BETA-INNERVATION AND RECURRENT INHIBITION - A HYPOTHESIS FOR MANIPULATORY AND POSTURAL CONTROL

The innervation of muscle spindles by skeleto-fusimotor (beta) axons i n functionally different muscles of the cat forelimb has been correlat ed with the neural circuitry of the respective motor nuclei, morpholog ical characteristics of their motoneurones and the innervated muscle f ibres. In long digt extensor muscles a high degree of beta-innervation (more than 70%) and of fast contracting muscle fibres (more than 70% IIB fibres) correlates with specific projections of their Ia muscle sp indle afferents, with the absence of a recurrent inhibitory system, wi th cell body diameters in the range of small alpha-motoneurones and wi th a short duration of their after-hyperpolarisations. In contrast, th e investigated elbow muscles display a low degree of beta-innervation (41-47%) irrespective of their fibre type composition, their Ia affere nt fibres show a divergent projection pattern and their motor nuclei a distinct recurrent inhibitory system. We suggest that for the distal muscles the specific combination of these different characteristics se rves the control of manipulative movements, whereas for the proximal m uscles the contrasting characteristics serve the control of posture an d locomotion. This hypothesis is discussed in view of the phylogenetic development of motor control.