STRIATAL AND CORTICAL PROJECTIONS OF SINGLE NEURONS FROM THE CENTRAL LATERAL THALAMIC NUCLEUS IN THE RAT

Striatal and cortical projections arising from the central lateral tha lamic nucleus were studied in rats by tracing the axons of small pools of neurons labeled anterogradely with biocytin. Cells of the central lateral nucleus have a morphology that conforms to the classic descrip tions of the bushy cells which represent the main neuronal type of mos t thalamic nuclei. They display many short radiating dendrites studded with sessile spines, protrusions and grapelike appendages. The total extent of their dendritic fields is about 250 mu m. After leaving the nucleus, all central lateral axons course through the rostrolateral po le of the thalamic reticular nucleus, where they branch profusely, ent er the striatum, where they distribute collaterals, and arborize in th e motor cortex. At striatal level, central lateral fibers form a loose ly organized network composed of varicose axonal branches that appear to contact en passant several striatal neurons. In the cortex, central lateral axons form multiple (four to five) patches of terminations in layers Va and III aligned along the rostrocaudal extent of the motor area. The projection to layers I and II is very sparse, consisting of occasional branches which show few ramifications. Our results indicate that most, and perhaps all, central lateral relay neurons project to both the striatum and cerebral cortex. The patchy innervation of mid c ortical layers of the frontal motor areas by central lateral afferents strongly argues against the nonspecific character of this projection. It is proposed that the central lateral nucleus, which receives a str ong innervation from brainstem cholinergic afferents, takes part in a mechanism of attention related to the central initiation of directed p atterns of movements.