Somatosensory and movement-related properties of red nucleus: a single unit study in the turtle.

Extracellular recordings were performed from turtle red nucleus neurons to examine their responsiveness to peripheral somatic stimulation and to study differences between rubral sensory and movement-related responses. In pentobarbital sodium-anesthetized or decerebrate turtles, red nucleus neurons could be divided into two categories based on their response characteristics. The first group, which included 87% of neurons studied, had low spontaneous rates of activity and responded with excitation to electrical stimulation of the spinal cord or the cerebellum, or during active movement of the contralateral limbs. Neurons in this category were likely to be rubrospinal cells. The remaining 13% of cells studied had higher rates of spontaneous discharge and were inhibited by electrical stimulation or during active movement. These cells might be rubral GABAergic interneurons. Single red nucleus neurons responded with excitation and/or inhibition to somatosensory stimulation. Unlike the motor fields, which were restricted to a single contralateral limb, red nucleus sensory receptive fields were wide and often bilaterally distributed. Rubral responsiveness to sensory stimulation was found to be significantly diminished during active limb movements, thereby suggesting that sensory inputs to the red nucleus are not used for the on-line modification of motor commands. Inactivation of the cerebellar cortex enhanced the sensory responsiveness of rubral neurons and expanded the size of red nucleus receptive fields. These results suggest that the red nucleus receives substantial sensory input, and that the cerebellar cortex can modify the flow of sensory information to the red nucleus.