Spinal neurons reaching the lateral reticular nucleus as studied in the rat by retrograde transport of horseradish peroxidase

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Abstract
An anatomical technique based on the retrograde transport of horseradish peroxidase (HRP) was used to investigate the projections of spinal cord neurons to the lateral reticular nucleus (LRN). Labeled cells were found at all spinal levels and in particular large numbers in cervical and lumbar segments. Various spinal areas gave rise to cells of origin of this tract, which appears to be more prominent than any other tract previously studied with a similarapproach. Labeling common to all spinal segments was observed in (1) ventro‐medial parts of both intermodiate zone and ventral horn (laminae VII, V and X), mainy contralalerally; (2) the reticular extension of the neck of the dorsal horn partly bilateral; and (3) superficial layers of the dorsal horn and nucleus ofthe dorsolaleral funiculus (NDLF), mainly contralateral and projecting essentially to the lateral zone of the LRN. Additional labeling was observed at cervical and lumbar levels, each with specific qualities: (1) the cervical enlargement, which displayed labeling in the central part of the ipsilateral intermediate zone (lamina VII): (2) the rostral lumbar levels, which had projections from the contralateral median portion of the neck of the dorsal horn. These latter projections appear to be specific to pathways reaching the lateral reticular nucleus and the inferior olive. Control injections in neighboring structures demonstrated the similarity between the afferents to the lateral reticular nucleus and the inferior olive (except lamina I and NDLF projections) and the differences between these afferents and those projecting to the dorsal reticular formation, i.e., the nucleus reticularis ventralis.