Spinal projections from the midbrain in monkey

Abstract

Neurons descending from the midbrain to the spinal cord in the monkey were identified with the retrograde horseradish peroxidase technique. Beginning in the caudal midbrain and extending anteriorly beneath the superior colliculus, large numbers of neurons of the nucleus cuneiformis and lateral central gray were found to project ipsilaterally to the spinal cord. In the posterolateral superior colliculus, neurons of the intermediate and deep layers, stratum griseum intermediale and stratum griseum profundum, were found to give rise to contralateral projections to the upper cervical spinal segments. An ipsilateral tectospinal projection from the anteromedial part of the collicus may also exist. In the red nucleus, neurons of the magnocellular division were shown to give rise to a somatotopically organized projection to the upper cervical cord and spinal enlargements. No neurons of the parvocellular red nucleus were labeled from the spinal cord. In the anterior midbrain, neurons of the interstitial nucleus of Cajal, nucleus of Darkschewitsch, and the adjacent dorsomedial and ventromedial midbrain tegmentum were found to give rise to an extensive ipsilateral descending spinal projection. Neurons located in various midline nuclei including the supratrochlear nucleus, oculomotor nucleus, Edinger‐Westphal nucleus, and the ventral part of the central gray were also labeled from the spinal cord. These findings indicate that the primate midbrain is the origin of an extensive system of descending spinal pathways, some of which are likely to be involved in mediating descending influences involved in complex motor and sensory behavior.