MORPHOLOGY AND SPATIAL-DISTRIBUTION OF CORTICOTHALAMIC TERMINALS ORIGINATING FROM THE CAT AUDITORY-CORTEX

In this paper we studied the morphology and spatial distribution of co rticothalamic axons and terminals originating from the auditory cortic al fields of the cat. The anterograde tracer biocytin was injected at electrophysiologically characterized loci in the primary (AI) (N = 2), anterior (AAF) (N = 1), posterior (PAF) (N = 1) and secondary (AII) ( N = 2) auditory fields. In all cases, two different types of labeled t erminals were found in the auditory thalamus: small spherical endings (1-2 mu m) and giant, finger-like endings (5-10 mu m). After biocytin injections in AI and AAF, the majority of anterogradely labeled axons terminated in the rostral half of the pars lateralis (LV) of the ventr al division of the medial geniculate body (vMGB). In LV, the corticoth alamic axons ramified profusely, giving rise to dense terminal fields forming well delineated curved stripes, with small spherical endings. Additional terminal fields formed by small endings were observed in th e medial division of the medial geniculate body (mMGB). Giant endings were observed in a small area in the dorsal nucleus (D) of the dorsal division of the medial geniculate body (dMGB), near its border with th e vMGB. PAF projections were located in the caudal half of vMGB and in mMGB, where only small terminals were found. Giant endings were seen in the superficial part of dMGB emerging from labeled corticothalamic axons oriented in parallel to the dorsal surface of the MGB. Projectio ns from AII gave rise to a main terminal field of small endings in D; a second terminal field consisting of giant endings intermingled with small endings was found in the deep dorsal nucleus (DD) of dMGB. We co nclude that small terminals serve the feedback projection to the thala mic nucleus from which the injected cortical field receives its main i nput, whereas giant terminals cross the borders between the parallel a scending auditory pathways.