Pattern of monosynaptic Ia connections in the cat forelimb.

Abstract

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).

Limited independent flexion of the thumb and fingers in human subjects.

“…However, both classes of afferents have divergent connections onto homonymous and heteronymous motoneurone pools (e.g. Eccles, Eccles & Lundberg, 1957;Brink, Jankowska, McCrea & Skoog, 1983;Fritz, Illert, de la Motte, Reeh & Sagau, 1989). As yet, no data on sensory and reflex partitioning of muscle afferent input in humans are available for FDP to assess the possibility critically.…”

“…As yet, no data on sensory and reflex partitioning of muscle afferent input in humans are available for FDP to assess the possibility critically. In the cat, reflexes from spindle afferents in FPL and the digital components of FDP are bidirectionally interconnected and any reflex partitioning within FDP is not strong (Fritz et al 1989). In humans, the extent of reflex connections from muscle spindle afferents in FDP is not clearly established (cf.…”

Formation of transient inappropriate sensorimotor synapses in developing rat spinal cords

“…mature spinal cords (Eccles et al, 1957a;Mendell and Henneman, 197 1;Binder, 1980;Kirkwood and Sears, 1982;Fritz et al, 1989; reviewed by Burke, 1990;Munson, 1990). Our data support preliminary findings indicating that in the spinal cords of O-lo-d-old neonates, the latencies of EPSPs in extensoi motoneurons were similar for homonymous and antagonistic nerve stimulation (Navarrete et al, 1987).…”

“…Primary afferents of a particular muscle monosynaptically excite two groups of motoneurons: those that innervate the same muscle (homonymous) and those that contact muscles with related function (synergists). These primary afferents also project onto interneurons that inhibit motoneurons innervating muscles with opposite function (antagonists) (Eccles et al, 1957a;Mendell and Henneman, 1971;Binder, 1980;Kirkwood and Sears, 1982;Fritz et al, 1989). Therefore, a specific primary afferent monosynaptically excites homonymous and synergistic motoneurons and disynaptically inhibits antagonistic motoneurons.…”

The G. L. Brown Prize Lecture. Cortical control of the primate hand

“…These include the absence of recurrent axon collaterals from intrinsic hand muscle motoneurones and of the Renshaw inhibition they induce, the generally weak spinal reflex excitatory or inhibitory effects from muscle spindles, and the absence of any fixed pattern of reciprocal Ta inhibition amongst pairs of 'antagonist' muscles. It would also appear that many distal muscles are characterized by a much higher proportion of fl-innervated spindles than are proximal muscles (Marsden, Merton & Morton, 1972;Buller, Garnett & Stephens, 1980;Fritz, Illert, de la Motte, Reeh & Saggau, 1989; Homer, Illert & Kiimmel, 1990). While the precise function of these different features has yet to be fully elucidated, they are consistent with a much greater flexibility of action amongst the twenty-nine muscles which move the hand and fingers, a flexibility that is played upon and enhanced by the corticospinal projection.…”