Afferent organization of the lateral reticular nucleus in the rat: an anterograde tracing study.

The organization of the afferent projections to the lateral reticular nucleus of the rat was investigated following placement of horseradish peroxidase-conjugated wheatgerm agglutinin into the red nucleus, fastigial nucleus, various levels of the spinal cord or the sensorimotor area of the cerebral cortex. The pattern of distribution of anterogradely labelled profiles visualized with tetramethylbenzidine revealed that the caudal three-fourths of the lateral reticular nucleus received a large, topographically organized projection from the entire length of the contralateral spinal cord. The lateral part of the rostral half of the lateral reticular nucleus received a small projection from the contralateral red nucleus, the dorsal part of the middle third of the nucleus received a diffuse projection from the contralateral fastigial nucleus, and the extreme rostromedial part of the nucleus received a sparse projection from the contralateral cerebral cortex. The dorsal part of the middle third of the lateral reticular nucleus also received a small projection from the ipsilateral cervical spinal cord. The distribution of afferent fibres from different levels of the spinal cord, red nucleus, and fastigial nucleus overlapped substantially in the middle third of the lateral reticular nucleus, whereas the cerebral cortical receiving area was separate. These data suggest that the middle third of the lateral reticular nucleus integrates spinal and supraspinal impulses to the cerebellum, while the rostral part of the nucleus is involved in a separate cerebral cortico-cerebellar pathway.