The structure of tendon organs in the cat: a proposed mechanism for responding to muscle tension

By reconstructing tendon organs in detail from serial cross sections, a prediction is made on how these sensors might affect their nerve endings during changes in tension at musculotendinous junctions. Tendon organs are composed of specialized encapsulated fascicles of dense collagen which are offshoots from the primary tendon of origin or insertion of a muscle. Fascicles project into the muscle mass on their way to becoming attached to a bundle of extrafusal muscle fibers. The main spur of dense collagen rapidly divides into fine bundles with septa between. Within this encapsulated region, branches of the entering sensory nerve intertwine among the fine bundles of collagen fibers via the septa. Towards the muscle end of the tendon organ, these fine bundles reorganize again into heavy, dense bundles before insertion of extrafusal muscle fibers. The hypothesis is presented that the spaces between collagen bundles in a relaxed tendon organ might spread open reducing pressure upon nerve endings lying between; during tension, these bundles would straighten and crowd together, leading to compression of nerve endings. Variable compression of nerve endings would excite electrochemical events in sensory axons; thus, tension information would be coded and transmitted to the central nervous system. The arrangement of a tendon organ in series with muscle fibers would imply that the organ sampled tension primarily of that group of extrafusal muscle fibers attaching to the organ.