Connections of higher order visual relays in the thalamus: A study of corticothalamic pathways in cats

Axonal markers injected into layers 5 and 6 of cortical areas 17, 18, or 19 labeled axons going to the lateral geniculate nucleus (LGN), the lateral p art of the lateralis posterior nucleus (LPI), and pulvinar (P). Area 19 sen ds fine axons (type 1, Guillery [1966] J Comp Neurol 128:21-50) to LGN, LPl , and P, and thicker, type 2 axons to LPl and P. Areas 17 and 18 send type 1 axons to LGN, and a few type 1, but mainly type 2 axons to LPl and P. Typ e 1 and 2 axons from a single small cortical locus distribute to distinct, generally nonoverlapping parts of LP and P; type 1 axons have a broader dis tribution than type 2 axons. Type 2 axons, putative drivers of thalamic rel ay cells (Sherman and Guillery [1998] Proc Natl Acad Sci USA 95:7121-7126; Sherman and Guillery [2001] Exploring the thalamus. San Diego: Academic Pre ss), supply small terminal arbors (100- to 200-mum diameter) in LPl and P, and then continue into the midbrain. Each thalamic type 2 arbor contains tw o terminal types. One, at the center of the arbor, is complex and multilobu lated; the other, with a more peripheral distribution, is simpler and may c ontribute to adjacent arbors. Type 2 arbors from a single injection are sca ttered around and along "isocortical columns" in LPl, (i.e., columns that r epresent cells having connections to a common cortical locus). Evidence is presented that the connections and consequently the functional properties o f cells in LP change along these isocortical columns. Type 2 driver afferen ts from a single cortical locus can, thus, be seen as representing function ally distinct, parallel pathways from cortex to thalamus.