Afferent influences on brain stem auditory nuclei of the chicken: effects of conductive and sensorineural hearing loss on n. magnocellularis.
Nucleus magnocellularis is the avian homologue of the spherical cell region of the mammalian anteroventral cochlear nucleus. Its primary excitatory synaptic input is from large end bulbs of Held from the eighth nerve ganglion cells. We have examined the effects of three peripheral manipulations--middle ear ossicle (columella) removal (monaural and binaural), columella removal and oval window puncture (monaural), and monaural earplug--on cross-sectional cell area ("cell size") of second-order auditory neurons in n. magnocellularis of the chicken. Manipulations were performed between embryonic day 19 and posthatch day 4. Survival time was varied from 2 to 60 days. Air conduction and bone conduction thresholds were determined to assess for conductive and sensorineural hearing loss associated with each of these manipulations. Hair cell counts were made from basilar papillae of each experimental group. We found that a columella removal alone, which produced a 50-55-dB purely conductive hearing loss, was not associated with changes in cell size of n. magnocellularis neurons. Similarly, chronic monaural earplugging did not affect the cross-sectional area of these second-order auditory neurons. Conversely, a combined columella removal and oval window puncture, which produced a mixed hearing loss with a 15-40-dB sensorineural component was associated with an 18-20% reduction in n. magnocellularis cell area. Hair cell counts for experimental ears were not significantly different from control ears. These results, in conjunction with measurements of multiunit activity recorded in n. magnocellularis, suggest that manipulations which markedly attenuate extrinsic auditory stimulation, but do not result in chronic change in the average activity levels, also do not influence the size of n. magnocellularis cell bodies. On the other hand, a manipulation which influences overall activity levels, but does not result in degeneration of receptor cells, resulted in marked changes in n. magnocellularis cell size.
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