Neurons at the origin of the medical component of the bulbopontine spinoreticular tract in the rat: An anatomical study using horseradish peroxidase retrograde transport

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Abstract
An anatomical technique based on the retrograde transport of horseradish perodidase (HRP) was used to investigate the progections of sponal cord neurions to the reticular formations in the rat. Both large and restricted injections were staggered all along the bulbar and pontine levels, involving the nucleus gigantocellularis, the nuclei reticularis pontis, pars oralis and cauladis and in some cases the nucleus raphé magnus. Labeled cells we constantly encountered in the reticular part of the neck of the dorsal horn throughout the whole length of the cord, mainly contralateral to the central core of the injection iste. This area was taken as the equivalent of lamina V in the cat. Other labeled cells were observed in the medial arts of the intermediate and ventral horns, in areas considered similar to laminae VII and VIII in the cat. The two most rostral cervical nating form the dorsolateral part of ventral horns. Thus, this study is a clear confirmation that the bulbopontine reticular formations constitute a target for various somatosensory inputs originating in spinal cord. It demonstrates that the medial spinoreticular tract (mSRT) differs from the other main ascending tracts by the absence of projections from (1) superficial layers and nucleus of the dorsolateral funiculus contrary to the spinomesencephalic tract; (2)ventromedial zone of the lumber dorsal horn unlike the spinothalamic tact; (3) the neck of the doral hor in its medial portuion contrary to the spinoreticular component reaching the lateral reticular nucleus; and (4) central cervical nucleus and Clarke's columns, unlike the spinocerebellar tracts. The difficulty in demonstrating retrograde labeling from discrete injections coulde result from the fact that mSRT neu‐rons have sparsely ramified collaterals on their terminal zones.