Critical points in the forelimb fictive locomotor cycle and motor coordination: evidence from the effects of tonic proprioceptive perturbations in the cat.

During locomotion, the limbs of one girdle must remain coordinated in different conditions. To understand the neural mechanisms underlying such coordination, tonic protraction/ retraction of one shoulder or tonic flexion/extension of one elbow was applied during fictive locomotion in high decerebrate and paralyzed cats. We studied bilateral changes in the timing and amplitude characteristics of electroneurographic (ENG) muscle nerve bursts of cleidobrachialis (ClB, elbow flexor and shoulder protractor) and the two heads of triceps (long, TriLo, elbow extensor and shoulder retractor and lateral, TriLa, elbow extensor). Perturbations induced bilateral changes in amplitude and timing of ENG bursts that were anchored on certain critical points in the cycle. These critical points could correspond to morphological characteristics within the bursts or to bilateral onsets or offsets of ENG bursts. For instance, in response to shoulder and elbow perturbations, burst changes occur in relation to a fixed point, labeled point C, occurring at about mid-extensor burst and corresponding to a simultaneous abrupt increase in TriLa amplitude and a decrease in amplitude of contralateral ClB. At a point labeled B, corresponding to about mid-flexor burst, ClB amplitude increases above control with elbow extension or starts decreasing with shoulder protraction. Although cycle reorganization is specific for each type of tonic perturbation, a common feature is that the changes in burst duration are achieved through discrete shifts between consecutive critical points. It is postulated that coordination may be based on a discrete temporal cycle structure along which critical points delimiting burst components are shifted.