A study of sensory innervation to long nuclear chain intrafusal fibers in the cat muscle spindle.

The sensory innervation of 46 poles of long chain intrafusal muscle fibers was studied histochemically by staining for NADH-TR in periodic, 8 micron thick transverse sections of cat muscle spindles. Each long chain fiber carried terminals of the primary sensory axon, and 23 of the fiber poles also displayed secondary sensory endings. With the NADH-TR reaction there was no apparent difference in the cross-sectional appearance of sensory endings on the long chain and on other nuclear chain fibers. However, the contact area between the secondary endings and the muscle fiber tended to be shorter on the long chain than on the neighboring chain fibers of shorter polar length. This was also the case for one long chain fiber in which the sensory innervation was examined in serial, 1 micron thick sections stained with toluidine blue. Discharges of the secondary sensory axons in cat spindles may be affected more by contraction of the shorter nuclear chain fibers than by activation of the long chain fibers.