An analysis of mechanisms controlling the reversal of crossed spinal reflexes.

In acute spinal cats injected with clonidine, a noxious stimulation applied to a hindlimb may evoke in the other hindlimb an extension or a flexion response depending on whether it is respectively flexed or extended passively at the time of stimulation. The contribution of various afferents in the control of such reflex reversal was investigated. After denervation of joints, reflex reversal could still be found. Reflex reversal could be obtained in a pair of antagonist muscles whose distal tendons were cut to prevent length changes during manipulations of the whole limb. This suggests that the response in either the flexor of the extensor of one muscle pair can appear without stretch signals originating from the pair itself and can be determined by the state of stretch of other muscles acting at the same or other joints. When the whole limb was practically denervated except for one muscle pair responses were found only in the extensor muscle indicating that stretch signals from that pair alone were not sufficient to reverse the reflex pattern. After complete rhizotomy, strong responses were also observed only in extensor muscles. These results indicate that crossed extensor responses are not evoked because of interactions with afferent impulses related to the position of the limb. On the other hand, when afferents are intact and the limb extended, crossed extension can be blocked and replaced by crossed flexion. Evidence has been obtained suggesting that the stretch of flexor muscles might indeed change the pattern of crossed extension to a pattern of crossed flexion. These findings are discussed in the context of a purposeful role in posture and locomotion.