CORTICOSTRIATAL PROJECTIONS FROM LAYER V-CELLS IN RAT ARE COLLATERALSOF LONG-RANGE CORTICOFUGAL AXONS

Corticostriatal projections arising from the infragranular layers of t he motor and second somatosensory cortices were studied in rats after labeling small pools of neurons with biocytin. Camera lucida reconstru ction of 263 fibers arising from laminae V and VI revealed that all co rticostriatal projections derive from collaterals of lamina V cells wh ose main axons descend into the cerebral peduncle. In contrast, lamina VI cells do not branch upon the striatum, but upon the thalamus. Toge ther with the results obtained in previous tracing studies, the presen t data raise the possibility that no neuron is exclusively corticostri atal. We therefore propose that all corticostriatal projections are co llaterals given off by the axons of two types of neurons: layer V cell s whose main axon project to the brainstem and/or spinal cord, and lay er III cells that project to the contralateral hemisphere.