A collateral pathway to the neostriatum from corticofugal neurons of the rat sensory-motor cortex: an intracellular HRP study.

A projection from large pyramidal cells in layer V of the rat somatic sensory-motor (SSM) cortex both to the neostriatum and the brainstem was demonstrated by intracellular recording and injection of horseradish peroxidase (HRP). Layer V neurons that project to the brainstem were identified either by antidromic activation from the cerebral peduncle or by tracing the HRP-labeled axon into the internal capsule in histochemically processed sections. Intracellular responses to stimulation of the hindlimb, forelimb or mystacial pad were also examined. Five of 20 HRP-injected neurons that project to the brainstem had a fine collateral branch within the striatum. These branched corticostriatal cells respond at short latency (7--12 msec) to somatic sensory stimulation. All of the injected corticofugal neurons that had a striatal collateral were large pyramidal neurons located in layer Vb of the forelimb and head areas of SSM cortex. Branched corticofugal neurons have a rich basal dendritic field and a prominent apical dendrite that arborizes in the superficial cortical layers. Intracortical axon collaterals from the branched cells ramify in layers V and VI, and also project to the upper layers of cortex near the apical dentrite. Beyond the cortex, the main axon has no collateral branches, except for a single laterally directed branch in the neostriatum. The diameter of the striatal collateral is small (about 0.5 micrometer) compared to that of the main axon (2.0--2.5 micrometers). It is concluded that these branched cells provided a parallel input to the neostriatum and to brainstem or spinal motor centers.