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

The vast majority of corticothalamic (CT) axons projecting to sensory- specific thalamic nuclei arise from layer VI cells but intralaminar an d associative thalamic nuclei also receive, to various degrees, a cort ical input from layer V pyramidal cells. It is also well established t hat all long-range corticofugal projections reaching the brainstem and spinal cord arise exclusively from layer V neurons. These observation s raise the possibility that the CT input from layer V cells may be co llaterals of those long-range axons projecting below thalamic level. T he thalamic projections of layer V cells were mapped at a single cell level following small microiontophoretic injections of biocytin perfor med in the motor, somatosensory and visual cortices in rats. Camera lu cida reconstruction of these CT axons revealed that they are all colla terals of long-range corticofugal axons. These collaterals do not give off axonal branches within the thalamic reticular nucleus and they ar borize exclusively within intralaminar and associative thalamic nuclei where they form small clusters of varicose endings. As layer V cells are involved in motor commands everywhere in the neocortex, these CT p rojections and their thalamic targets should be directly involved in t he central organization of motor programs.