Terminal morphology and distribution of corticothalamic fibers originating from layers 5 and 6 of cat primary auditory cortex

Two types of terminations were observed on corti-cothalamic fibers arising from cells in different layers of cat auditory cortex. Injections of the anterograde tracers Phaseolus vulgaris leucoagglutinin (PHA-L) or biocytin were made into single cortical loci that in-cluded both superficial layer 5 (5a) and layer 6 in the primary auditory cortex (Al). These resulted in labeling of terminal fibers with small (~1 (tm) and large (~2 fj.m) boutons in the medial geniculate complex (MG) and the lateral nucleus of the posterior complex. Large boutons were found in the deep and superficial dorsal nuclei, in the ventrolateral nucleus, and, less frequent-ly, in the medial nucleus of the MG. They usually ended in grape-like clusters of boutons. By contrast small boutons were found densely in the pars lateralis an