Identification of cells of origin of non-primary afferents to the dorsal column nuclei of the cat

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Abstract
In order to identify cells of origin in the spinal cord of non-primary afferents to the dorsal column nuclei (DCN), the retrograde transport of horseradish peroxidase (HRP) has been utilized in adult cats. 10 to 30% HRP was injected bilaterally (0.6 μl per side) in the dorsal medulla of nine cats. In most instances the spread of the injected enzyme extended a few millimeters rostrocaudally and infiltrated the DCN as well as other nuclei and fiber tracts. Labelled cells in these cases are numerous in the upper cervical, brachial and lumbosacral cord but are sparse in thoracic segments below T1. At upper cervical levels (C1-C4) HRP-positive neurons are distributed throughout the grey matter but are especially concentrated in the medial part of lamina VI. Cells projecting to the dorsal medulla are mainly localized in lamina IV and, more ventrally, along the medial border of the dorsal horn in the brachial and lumbosacral cord. Labelled cells at these levels are also scattered within lamina I and laminae VI through VIII in cases in which the focus of the injection involved extensive portions of the medulla. From cases in which bilateral HRP injections were preceded by spinal tractotomy, it appears that the axons of at least the majority of labelled cells in lamina IV ascend in the ipsilateral dorsal quadrant of the spinal cord. In another group of adult cats, 0.1 to 0.25 μl of 30 to 50% HRP was injected unilaterally in the dorsal medulla at the level of, or rostral to, the obex. With these volumes of exogenous enzyme, an intense reaction product is largely confined within the limits of the DCN. Labelled cells in these cases are found almost exclusively in the medial part of lamina VI at upper cervical levels and, at brachial and lumbosacral levels, throughout lamina IV and medially in lamina V on the side of the cord ipsilateral to the injection. The results are discussed in relation to the organization of the dorsal horn and ascending pathways with special reference to cells of origin of the spinocervical tract.