INSERTIONS OF INTRAFUSAL FIBERS IN MUSCLE SPINDLES OF CAT AND OTHER MAMMALS

The manner of insertion of intrafusal (IF) fibers of muscle spindles has been studied in serial histological sections of spindles from hindlimb muscle of the cat, opossum, mouse, rat, bat and gibbon. Four basic types of IF terminations in relation to the capsule were recognized: (A) direct insertion into the capsule; (B) strong lateral attachments to the capsule, though the fiber may continue beyond the capsule; (C) termination in the connective tissue beyond the limits of the capsule; (D) free extension of the fiber beyond the capsule. Bag fibers in all species generally extended beyond the capsule. The pattern of chain fiber insertion, however, varied with the species. Most rat spindles, for example, followed the D pattern, those in the cat pattern A, and the gibbon chiefly A and B. The bearing of these findings upon a postulated mechanism by which spindles may detect intramuscular pressures, as changes in length, is discussed. Copyright © 1969 Wiley‐Liss, Inc.