A collateral pathway to the neostriatum from corticofugal neurons of the rat sensory‐motor cortex: An intracellular HRP study

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Abstract
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 dendrite. 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 μm), compared to that of the main axon (2.0–2.5 μm). It is concluded that these branched cells provide a parallel input to the neostriatum and to brainstem or spinal motor centers.