The lateral cervical nucleus in the cat: anatomic organization of cervicothalamic neurons.

The morphology of the lateral cervical nucleus (LCN) and the organization of the cervicothalamic projection neurons were studied in cats which had received thalamic injections of horseradish peroxidase (HRP). The boundaries of the LCN were defined following very large thalamic (HRP injections. Roughly 92-97% of LCN cells project contralaterally to thalamus; an additional 1.5% project ipsillaterally. Computer-assisted measurements of perikaryal areas demonstrated that there are two sizes of LCN cells, large (175-900 micrometer 2) and small (less than 175 micrometer 2); the small cells are localized in the medial third of the LCN. LCN cells which are not labeled after large thalamic HRP injections are predomininantly small, medially-located neurons. Small HRP injections into physiologically identified regions of ventroposterior thalamus demonstrated that cervicothalamic neurons are organized in a topography consistent with that observed physiologically in the LCN (Craig and Tapper, '78). Dorsolateral LCN cells are retrogradely labeled from nucleus ventroposterolateralis, pars lateralis (VPL1), ventromedial LCN cells are labeled from pars medialis (VPL m), and a few medial cells are labeled from nucleus ventroposteromedialis (VPM). A few cells in the medial portion of the LCN are also labeled from each part of ventroposterior thalamus. Some interspersion was observed even in the cases with the most well-restricted labeling. We conclude that the LCN maintains a basic somatotographic organization with an inherent variability, certain aspects of which are consistently demonstrable both physiologically and anatomically. Evidence was also obtained suggestive of a rostrocaudal inversion in the cervicothalamic projection. The cervicothalamic projection, the differentiation of the medial LCN subpopulation, and the possible redefinition of the LCN are discussed in light of these results.