[Muscle spindles in denervated and reinnervated m. soleus of the rat. II. Changes in the extra- and intrafusal muscle fibers].

After a transient or permanent unilateral denervation of the soleus muscle of the rat changes were investigated of extra- and intrafusal muscle fibres of the denervated (reinnervated) muscle as well as its contralateral still innervated muscle. Those data which were obtained from normal muscles of uninjured rats served as control. The changes of permanent denervated muscles were clear and statistically significant. The extrafusal muscle fibres show a considerable atrophy. The nuclear-chain fibres exhibit a decrease of their calibres. The extent of this atrophy is not as pronounced as in extrafusal muscle fibres. The nuclear-bag fibres show 12 weeks after denervation a small atrophy (or pseudoatrophy) and 18 weeks after denervation a significant hypertrophy. At the same time the number of nuclear-bag fibres is increasing. Besides the increase in number of intrafusal muscle fibres per muscle spindle, a change is observed of proportion of both intrafusal fibre types in favour of nuclear-bag fibres. The hypertrophy and the increase in number of the nuclear-bag fibres are discussed in connection with their functional properties.