Neuronal organization of the cochlear nuclei in alligator lizards: a light and electron microscopic investigation

The organization of neurons and fibers in the cochlear nuclei of the alligator lizard (Gerrhonotus multicarinatus) was examined with light and electron microscopy. In this species, much is known about the anatomy and physiology of the inner ear including the cochlear nerve, but little is known about the synaptic connections of cochlear fibers on second-order neurons. These data will help to develop general principles addressing the cellular organization of the vertebrate auditory system. Subdivisions of the cochlear nuclei were defined on the basis of their histologic appearance and neuronal composition. Neuron classes were proposed from their light microscopic and ultrastructural features. Nucleus magnocellularis medialis consists of a homogeneous population of neurons called "lesser ovoid" cells. Nucleus magnocellularis lateralis consists of "greater ovoid" and "small" cells. Nucleus angularis lateralis consists of "spindle" cells. Lastly, nucleus angularis medialis contains a population of large neurons called "duckhead" and "multipolar" cells, and a population of smaller neurons called "bulb" and "agranular" cells. These neuron populations are differentially innervated by tectorial and free-standing cochlear fibers that are associated with separate frequency ranges. All neuronal populations except agranular cells were observed to receive synaptic input from cochlear nerve fibers. In nucleus magnocellularis medialis and nucleus angularis medialis, primary afferents form both chemical and electrical synapses with resident neurons. These observations imply that acoustic information is synaptically processed in fundamentally distinct ways in the cochlear nuclei of alligator lizards and distributed along separate neural circuits. Thus, the characteristic structural and functional dichotomy of the alligator lizard inner ear is extended to central auditory pathways by way of cochlear nerve projections.