Arborisation and termination of single motor thalamocortical axons in the rat

The aim of this study was to examine the arborisations and terminations of individual thalamocortical axons in the motor system of the rat. Small, extracellular injections of an anterograde tracer (dextran-biotin) were made into the ventrolateral (VL) or ventral posterolateral (VPL) thalamic nuclei to label thalamocortical projections. Eleven motor axons and one somatosensory axon were reconstructed through serial sections just rostral from the injection site to their terminations in sensorimotor cortex. The smallest arbor arising from a single motor axon extended approximately 0.9 mm rostrocaudal and 0.9 mm mediolateral, the largest extended 3.9 mm rostrocaudal and 1.0 mm mediolateral. In some cases, two distinct plexuses of terminals were formed by an axon. In addition, motor axons formed terminals in cortical layer V only or in layers I, III, and V. By contrast (and in keeping with previous reports), the somatosensory axon formed a single plexus of terminals in layer IV of the cortex that extended approximately 0.3 mm rostrocaudal and 0.4 mm mediolateral. It is concluded that individual motor thalamocortical neurones are in a position to influence much more widespread cortical regions than somatosensory thalamocortical neurones.