The neuronal architecture of the inferior colliculus in the cat: defining the functional anatomy of the auditory midbrain.

This study defines anatomical subdivisions in Golgi-impregnated material from the inferior colliculus of the cat. The findings demonstrate that the inferior colliculus consists of a mosaic of morphologically distinct parts of neuropil. Each part is also characterized by a unique set of neuronal types. Each part of the inferior colliculus can be defined as tectal or tegmental on the basis of the fundamental pattern of dendritic branching. The main subdivisions of the auditory tectum are the central nucleus, the cortex, and the paracentral nuclei. The central nucleus is distinguished by its laminated neuropil composed of neurons with disc-shaped dendritic fields oriented in parallel arrays with the lemniscal axons. In contrast, the cortex is identified by its broad layers of loosely woven neuropil, which are orthogonal to those in the central nucleus and lack neurons with disc-shaped dendritic fields. The paracentral nuclei, so called because of their scattered arrangement around the central nucleus, are the commissural, dorsomedial, rostral pole, lateral, and ventrolateral nuclei. The main subdivisions of the auditory tegmentum are the pericollicular areas, the nucleus of the brachium of the inferior colliculus, and the sagulum. The pericollicular areas are intercollicular or subcollicular and separate the tectal division from the superior colliculus, central gray, and remaining portions of the tegmentum. The afferent projections to each tectal and tegmental subdivision, as observed in silver-degeneration experiments, distinguish the parcellations based on the Golgi findings. Subdivisions containing tectal cell types receive afferents predominantly from the auditory pathways, in contrast to subdivisions with tegmental cell types, which receive inputs from a wide variety of sources. This suggests a correlation between neuronal types and the nature of their inputs. This analysis of the subdivisions of the inferior colliculus differs from previous studies, especially those relying on Nissl stains. It is likely that subdivisions distinguished by the pattern of the neuropil differ functionally, since the structural components identified in the Golgi-impregnated material are essential parts of the synaptic organization of the auditory midbrain. Future physiological studies should benefit from approaches in which the cell types serve as the focus for the analysis.