Axon collaterals of Meynert cells diverge over large portions of area V1 in the macaque monkey

Patchy intrinsic connections, originating mainly from horizontal collateral s of pyramidal neurons, have been demonstrated in area V1 and many other co rtical areas. In fl-lis article, we identify a network of intrinsic connect ions concentrated in layer 6 of area V1. These are visualized by extracellu lar injections of anterograde tracers in V1, which label small clusters of large terminal boutons in layer 6, in conjunction with thick axon segments. These segments can be traced back to infragranular Meynert cells (n = 10), which are retrogradely labeled from the injections. By using serial sectio n analysis, we identified the following features of this distinctive system of Meynert cell collaterals: (1) terminal clusters are relatively small (< 100 <mu>m); (2) each cluster has a small number of rather large boutons (up to 3.0 mum); (3) there is typically a termination-free zone in the immedia te vicinity (0.5-2.0 mm) of the cell body; (4) a single neuron has multiple branches that can extend up to 8.0 mm from the soma; and (5) the collatera ls are concentrated in layer 6. These features are different fi-om those of horizontal intrinsic connections in the supragranular layers of area V1. T hey are consistent with fast dynamics and a possible role in wide-field mot ion processing, such as has been associated with Meynert cells from other s tudies. J. Comp. Neurol. 441:134-147, 2001. (C) 2001 Wiley-Liss, Inc.