Participation of periolivary neurons in the auditory brainstem circuitry. A Golgi, tracer and degeneration study.

Neurons in the periolivary areas of cat were described using Golgi methods and HRP labeling, and spineless neurons with distant projecting axons and spinous with local ramifying axons were distinguished. Spineless neurons fell into two classes, fusiform and multipolar cells characterised by the large number of terminals of various size, shape, vesicle content and membrane attachment articulating with them and both were identified as projection cells by retrograde tracer. They were, however, distinguished by the shape and orientation of their perikarya, the number, direction, branching pattern and composition of the presynaptic complement of their dendrites and their numerical occurrence and distribution among the periolivary areas. Spinous neurons had multipolar somata, wavy spinous dendrites spherically displayed and local, profusely ramifying axon arbor. Their somata and dendrites had few synaptic contacts with afferents, their axonal terminals escaped degeneration and since they failed to get labelled retrogradely from external sources they were considered as local interneurons. Based on morphological criteria and experimental results four types of axons were described. The large terminals with asymmetric membrane contacts and round vesicles took origin from the cochlear nuclei, and terminals of symmetrical membrane specialisation with pleomorphic vesicle content were traced from the inferior colliculus. Small profiles containing ovoid vesicles were considered as terminals of interneurons and the few, with round vesicles as local axoncollaterals.