Afferent influences on brain stem auditory nuclei of the chicken: Neuron number and size following cochlea removal

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Abstract
The consequences of cochlea removal on neuron number and soma cross‐sectional area were examined in the second order auditory nucleus (n. magnocellularis) of chickens. Both the age of the subjects at the time of cochlea (basilar papilla) removal (1–66 weeks) and the survival period (1–45 days) were varied. Neuron number and soma cross‐sectional area were determined from Nissl stained sections. Additional material was processed to examine the relationship of ganglion cell loss to changes in n. magnocellularis. Neuron number decreased by 25–30% and soma cross‐sectional area decreased by 10–20% ipsilateral to the cochlea removal in chickens operated on during the first 6 weeks after hatching. In contrast, in chickens operated on at 66 weeks posthatch neuron number decreased less than 10% and there was no change in soma area. The changes were rapid, being nearly complete 2 days after cochlea removal. An initial change (1 and 2 days after surgery) observed in animals operated on up to 6 weeks posthatch was the presence of a large number of neurons in which no Nissl substance could be detected. These results demonstrate an age‐dependent change in the susceptibility of NM neurons to deafferentation. This change is not temporally related to other measures of functional maturation of the auditory system.