Fusimotor axons in the kitten

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

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.

Extrafusal and intrafusal motor units in the kitten

In the lateral flexor digitorum longus muscle (FDL) of anaesthetized kittens aged 3-21 days, alpha (alpha) and gamma (gamma) motoneurone conduction velocities were measured together with motor unit tensions. Conduction velocity lay in the range 8-27 msec-1 for alpha motoneurones and 2-10 msec-1 for gamma motoneurones. Motor unit tetanic tensions were 3-47 mN with the largest units being 2-2.2% of whole muscle tension. The hypothesis was tested that motor axons conducting within the gamma range and presumed to be innervating the intrafusal fibres of muscle spindles, on stimulation, produced measurable tension. Stimulating gamma motoneurones in the adult produces no tension. The hypothesis was based on histological observations that while in the adult, intrafusal fibres were about one third of the size of extrafusal fibres, in kittens the two were nearly equal, both in length and diameter. It was shown by means of signal averaging during stimulation of ventral root filaments that whenever tension was recorded in the muscle an impulse could be detected in the muscle nerve conducting at alpha motoneurone tempo. It is concluded that in the kitten, despite the fact that intrafusal and extrafusal fibres are of similar size, stimulating single gamma motoneurones does not develop measurable tension.

Postnatal development of peroneal motoneurons in the kitten

In 1- to 72-day-old kittens, motoneurons of the 3 peroneal muscle nuclei were labeled by retrograde axonal transport of horseradish peroxidase from individual muscles. At birth, the locations of peroneal nuclei were similar to those of the adult cat. Counts of motoneurons at different ages indicated that postnatal cell death does not occur in peroneal motor nuclei. Primary dendrites were as numerous in motoneurons of newborn kittens as in adult motoneurons but they were thinner, shorter and poorly ramified. The number of recurrent axon collaterals was higher in the first postnatal week than at later stages. The growth of motoneurons followed similar rates in the 3 peroneal nuclei. Distributions of cell body diameters and volumes were unimodal at birth and became bimodal between 15 and 20 days postnatal. The separation of peroneal motoneurons in two size subgroups, presumably corresponding to alpha and gamma populations, was followed by an increase in growth rate which became faster for alpha than for gamma motoneurons.

The responses of muscle spindles in the kitten to stretch and vibration

Discharges of muscle spindle afferents from the soleus muscle were studied in kittens aged 1-21 days and in adult cats. Vibration applied longitudinally to the tendon elicited one impulse for each cycle of vibration over the range 1-200 Hz for the kittens and up to 450 Hz for the adult. Threshold amplitudes were generally higher in the kitten than in the adult. In response to large ramp and hold stretches applied at long muscle lengths kitten spindles showed rate saturation during the length change. Dynamic index, that is the peak rate during the length change minus the rate at the final length became progressively smaller at longer muscle lengths. No sign of saturation was seen at comparable muscle lengths in the adult. It is suggested that in the newborn the bag1 intrafusal fibre is not functional and that the dynamic response is produced only by the afferent terminals on the bag2 fibre. Another difference between kitten and adult was the length sensitivity measured under dynamic conditions. This increased much more steeply with stretch rate in the kitten. One possible explanation for the higher dynamic length sensitivity is a lack of elastic fibres surrounding intrafusal fibres of immature spindles.