Neuronal mechanisms of spino-bulbo-spinal activity

Abstract

Mechanisms of spino-bulbo-spinal motor activity evoked by stimulation of somatic (intercostal nerves) and visceral (the greater splanchnic nerve) afferents were studied in cats anesthetized with chloralose. It was found that thoracic interneurons activated by spino-bulbo-spinal mechanisms are distributed mainly in Rexed's laminae VII and VIII. They are excited either by both segmental and spino-bulbo-spinal influences or only by the latter. This neuronal population seems to form the spinal (segmental) pathways of spino-bulbo-spinal reflexes. Supraspinal structures related to the transmission of spino-bulbo-spinal activity are widespread in both the bulbar and the pontine reticular formations. They comprise reticulo-spinal neurons as well as neurons without projections to the spinal cord. Reticulo-spinal axons of ventral (conduction velocities 16–120 m/s) and lateral (17–100 m/s) funiculi can possibly participate in descending transmission of spino-bulbo-spinal activity. Stimulation of certain brain stem structures, especially pontine structures, results in long lasting (300–800 ms) depression of spino-bulbo-spinal reflexes. Conditioning spino-bulbo-spinal volleys also produce long-lasting (300–850 ms) depression of spino-bulbo-spinal reflex activity, as well as of descending spino-bulbo-spinal activity and responses evoked by direct stimulation of the reticular formation. Evidence is presented that this depression can be localized at the supraspinal level. Possible mechanisms of initiation and inhibition of spino-bulbo-spinal reflex activity evoked by somatic and visceral afferents are discussed.