SYNAPTIC CONNECTIONS FROM LARGE AFFERENTS OF WRIST FLEXOR AND EXTENSOR MUSCLES TO SYNERGISTIC MOTONEURONS IN MAN

Short-latency excitatory Ia reflex connections were determined between pairs of human wrist flexor and extensor muscles. Spindle Ia afferent s were stimulated by either tendon tap or electrical stimulation. The activity of voluntarily activated single motor units was recorded intr amuscularly from pairs of wrist flexor or extensor muscles. Cross-corr elation between stimuli and the discharge of the motor units provided a measure of the homonymous or heteronymous excitatory input to a moto neurone. Homonymous motoneurone facilitation was generally stronger th an that of the heteronymous motoneurones. The principal wrist flexors, flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU), were tigh tly connected through a bidirectional short-latency reflex pathway. In contrast, the extensor carpi ulnaris (ECU) and the extensor carpi rad ialis (ECR) did not have similar connections. ECU motoneurones receive d no short-latency excitatory Ia input from the ECR. ECR motoneurones did receive excitatory Ia input from ECU Ia afferents; however, its la tency was delayed by several milliseconds compared with other heterony mous Ia excitatory effects observed. The wrist and finger extensors we re linked through heteronymous Ia excitatory reflexes. The reflex conn ections observed in humans are largely similar to those observed in th e cat, with the exception of heteronymous effects from the ECU to the ECR and from the extensor digitorum communis (EDC) to the ECU, which a re present only in humans. The differences in the reflex organization of the wrist flexors versus the extensors probably reflects the import ance of grasping.