Somatosensory projection to the mesencephalon: An anatomical study in the monkey

Abstract The terminal areas and cells of origin of the somatosensory projection to the mesencephalon in the monkey were investigated by the intraaxonal transport method. Following injection of wheat germ agglutinin‐horseradish peroxidase conjugate (WGA‐HRP) into the spinal enlargements, the lateral cervical nucleus (LCN), the dorsal column nuclei (DCN), or the spinal trigeminal nucleus, anterograde labeling was observed in several regions of the mid‐brain. (1) Injection of tracer into the spinal enlargements resulted in dense terminal labeling in the parabrachial nucleus (PBN) and the periaqueductal gray matter (PAG); moderate termination was observed in the intercollicular nucleus (Inc), the intermediate and deep gray layers of the superior colliculus (SGI, SGP), the posterior pretectal nucleus (PTP), and the nucleus of Darkschewitsch (D); and scattered terminal fibers were seen in the cuneiform nucleus (CNF) and the pars compacta of the anterior pretectal nucleus (PTA C ). The projections from the cervical enlargement to PAG, Inc, and the superior colliculus terminated more rostrally than those from the lumbar segments, indicating a somatotopic organization. (2) Terminal labeling after injection of tracer into LCN was found mainly in Inc, SGI, and SGP, but sparse labeling was also observed in the nucleus of the brachium of the inferior colliculus (BIN), PAG, PBN, PTP, and D. (3) The projection from DCN terminated densely in the external and pericentral nuclei of the inferior colliculus (ICX, ICP), Inc, SGI, SGP, PTP, PTA c , the nucleus ruber, and D, and weak terminal labeling was seen in BIN, PAG, and PBN. Comparisons of the anterograde labeling following injections involving both the gracile nucleus and the cuneate nucleus with that after injection restricted to the gracile nucleus alone suggested a somatotopic termination pattern in Inc, the superior colliculus, and the pretectal nuclei. (4) The patterns of projection from the laminar and alaminar parts of the spinal trigeminal nucleus differed: injection of tracer into the caudal part of the alaminar spinal trigeminal nucleus (nucleus interpolaris) resulted in dense anterograde labeling in SGI and SGP, moderate termination in Inc, and minor projections to PBN, PAG, and PTP, whereas after tracer injection into the laminar trigeminal nucleus (nucleus caudalis) terminal labeling was present only in PBN and PAG. Following injection of tracer into the midbrain terminal areas retrogradely labeled neurons were found in the spinal cord, LCN, DCN, and the spinal trigeminal nucleus, with the majority of labeled cells situated on the side contralateral to the injection site. (1) In the spinal cord the retrogradely labeled neurons were concentrated to the cervical segments and to the lumbosacral enlargement, with more sparse retrograde labeling in the thoracic and upper lumbar segments. The majority of the labeled neurons were situated in lamina I, with smaller fractions in laminae IV and V, and, in the upper cervical, thoracic, and sacral segments, also in the deeper laminae. (2) Heavy retrograde labeling was observed throughout the rostrocaudal extent of LCN . (3) The retrogradely labeled neurons in DCN were located along the periphery of the nuclei; in the cuneate nucleus the labeled neurons were found in the pars triangularis, but not in the pars rotunda, and in the gracile nucleus they were situated along the dorsal and medial borders of the nucleus. (4) In the sensory trigeminal nuclei peroxidase‐positive neurons were present throughout the entire nuclear complex except for the principal sensory trigeminal nucleus. The most dense labeling was observed in the laminar spinal trigeminal nucleus (nucleus caudalis), with the majority of the labeled neurons situated in its marginal layer (lamina I) and along the ventral and lateral parts of the alaminar spinal trigeminal nucleus (nuclei interpolaris and oralis). The results of this study show that the somatosensory projection to the midbrain in the monkey is organized in a way similar to that previously demonstrated in the cat. The possible functional role of these projections is discussed in relation to the orientational behavior and central pain mechanisms.