SUMMARY1. 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 I a 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 05. 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 I a 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 unidirectional projections onto supinator and abductor pollicis longus motoneurones).
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