BackgroundMammalian and avian auditory hair cells display tonotopic mapping of frequency along the length of the cochlea and basilar papilla. It is not known whether the auditory hair cells of fishes possess a similar tonotopic organization in the saccule, which is thought to be the primary auditory receptor in teleosts. To investigate this question, we determined the location of hair cell damage in the saccules of goldfish (Carassius auratus) following exposure to specific frequencies. Subjects were divided into six groups of six fish each (five treatment groups plus control). The treatment groups were each exposed to one of five tones: 100, 400, 800, 2000, and 4000 Hz at 176 dB re 1 μPa root mean squared (RMS) for 48 hours. The saccules of each fish were dissected and labeled with phalloidin in order to visualize hair cell bundles. The hair cell bundles were counted at 19 specific locations in each saccule to determine the extent and location of hair cell damage. In addition to quantification of anatomical injury, hearing tests (using auditory evoked potentials) were performed on each fish immediately following sound exposure. Threshold shifts were calculated by subtracting control thresholds from post-sound exposure thresholds.ResultsAll sound-exposed fish exhibited significant hair cell and hearing loss following sound exposure. The location of hair cell loss varied along the length of the saccule in a graded manner with the frequency of sound exposure, with lower and higher frequencies damaging the more caudal and rostral regions of the saccule, respectively. Similarly, fish exposed to lower frequency tones exhibited greater threshold shifts at lower frequencies, while high-frequency tone exposure led to hearing loss at higher frequencies. In general, both hair cell and hearing loss declined as a function of increasing frequency of exposure tone, and there was a significant linear relationship between hair cell loss and hearing loss.ConclusionsThe pattern of hair cell loss as a function of exposure tone frequency and saccular rostral-caudal location is similar to the pattern of hearing loss as a function of exposure tone frequency and hearing threshold frequency. This data suggest that the frequency analysis ability of goldfish is at least partially driven by peripheral tonotopy in the saccule.

As a universal, evolutionarily conserved phenomenon, sleep serves many roles, with an integral role in memory. This interplay has been examined in a variety of research. The purpose of this article will be to review the literature of sleep, aging, cognition, and the impact of two common clinical conditions (obstructive sleep apnea and insomnia) on cognitive impairment. This article will review data from meta-analyses, population studies, smaller cohort studies, neuropsychological studies, imaging, and bench data. Considerations are given to the current data trends and their limitations. This paper will explore the impact of sleep on cognitive impairment. Finally, we will conclude with integrating the separate mechanisms towards more generalized common pathways: disruption of sleep quality and reduction in sleep quantity lead to excessive neuronal activity without sufficient time for homeostasis. Sleep apnea and chronic insomnia can lead to oxidative stress and neuronal damage. These changes predispose and culminate in the development of cognitive impairment.

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