The neuronal organization of the inferior colliculus of the adult cat. II. The pericentral nucleus

Abstract

The intrinsic organization and the connections of the pericentral nucleus of the inferior colliculus have been studied with the Golgi and Nauta techniques. The pericentral nucleus is a thin sheet of densely packed cells extending over the dorsal and posterior surfaces of the inferior colliculus. The pericentral nucleus is composed of spiny and non‐spiny cells. There are large and small spiny neurons, all of which have numerous dendritic spines. The large cells are the most spiny and frequently have spines on their somata. The small spiny cells in most cases have spines on the initial segments of their axons as well as on their somata. The small cell type has a locally ramifying axon and should probably be regarded as an interneuron. The non‐spiny cell‐types are large, and multipolar or fusiform. The dendrites of the multipolar cells extend through the thickness of the pericentral nucleus and the parent cell probably gives rise to the efferents of the nucleus. Although the pericentral nucleus has previously been described as having a layered “cortical” structure, there is little evidence to justify this, since no clearly defined axonal or cellular layers are present. The pericentral nucleus receives a heavy descending projection of fibers from the auditory cortex, and an ascending projection from the upper brainstem (probably only from the dorsal nucleus of the lateral lemniscus). The ascending system of fibers may also contribute branches to the central nucleus of the inferior colliculus. The ascending and descending afferents run parallel to the surface of the pericentral nucleus but at right‐angles to each other, thus forming a rectangular grid. The possible functional significance of this grid‐like arrangement is discussed. No evidence was found for a projection from the central nucleus to the pericentral nucleus, but axons were seen to pass in the opposite direction, from the pericentral nucleus to the central nucleus, running parallel to the cellular laminae in the latter nucleus.