Non-uniformity of extrinsic connections and columnar organization.

Afferent columns (>200 microm in diameter) have been intensively investigated in the context of thalamocortical and intrinsic connections. Many extrinsic cortical connections also form columnar terminations, but less is known about their fine organization. Results from intracellular injections of neighboring neurons (in rats: Johnson et al., 2000) suggest that even neurons within a common domain may have non-stereotyped projection patterns, with only partial overlap of terminal arbors. The issue of non-stereotyped projections at the columnar level is further considered by analysis of V1 axons terminating in primate area MT/V5 (an early visual area), and of an axon from temporal cortex terminating in area 7b (a higher cortical area). Both these axons have multiple non-uniform arbors. The implication is that each arbor recruits different numbers and possibly different combinations of postsynaptic elements. While more data are needed concerning convergence of connectional systems, and the actual identity and numbers of postsynaptic targets, the distributed spatial and laminar patterns do not evoke a repetitive uniformity, but rather a columnar substructure and the combinatoric possibilities of the 3-dimensional cortical organization.