Processing of vestibular and other inputs by the caudal ventrolateral medullary reticular formation

Lesions of the lateral medullary reticular formation caudal to the obex abolish vestibulosympathetic and somatosympathetic responses; this area also contains neurons that mediate baroreceptor reflexes. Recordings were made from neurons in the caudal medullary reticular formation of cats that were decerebrate or anesthetized using alpha-chloralose-urethan to determine whether common neurons responded to electrical stimulation of vestibular and hindlimb afferents and had cardiac-related (i.e., baroreceptor) inputs. Many neurons in the ventrolateral portion of the caudal reticular formation received labyrinthine inputs, and they were interspersed with neurons that received baroreceptor signals. However virtually none of the units received convergent baroreceptor and vestibular inputs, suggesting that separate pathways from the caudal ventrolateral medulla mediate baroreceptor and vestibulosympathetic reflexes. Furthermore, the neurons that received labyrinthine signals could not be antidromically activated from electrodes inserted into the rostral ventrolateral medulla, which is known to mediate vestibulosympathetic responses; thus an indirect pathway must convey vestibular inputs from the caudal to rostral medullary reticular formation. Over 75% of both neurons with baroreceptor inputs and cells with vestibular signals responded to sciatic nerve stimulation, suggesting that more than one pathway from the caudal medulla may mediate somatosympathetic responses.