Monosynaptic Ia pathways at the cat shoulder.

1. The study aimed to describe in cat forelimb and shoulder motoneurones the convergence and projection patterns from large muscle spindle afferents (Ia). In 11 chloralose-anaesthetized cats maximum Ia EPSPs evoked by electrical stimulation of ipsilateral forelimb nerves were obtained in 309 intracellularly recorded alpha-motoneurones. 2. Groups of motor nuclei displayed similar Ia patterns. As in the distal forelimb they were often interconnected by bidirectional pathways, which were used to combine Ia synergistic groups. Three such groups are described at the shoulder. 3. The first group was composed of the main flexors of the scapulo-humeral joint. Regular disto-proximal Ia excitation from elbow extensors (and median afferents) indicates a coupling of flexion in the scapulo-humeral joint to the angular position of the elbow. 4. The second group comprised the outward rotators of the humerus with differentiated Ia convergence onto the different group members. The patterns of Ia excitation received and sent by the group members demonstrate that the outward rotators are incorporated in versatile synergisms and may occupy a central position in steering forelimb movements. 5. The third group was formed by the spinatus muscle and the subscapularis. This arrangement is suggested by the common convergence onto them from the elbow extensors and flexors. The pattern may serve to guide and keep the humeral head in the joint capsule. 6. The Ia synergistic groups receive Ia convergence from muscles acting at distant joints and also project to distant muscles. This is discussed as part of an extended pattern of Ia connections along the forelimb. In this way the shoulder muscles would be incorporated in flexor and extensor oriented synergisms which are needed to co-ordinate the muscular activation along the multijoint forelimb during locomotion. When the shoulder Ia pathways are compared with those in the distal forelimb, organization of the Ia system apparently follows a few basic principles which have adapted to the mechanical situation at the particular joints and their mechanical interaction.