Skeletofusimotor (β) innervation of proximal and distal forelimb muscles of the cat

The innervation of muscle spindles by skeletofusimotor (β) axons has been compared in two long digit extensor muscles with three elbow extensor muscles of the cat forelimb. The proportion of muscle spindle poles with p1-endplates was analysed in silver impregnated teased material. The proximal and distal muscles displayed a significant difference in their proportion of muscle spindle poles with p1-endplates. The distal muscles had an estimated proportion of more than 70% β innervation of their muscle spindles, the proximal muscles of 41–47%. This difference is discussed with respect to the different spinal circuitry of the motoneurones. We suggest that the high proportion of β-innervated spindles in distal muscles combined with the absence of a recurrent inhibitory system in the innervating motoneurones [18] serves the execution of manipulative movements.