Rostro-Caudal Specificity of Corticospinal Tract Projections in Mice

Rostro-caudal specificity of corticospinal tract (CST) projections from different areas of the cortex was assessed by retrograde labeling with fluorogold and retrograde transfection following retro-AAV/Cre injection into the spinal cord of tdT reporter mice. Injections at C5 led to retrograde labeling of neurons throughout forelimb area of the sensorimotor cortex and a region in the dorsolateral cortex near the barrel field (S2). Injections at L2 led to retrograde labeling of neurons in the posterior sensorimotor cortex (hindlimb area) but not the dorsolateral cortex. With injections of biotinylated dextran amine (BDA) into the main sensorimotor cortex (forelimb region), labeled axons terminated selectively at cervical levels. With BDA injections into caudal sensorimotor cortex (hindlimb region), labeled axons passed through cervical levels without sending collaterals into the gray matter and then elaborated terminal arbors at thoracic sacral levels. With BDA injections into the dorsolateral cortex near the barrel field, labeled axons terminated at high cervical levels. Axons from medial sensorimotor cortex terminated primarily in intermediate laminae and axons from lateral sensorimotor cortex terminated primarily in laminae III-V of the dorsal horn. One of the descending pathways seen in rats (the ventral CST) was not observed in most mice.