Synaptic relationships of Golgi type II cells in the medial geniculate body of the cat

Abstract

A combined analysis with the Golgi and silver-degeneration methods and electron microscopy in the ventral nucleus of the medial geniculate body has confirmed that the Golgi type II neuron forms dendro-dendritic synapses with the principal neuron in terminal aggregates called synaptic nests. Both types of neurons receive synaptic contacts from the afferent axons that ascend from the posterior colliculus and from those that descend from the auditory cortex. Only the principal neuron projects to the auditory cortex. The Golgi type II cells that receive endings from afferent axons send presynaptic processes to principal cells that are also contacted by the very same afferent axons. The axons of Golgi type II cells project to synaptic nests other than those supplied by the dendrites of the parent cell and link the Golgi type II cells with each other. On the surface of the Golgi type II cell there is a segregation of the different types of synaptic endings and a consistent sequence in their synaptic relationships. The endings of colliculogeniculate and Golgi type II axons predominate on the distal dendrites in the synaptic nests. Corticogeniculate endings congregate more on the soma and proximal dendrites. In the synaptic nests the Golgi type II dendrites are presynaptic to the principal cell dendrites, whereas both kinds of dendrites are postsynaptic to the very same axons, which project either from the posterior colliculus or from Golgi II cells. The endings of the Golgi type II dendrites have pale, irregular, often quite big profiles that contain large pleomorphic vesicles and make nearly symmetrical synaptic contacts. The synaptic endings of the Golgi type II axons could resemble the dendritic endings, but many of the axonal endings appear to have smaller profiles containing smaller, flatter vesicles. The endings of the colliculogeniculate axons are generally darker, often contain smaller, rounder synaptic vesicles than the dendritic endings do, and make strongly asymmetric synaptic contacts. The terminals of the corticogeniculate axons resemble the smallest colliculogeniculate endings but have darker profiles and contain some flattened vesicles. These identifications are consistent with electron microscopic observations of the synaptic degeneration following destruction of the sources of the afferent axons in the posterior colliculus and cerebral cortex. The segregation of inputs on the Golgi type II cell implies that its axon would be influenced more directly by the corticogeniculate pathway, while the dendrites of the same cell would be more directly accessible to the colliculogeniculate pathway. The Golgi type II cells might be either inhibitory or excitatory interneurons or both. As inhibitory interneurons, Golgi type II cells could provide for response patterns that would render the principal neurons particularly sensitive to stimulus onsets and to temporal changes in the afferent input. As excitatory interneurons, Golgi type II cells could provide for incremental or sustained response patterns that would tend to prolong particular signal trains. In either case the synaptic relationships of the Golgi type II cells would be critical in defining the spatial and temporal aspects of stimulus coding involved in the cortical analysis of temporal patterns and locations of sounds.