Morphology and morphometry of motor endings in primate intrafusal fibers.

The pre- and postsynaptic structure of 243 axon terminals of bag 1, bag 2, and chain fibers were studied in cynomolgus monkey skeletal muscle spindles. The motor endings of the biceps and gastrocnemius spindles (long limb muscle) were compared to the motor endings of lumbricals and opponens pollicis (intrinsic hand muscle) spindles. In both muscle groups the only significant difference observed in the presynaptic features was in the presynaptic membrane length. The postsynaptic features of bag 1, bag 2 and chain endings were similar in the long limb muscle spindles. In the intrinsic hand muscle, however, the bag 1 and chain endings showed complex postsynaptic structure which resembled the extra fusal endings while the postsynaptic structure of bag 2 endings was much simpler. From these studies we conclude that the postsynaptic structure of various intrafusal fiber types is dissimilar in different muscles.