Sprouting of fusimotor neurones after partial denervation of the cat soleus muscle

Summary

Normally, γ motoneurones innervate only the intrafusal fibres of muscle spindles. This is a report of sprouting of γ motoneurones to innervate extrafusal muscle fibres following partial denervation of the soleus muscle of kittens. In eight newborn animals, the L7 ventral root was cut on one side under anaesthesia and the animals were then allowed to recover. At approximately 100 days of age animals were reanaesthetised and a study made of mechanical properties of motor units whose axons ran in the S1 ventral root and supplied the partially denervated soleus muscle. Evidence was obtained for sprouting of all surviving α motoneurones. In addition, in four experiments axons conducting within the γ range, on stimulation, produced measurable tension. In one experiment, stimulation of one such γ axon also produced specific fusimotor effects on four afferents identified as coming from primary endings of muscle spindles. The γ axon was therefore a fusimotor axon. The effect observed on stimulation of the γ axon suggested a largely dynamic action. Other examples of γ axons were encountered that on stimulation produced tension, but which could not be specifically associated with spindles. In addition, a number of γ axons that did not develop tension were shown, on stimulation, to have fusimotor effects that were static in action. It is concluded that in extensively denervated muscles γ motoneurones may sometimes sprout to innervate extrafusal fibres. The mechanical properties of the extrafusal fibres innervated by such γ axons were similar to those of ordinary α motor units.