Complexities in the thalamocortical and corticothalamic pathways.

It is now a century since Kölliker (Handbuch der Gewebelehre des Menschen. Nervensystemen des Menschen und der Thiere, Vol. 2, 6th edn. Engelmann, Leipzig, 1896) described the thalamic reticular nucleus as the 'Gitterkern' or lattice nucleus on the basis of the fibrous latticework that is the characteristic feature of this part of the ventral thalamus and adjacent parts of the internal capsule. We suggest that the fibre reorganization produced in this lattice is a fundamental requirement for linking orderly maps in the thalamus to corresponding cortical maps by two-way thalamocortical and corticothalamic connections; these connections involve divergence, convergence and mirror reversals, which all have to occur between the thalamus and the cortex. Apart from the thalamic reticular nucleus, two transient groups of cells, the perireticular nucleus (located in the internal capsule lateral to the reticular nucleus) and the cells of the cortical subplate, are prominent along the course of axons linking the cortex and thalamus early in development. The functions of these two cell groups are not known. However, since early in development complex patterns of reorganization, defasciculation and crossings occur in the regions of these cells, it is likely that they play a role in creating the latticework of the adult. The latticework that characterizes the thalamic reticular nucleus of mammals can also be identified in the ventral thalamus of non-mammalian brains, formed along the course of the fibres that join the dorsal thalamus to the telencephalon. We suggest that the ubiquitous presence of such a zone of fibre reorganization is integral to the functioning of the thalamocortical pathways, and that the complexity of thalamic connections produced in the lattice has been central to the evolutionary success of the thalamotelencephalic system.