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

Abstract

Normally, gamma motoneurones innervate only the intrafusal fibres of muscle spindles. This is a report of sprouting of gamma 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 alpha motoneurones. In addition, in four experiments axons conducting within the gamma range, on stimulation, produced measurable tension. In one experiment, stimulation of one such gamma axon also produced specific fusimotor effects on four afferents identified as coming from primary endings of muscle spindles. The gamma axon was therefore a fusimotor axon. The effect observed on stimulation of the gamma axon suggested a largely dynamic action. Other examples of gamma axons were encountered that on stimulation produced tension, but which could not be specifically associated with spindles. In addition, a number of gamma 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 gamma motoneurones may sometimes sprout to innervate extrafusal fibres. The mechanical properties of the extrafusal fibres innervated by such gamma axons were similar to those of ordinary alpha motor units.

Botulinum toxin changes intrafusal feedback in dystonia: A study with the tonic vibration reflex

“…In this regard, our data from the long‐term evaluation of Patient 5 and 6 might be tentatively interpreted taking into account that the reinnervation of intrafusal fibers after partial denervation is a long and defective process. Gamma‐motoneurons may sometimes sprout to innervate extrafusal fibers, thus leaving the muscle spindle without motor drive 33…”

Intrafusal effects of botulinum toxin in post‐stroke upper limb spasticity

“…Owing to this twofold mechanism, the toxin should reduce the TVR more than the M‐max. Because the reinnervation of intrafusal fibres is a long and defective process [10], the TVR reduction should also last longer than the M‐max reduction. The difference between the BoNT‐A‐induced reduction of the TVR and the M‐max could therefore be the physiological marker of the toxin’s action at the level of the pre‐synaptic terminal of the γ‐motoneurons.…”

Neuronal regeneration in denervated muscle following sensory and muscular neurotization

“…Moreover, studies have shown that sensory axons can also release acetylcholine (Falempin et al 1989). Fusimotor neurons in extensively denervated muscles have been observed to sprout and innervate extrafusal fibers (Einsiedel et al 1992). Thus, a sensory nerve, with some fibers resembling motor nerves, may be capable of preserving a degree of muscle bulk in a muscle flap with no other physical stimulation.…”