Pattern of monosynaptic Ia connections in the cat forelimb.

1. In anaesthetized cats intracellular records were obtained from antidromically identified motoneurones. The motor nuclei to the elbow extensor and flexor muscles and to the muscles innervated by the deep radial, ulnar and median nerves were investigated. The maximum Ia EPSPs from electrical stimulation of various peripheral nerves were measured. The characteristic convergence and projection patterns to each motor nucleus were established from pooled data. 2. The total aggregates of the Ia EPSPs between the different motor nuclei ranged from 3.5 to 11.7 mV. The smallest aggregates were found in the nuclei to the digit muscles. The ratio of the heteronymous versus homonymous EPSP amplitudes varied between 3.9 and 0.5. A general rule which would govern the distribution of the EPSP aggregates, such as a proximo‐distal gradient, was not observed. 3. The Ia connections followed a complex but highly organized pattern. Bidirectional and unidirectional pathways were present. In many cases the convergence pattern of a motor nucleus included muscles acting at different joints. The connections of one nucleus were not necessarily restricted to one side of the limb, but could cross the radio‐ulnar plane. 4. Muscles with similar actions onto the same joint were interconnected with bidirectional, rather balanced Ia pathways. Such relations were also present between close functional synergists and then often extended across several joints. The relations between the anatomical extensors of wrist and digits were graded according to the neighbourhood of these muscles. It is suggested that this reflects the graded mechanical synergism in the wrist action of these muscles. 5. A large number of unidirectional or strongly skewed bidirectional Ia pathways project from proximal to distal muscles. It is suggested that they may serve a readjustment of distal joints during changes in the position of proximal ones (e.g. stabilization of the position of the radio‐ulnar plane during elbow extension in case of the undirectional projections onto supinator and abductor pollicis longus motoneurones). 6. The motor nuclei to some multifunctional muscles display a negative correlation between different heteronymous Ia inputs: motoneurones with a large input from one muscle show a significant tendency to receive a smaller input from another muscle and vice versa. This organization leads to subpopulations of neurones with different convergence patterns within the same motor nucleus. 7. Motor nuclei with bidirectional Ia relations between each other displayed similar convergence and projection patterns. They were combined into ‘Ia synergistic groups.’ One motor nucleus may belong to several groups.(ABSTRACT TRUNCATED AT 400 WORDS)