Electrophysiological identification of a somaesthetic pathway to the red nucleus.

In awake chronically implanted cat, the cells in the red nucleus (RN) can be either activated or inhibited by natural stimulation on periphery. The effective stimuli are touching the fur, rotating the joints and tapping the muscles. A somaesthetic map has been constructed with the face area dorsally, the forelimb more ventrally and the hindlimb lateroventrally in the RN. In acute preparations, after ablation of the motor cortex and the cerebellum and section of the dorsal columns of the spinal cord at cervical level, the RN cells were still reacting to natural stimulation of the skin or electrical stimulation of peripheral nerves. The course of the somaesthetic pathway was systematically mapped by microstimulation of the spinal cord. It was shown that it follows the primary afferents which enter the dorsal columns, where they give off collaterals which relay at segment levels. After decussation the fibres ascend the ventromedial quadrant of the cord. A large portion of the fibres relay a second time in the medulla. The existence of such a pathway can account for the somaesthetic responses recorded in the RN in awake cats.