The organization of the lateral thalamus of the hooded rat.

Analysis of cytoarchitecture and connectivity showed that the lateral thalamus of the hooded rat is composed of eight nuclei. An examination of the cytoarchitecture permitted the identification of seven cellular fields: nucleus suprageniculatus (sg), nucleus lateralis posterior pars caudomedialis (lpcm), nucleus lateralis posterior pars lateralis (lpl), nucleus lateralis posterior pars rostromedialis (lprm), intramedullary area (ima), nucleus lateralis dorsale pars ventrolateralis (ldvl), and nucleus lateralis dorsale pars dorsomedialis (lddm). An analysis of the connectivity showed that lpl is further divisible into a rostral (lplr) and a caudal (lplc) sector, bringing the total number of nuclei to eight. Nucleus suprageniculatus, the most caudal element of the lateral thalamus, is composed of medium to large, fusiform, and multipolar neurons. It contains a terminal field of the projection of the superficial layers of the ipsilateral superior colliculus. Nucleus lpcm, found rostrolateral to sg, is loosely packed with large multipolar neurons. A terminal field of the superficial layers of the superior colliculus of both sides fits precisely within its cytoarchitectural boundaries. Nucleus lpl, a long cellular territory found lateral to lpcm, extends from the caudal pole of the dorsal lateral geniculate nucleus to the caudal pole of ldvl and contains round cells which are smaller and more densely packed than those of lpcm. Its caudal portion (lplc) contains another terminal field of the ipsilateral superior colliculus while its rostral portion (lplr) contains a terminal field of the projection of Area 17. Area 18 also projects to lplr, whereas Area 18a projects to both lplr and lplc. The intramedullary area, which occupies the fibrous zone between lpl and the dorsal lateral geniculate nucleus, contains round and fusiform neurons and is innervated by Area 18a. Nucleus lprm, situated medial to lpl, is characterized by round neurons which are frequently found in clusters. It is innervated by Areas 17, 18, and 18a. Nucleus ldvl is evenly packed with moderately large, polygonal cells and contains the complete terminal fields of both Areas 17 and 18. It also receives inputs from Area 18a. Finally, lddm, tightly packed with small, round cells and lying medial to ldvl, receives inputs from Area 4.