Projection from the inferior colliculus to the superior olivary complex in the albino rat.

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.