Electron microscopic study of muscle spindle in human interarytenoid muscle

Using transmission electron microscopy, the fine structure of the muscle spindle in the interarytenoid muscle was studied. The specimen was made from a fresh adult male larynx which had been surgically removed. The muscle spindle showed the following characteristic features: (1) the capsule was thin, with only a small number of layers of capsular sheet cells; (2) the periaxial space was narrow and there were abundant collagenous fibrils; (3) the sensory endings consisted of irregularly coiled terminal axons with varicose swellings, and some endings penetrated the sarcoplasm; (4) one nuclear chain fiber terminated on the capsule, while the other branched into two bundles; and (5) special sarcoplasmic inclusions, possibly related to the sensory endings, were also observed. These findings suggest that the muscle spindle in the interarytenoid muscle is not only a very sensitive stretch receptor, but also a pressure receptor.