Histochemical study of an unusual cat intrafusal muscle fiber.

A cat tenuissimus muscle spindle that contained two long chain intrafusal fibers in its distal pole is described. One of the fibers (lc1) had a histochemical profile (ATPase, NADH-TR, ChE reactions) of the kind which is characteristic for long chain fibers. The other fiber (lc2) consisted of two separate segments. The inner lc2 segment included the sensory equatorial region and was histochemically normal. The outer lc2 segment carried a motor plate, and did not stain for NADH-TR in the same way as the inner lc2 segment and the lc1 fiber. It is suggested that the unusual enzyme staining properties of the outer lc2 segment stemmed from its lack of sensory innervation, a situation which may have permitted the full expression of influences mediated by its motor nerve supply.