This paper defines the pattern of subdivision of the inferior colliculus in rat. It is based on serial sections of brains of albino and hooded rats cut in the frontal, sagittal and horizontal planes using Golgi, Nissl and a combined cell-myelin method. In rat, like in other mammals, the inferior colliculus consists of a central nucleus, an external cortex, and a dorsal cortex. The central nucleus is flattened in the frontal plane and confined to the caudomedial part of the inferior colliculus. It is characterized by a lamellar organization of disc-shaped neurons interspersed with multipolar cells. The cells are small to medium-sized. Although there is a dorsoventral gradient in size and packing density of cells within the nucleus, the overall size is smaller and the packing density larger than in adjacent subdivisions. The two cortices each consists of three layers. The outer-most layer is common to the two cortices, forming a fibrocellular capsule continuous along most of the circumference of the inferior colliculus. The external cortex is located lateral, rostral, ventral and ventrocaudal to the central nucleus. Its second layer, deep to the superficial capsule, is characterized by clusters of many small and a few medium-sized neurons in a myelin-dense neuropil. Layer 3, which constitutes the major portion of the subdivision, consists of relatively scattered, small, medium and large cells, the most characteristic element being large multipolar neurons with coarse Nissl granules. The dorsal cortex is located dorsocaudal and dorsomedial to the central nucleus. Its second layer is composed of small neurons, while the third, deep layer in addition contains medium-sized neurons. The cell density is intermediate to that of the central nucleus and the deep part of the external cortex. We have tried to facilitate the parcellation by reference to easily recognizable, nearby structures and to standard stereotaxic coordinates.

The origin and termination of the fibers projecting from the inferior colliculus to the superior olivary complex have been studied in rat by means of the Fink and Heimer and horseradish peroxidase techniques (anterograde and retrograde transport of free horseradish peroxidase and peroxidase conjugated to wheat germ agglutinin). The colliculoolivary fibers originate in layer 3 of the external cortex and the adjacent part of the central nucleus, particularly in the former. Via the lateral lemniscus the fibers reach the ipsilateral periolivary region where they terminate in the rostral and medioventral zones. The terminal field contains at least two types of cells which could constitute the next link in the descending projection to the cochlea and/or the cochlear nuclei. One of these is the large olivocochlear neuron, and the other a smaller neuron projecting to the cochlear nuclei. Judged by their topographic relationship in the inferior colliculus and in the superior olive, the colliculoolivary neurons may form a link in a oligosynaptic projection from the auditory cortex to the cochlea and/or the cochlear nuclei. The observations are based on light microscopy, however, and do not allow conclusions concerning synaptic contacts.

The purpose of the present study was to define the field of termination of the neocortical projection to the inferior colliculus in rat. The study was based on fiber degeneration following large lesions of the cerebral cortex, and anterograde transport of wheat germ agglutinin horseradish peroxidase ejected iontophoretically into more restricted neocortical loci. Neocortical fibers were found to supply the dorsal and external cortices of the inferior colliculus. The central nucleus, in contrast, did not receive such fibers. The results speak in favor of three separate projections, one partly bilateral to the deeper part of the dorsal collicular cortex, a second ipsilateral to the superficial part of this subdivision, and a third ipsilateral to the external collicular cortex.

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