The afferents and projections of the ventroposterolateral thalamus in the monkey.

In seven monkeys (M. fascicularis), recordings were made from neurones in subnuclei VPLo and VPLc of the thalamus. The peripheral inputs to these cells from the forelimb were established by stimulating muscle, skin and joint nerves, and cutaneous receptive fields were examined by natural stimulation of the skin. The projection of the same cells to motor and sensory cortex was examined by stimulation of the cortex to demonstrate antidromic responses and by retrograde axonal transport of horseradish peroxidase injected into these regions of cortex. Of the 139 neurones which were driven by stimulation of peripheral nerves, 73 were located in VPLo. This suggests that there is a significant projection of forelimb afferents to VPLo in the monkey. Of the 73 neurones located in VPLo, 60 responsed to stimualtion of muscle or joint nerves. The projection to VPLo is therefore predominantly from deep receptors. Twenty-one of these 73 neurones had convergent inputs. The latencies of activation of units in VPLo were short (4--8 ms) and consistent with a lemniscal pathway via the dorsal column nuclei. Injection of horseradish peroxidase into the arm area of the motor cortex labelled cells in VPLo and not in VPLc. The neurones from which recordings were made in VPLo were located within the population of cells labelled with horseradish peroxidase after injections into the primary motor area. This suggests that this subnucleus is the thalamic relay for the sensory input from peripheral receptors to cells of the motor cortex.