Direct innervation of identified tectothalamic neurons in the inferior colliculus by axons from the cochlear nucleus.

The present study sought to identify tectothalamic neurons in the rat inferior colliculus that receive their innervation directly from the cochlear nuclei and to identify the axons that provide the innervation. A direct projection would bypass the binaural interactions of the superior olivary complex and provide the quickest route to the neocortex. Axons, primarily from the dorsal cochlear nucleus, were labeled with anterograde transport of dextran and terminated in the central nucleus of the inferior colliculus in a laminar pattern. Most labeled axons were thin and simply branched. Other axons were thicker, gnarly, less frequently observed and probably originated from the ventral cochlear nucleus. None had concentrated endbulbs or a nest of endings. Both types of axons terminated primarily in the central nucleus and layer 3 of the external cortex. This pattern suggests that the combination of these subdivisions in the rat are equivalent to the central nucleus as defined in other species. Tectothalamic neurons in the inferior colliculus in the same animals were identified by retrograde transport from the medial geniculate body and intracellular injection of Lucifer Yellow. A number of different cell types act as tectothalamic neurons and receive contacts from cochlear nucleus axons. These include flat cells (disc-shaped), less-flat cells and stellate cells. Two innervation patterns were seen: a combination of axosomatic and axodendritic contacts, and predominantly axodendritic contacts. Both patterns were seen in the central nucleus, but axosomatic contacts were seen less often in the other subdivisions. This is the first study to show direct connections between cochlear nuclear axons and identified tectothalamic neurons. The layers of axons from cochlear nuclei may provide convergent inputs to neurons in the inferior colliculus rather than the heavy inputs from single axons typical of lower auditory nuclei. Excitatory synapses made by axons from the cochlear nuclei on tectothalamic neurons may provide a substrate for rapid transmission of monaural information to the medial geniculate body.