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.