Skeleto-fusimotor axons in the hind-limb muscles of the cat.

1. Motor axons supplying various hind-limb muscles of the cat (flexor hallucis lingus, peroneus brevis, peroneus digiti quinti, tibialis anterior, soleus and tenuissimus) were identified as skeleto-fusimotor or beta axons because their repetitive stimulation elicited both the contraction of extrafusal muscle fibres and an increase in the rate of discharge of spindle primary endings which perisited after selective blockade of extrafusal neuromuscular junctions. 2. The conduction velocity of these axons ranged from 39 to 92 m/sec. 3. Of seventy-six beta axons, seventy-two had a dynamic action on the sensitivity to velocity of stretching of primary endings, four had a static action. 4. The dynamic action of six beta axons was observed only after the contraction of extrafusal muscle fibres was selectively suppressed. 5. Tendon organs can be activated by beta motor units.