An investigation into the site of termination of static gamma fibres within muscle spindles of the cat peroneus longus muscle.

1. The distribution of static fusimotor fibres to intrafusal muscle fibres of cat peroneus longus muscle spindles was investigated using the glycogen-depletion technique of Edström & Kugelberg (1968). Single static gamma fibres were stimulation intermittently at high rates for 3 hr with the blood supply occluded for some of this time. Subsequently the portion of muscle containing the activated spindles was fixed, sectioned and stained for glycogen with the periodic acid-Schiff (PAS) method. 2. Ten static axons caused depletion in eleven spindles. In five of these the only glycogen-depleted fibres were nuclear chain fibres. In the other six spindles one nuclear bag fibre was depleted in addition to chain fibres and this was always the larger of the two within the spindle. 3. These results on a medium-sized hind limb muscle are compared with findings concerning the distribution of static gamma fibre axons previously investigated only in very small muscles. The results agree in showing that nearly all static gamma fibres innervate nuclear chain fibres but that in 50-75% of the times in which static gamma fibres innervate spindles the distribution is to bag fibres as well as to chain fibres. The interpretation to be put upon this is uncertain. One possibility with which the results from peroneus longus are consistent is that the bag fibres which are usually innervated by static axons are the 'intermediate' bag fibres whose ultrastructure has recently been shown to resemble that of chain fibres.