The spinal route of sympatho-inhibitory pathways descending from the medulla oblongata.

1. The effect of making discrete lesions in the cervical spinal cord on the brainstem elicited inhibition of a spinal somato-sympathetic reflex response has been studied in anaesthetized cats. 2. Electrical stimulation within three areas of the medulla caused an inhibition of the spinal component of the reflex response elicited in thoracic white rami communicantes by stimulation of intercostal nerves. The three medullary areas studied were the ventrolateral medulla and the caudal rephe nucleus, from where bulbospinal monoamine neurones originate, and the ventromedial reticular formation. 3. The inhibitory effects of stimulation in the ventrolateral medulla and raphe nucleus were abolished by the destruction of parts of the ipsilateral dorsolateral funiculus of the cervical spinal cord, whereas the inhibition produced by ventromedial reticular formation stimulation was abolished by lesions which included part of the ventral quadrant of the cord. 4. The time course of the inhibitory effects of electrical stimulation of descending sympatho-inhibitory tracts in the cervical spinal cord was studied in unanaesthetized decerebrate cats spinalized at C1. Inhibition obtained from the dorsolateral funiculus characteristically had a longer time to onset than inhibition obtained from the ventrolateral and ventral funiculi.