Beta innervation and recurrent inhibition: a hypothesis for manipulatory and postural control

The innervation of muscle spindles by skeletofusimotor (beta) axons in functionally different muscles of the cat forelimb has been correlated with the neural circuitry of the respective motor nuclei, morphological characteristics of their motoneurones and the innervated muscle fibres. 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 spindle afferents, with the absence of a recurrent inhibitory system, with cell body diameters in the range of small alpha-motoneurones and with a short duration of their after-hyperpolarisations. In contrast, the investigated elbow muscles display a low degree of beta-innervation (41-47%) irrespective of their fibre type composition, their Ia afferent 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 serves the control of manipulative movements, whereas for the proximal muscles the contrasting characteristics serve the control of posture and locomotion. This hypothesis is discussed in view of the phylogenetic development of motor control.