Cortical cells projecting to the dorsal column nuclei of cats. An anatomical study with the horseradish peroxidase technique

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Abstract
The retrograde transport of horseradish peroxidase (HRP) has been used to label cortical neurons which give origin to descending projections to the dorsal column nuclei (DCN) in kittens and adult cats. HRP was injected unilaterally or bilaterally into the dorsal medulla at, or rostral to, the level of the obex, and, in most cases, retrogradely transported HRP was visualized in cortical neurons by incubation of serial 40 μm coronal or sagittal frozen sections. In the remaining cases, 1 mm coronal or sagittal slabs through the anterior part of the cerebral hemisphere were cut and incubated en bloc, and embedded in celloidin for sectioning (40 μm). HRP‐positive neurons identifiable as the source of cortical projections to the DCN were layer V pyramidal cells whose largest diameter ranged from 20 to 45 μm: no “giant” cells were labelled. The cells appeared concentrated in the fore‐and hindlimb regions of the sensorimotor cortex and, to a lesser extent, in the second somatosensory area contralateral to the injected side. Labelled neurons were especially numerous in the upper bank of the cruciate sulcus and in the medial wall of the posterior sigmoid gyrus which respectively form parts of areas 4 and 3a (Hassler and Muhs‐Clement, '64). The number of labelled neurons progressively diminished in the first somatosensory area proper (areas 3b, 1 and 2). Ipsilaterally, fewer labelled neurons were present, mainly in areas 4 and 3a. These results are in general agreement with previous anatomical and electrophysiological studies. It is suggested that previous results based upon lesions or electrical stimulation of the cerebral cortex in cats have failed to reveal adequately the cortical regions containing the bulk of the cells projecting to the DCN, since these regions have escaped selective experimental manipulation due to their relative inaccessibility.