AXONAL ARBORIZATIONS OF CORTICOSTRIATAL AND CORTICOTHALAMIC FIBERS ARISING FROM THE 2ND SOMATOSENSORY AREA IN THE RAT

Corticostriatal and corticothalamic projections arising from the secon d somatosensory area in the rat were studied after labeling small pool s of neurons in laminae V and VI with biocytin. Tracing the axon of si ngle neurons revealed the following principles of organization: (i) al l corticostriatal and corticothalamic projections arising from layer V cells are collaterals of long-range corticofugal axons that also proj ect below the thalamic level; (ii) all layer V cells that project to t he thalamus also project to the striatum; (iii) all layer VI corticoth alamic cells project exclusively to the thalamus; (iv) cells of the up per part of lamina VI send collaterals to the thalamic reticular nucle us and arborize in the ventrobasal complex forming rod-like terminal f ields; and (v) cells of the lower part of lamina VI also send collater als to the thalamic reticular nucleus, give off few branches in the ve ntrobasal complex and terminate principally in the caudal part of the posterior thalamic group. On the basis of these findings, and in the l ight of previous anatomical studies, it is proposed that the above men tioned organizing principles represent general rules that also apply t o corticostriatal and corticothalamic pathways arising from other area s of the neocortex.