measured by auditory brainstem responses are expected in aged Brown-Norway rats (25). In this study we found similar increases at the cortical level.…”

Age‐related changes in the guinea pig auditory cortex: relationship with brainstem changes and comparison with tone‐induced hearing loss

“…, 1991; Turner & Caspary, 2005), spiral ganglion cells (Keithley et al. , 1989) and alterations of the outer/middle ear (Gratton et al. , 2008).…”

“…Studies assessing peripheral deficits by auditory brainstem responses (ABRs) revealed that, in correlation with the level of age-related hearing loss, the response strength is reduced, the amplitude-intensity function is shallower, the latencies and interpeak intervals are prolonged, and the latency-intensity function is steeper (Cooper et al, 1990;Boettcher et al, 1993a,b;Harada et al, 1999;Boettcher, 2002). These phenomena would mainly result from a degeneration of outer hair cells and, to a lesser extent, of inner hair cells (Coleman, 1976;Tarnowski et al, 1991;, spiral ganglion cells (Keithley et al, 1989) and alterations of the outer ⁄ middle ear (Gratton et al, 2008). The second potential cause of age-related deterioration in speech intelligibility is the aging of the central auditory system.…”

Morphological Study of Auditory Ossicles in the Mouse

“…Analogical descriptions of the ear structures of animals, found in veterinary anatomy textbooks, are scare, schematic and do not go beyond a rough enumeration of the structures, often with no data on topography and variability. Previous researches established the morphology of these ossicles in rodents and some laboratory animals (Kurtul et aZ., 2003;Songer and Rosowski, 2006;Gratton et al, 2008;Ravicz et al, 2008).…”