Absence of the gamma-spindle loop in the reinnervated hind leg muscles of the cat: "alpha-muscle".

Degeneration of afferent and efferent fibres supplying leg muscles was elicited by applying dry ice to the sciatic nerve in the mid femoral region. Two months after the denervation the cats showed practically normal walking activity. After 3 months the twitch tension of the reinnervated muslce (triceps surae) was almost similar to the tension developed by the non-operated contralateral control muscle. Afferent discharges with a pattern typical for muscle spindles could rarely be found before the end of the 4th month. The muscle spindles which could be recorded before the end of the 6th month showed mainly phasic responses to ramp-and-hold stretches and only a few of them exhibited a pause during muscle contraction. None of them showed any sign of gamma innervation, as tested by several independent gamma-activating or gamma-blocking precedures. The conduction velocities of the regenerated afferent and effenent fibres were lower than those of the normal fibres; they gradually recovered with time after the operation but did not reach normal values by the end of the 6th month. Fibre histograms revealed that the afferent fibres of groups I, II and III had regenerated, while of the efferent fibres only those corresponding to the alpha group had regenerated. Since during the period of observation the reinnervated muscles lack the support of a functioning gamma innervation, it is proposed to call them "alpha muscles".