Some properties of sympathetic neuron inhibition by depressor area and intraspinal stimulation.

In Nembutal anesthesized cats single shock stimulation of the depressor area of the medulla oblongata evoked inhibition of spontaneous and glutamate-evoked activity of sympathetic preganglionic units. Single shocks to the lateral funiculus of the cervical or upper thoracic spinal cord in acute spinal cats evoked inhibition of the spontaneous and glutamate-evoked activity of single units and of the segmental reflex mass discharge evoked by spinal afferent stimulation. Cats studied 4 to 6 weeks after a complete transection of the spinal cord also showed, on stimulation of the lateral funiculus below the transection, an inhibition of the segmental reflex with time course similar to that seen in the acute spinal state, but of lower threshold and greater intensity. These results suggest that the inhibitory coupling between supraspinal levels and sympathetic preganglionic units is mediated, at least in part, by propriospinal neuronal system which survive after chronic spinal section. On the assumption that the observed changes in the properties of inhibition are due to plastic changes consequent to partial denervation the results also suggest that continuous descending tracts exist, and that both the continuous and the propriospinal descending tracts may be converging onto some common neural element.