Descending inputs to the octopus cell area of the cat cochlear nucleus: an electron microscopic study

Large, unilateral lesions of the superior olivary complex (SOC) were made in 18 adult cats. Terminal degeneration was studied electron micrsocopically in the octopus cell area (OCA) of the caudal cochlear nuclei both ipsilateral and contralateral to lesions, after 1 to 14 postoperative days. Three synaptic types (OCA types 1, 2, and 3) have been previously described upon octopus cell somas and dendrites and types 1 and 2 identified as cochlear in origin. The present study shows a new synaptic ending (OCA type 4) on small octopus cell dendrites as well as dendrodendritic contacts. Following SOC ablations, type 4 endings degenerated in the OCA ipsilateral to the lesion. In the COA contralateral to the same lesion, however, degeneration was found in type 3 terminals ending upon more proximal octopus cell dendrities and upon somas. Ipsilateral terminal degeneration occurred between two and four postoperative days, was rare by seven days, and was gone by 14 days after these lesions. However, contralateral terminal degeneration was rare until four days, was most abundant after seven days, and was still present after 14 postoperative days. The different synaptic types and time courses of degeneration in the ipsilateral versus the contralateral OCA, suggested that type 4 endings originate from an ipsilateral source, such as the lesioned periolivary region, while type 3 endings originate from the contralateral SOC or from higher contralateral nuclei. Other evidence for these sources and possible functions of these descending inputs are briefly discussed.