The neuronal organization of the inferior colliculus of the adult cat. I. The central nucleus

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Abstract
The intrinsic organization of the central nucleus of the inferior colliculus has been studied in the adult cat by means of the Golgi and Nauta techniques. On the basis of cytoarchitecture and fiber connections, the central nucleus can be divided into: (i) a smaller, dorsomedial division consisting mainly of large cells and receiving fibers from the auditory cortex, lateral lemniscus and probably the central nucleus of the other side; (ii) a larger, ventrolateral division consisting mainly of medium and small cells but with some intermingled larger types. This part receives fibers from the lateral lemniscus only.The ventrolateral division has a pronounced laminar arrangement of cells, dendrites and axons. The laminae, although overlapping extensively, form an onion‐like series of concentric, curved shells, most of which are incomplete except for those closest to the center of curvature in the dorsolateral part of the nucleus. It is probable that these laminae provide a basis for the pronounced tonotopic organization of neurons in the nucleus. The thickness of the laminae is determined by the dendritic ramifications of two principal cell types which are fusiform or bi‐tufted. Other, multipolar and large cells have dendrites which lie across one or several laminae and may form a basis for interaction between laminae.Four axon types can be distinguished: Type I (lateral lemniscal) and type II (corticofugal) axons run parallel to the laminae but in opposite directions, though type II axons are confined to the parts of the laminae projecting into the large‐celled, dorsomedial division. Type III axons are widely ramifying and may arise within the central nucleus or be another form of afferent fiber. Type I and III axons end in dense clusters of terminals on principal cells but as single en passant terminals on multipolar cells. Type IV axons are the efferents of the inferior colliculus and run a recurrent course through the central nucleus giving off collaterals which sometimes end on the parent cell.