Innervated myotendinous cylinders in human extraocular muscles

PURPOSE. To analyze palisade endings and their end organs, the so-called in nervated myotendinous cylinders (IMCs), of human extraocular muscle (EOM) i n more detail and to clarify with the help of double-fluorescent labeling a nd electron microscopy whether terminals in IMCs are sensory, serving propr ioception. METHODS. EOMs obtained from a donated cadaver (66 years) and distal parts o f EOMs from multiorgan donors (35, 53 years) were processed for double-fluo rescent labeling. Antibodies against the protein gene product 9.5 and alpha -bungarotoxin labeling were used on cryostat sections of distal myotendons. EOMs from multiorgan donors (2, 17 years) were prepared for electron micro scopy. RESULTS. Palisade endings investing muscle fiber tips established contacts with tendon fibrils and the muscle fiber attached. alpha-Bungarotoxin bound to myoneural contacts but not to axonal varicosities in the tendon compart ment. Ultrastructural analysis revealed that palisade endings form IMCs, wh ich were associated exclusively with multiply innervated global layer muscl e fibers. IMCs consisted of a muscle fiber tendon junction, tightly enclose d by fibrocytes, and a supplying axon with preterminals and terminals. Term inals contained mitochondria, few neurotubuli, few neurofilaments, and accu mulations of clear vesicles of uniform size. A basal lamina always interven ed between axolemma and tendon fibrils as well as between axolemma and musc le fiber cell membrane. CONCLUSIONS. Palisade endings of human EOM form IMCs as in cat, monkey, and sheep. In contrast to animals, myoneural contacts in human IMCs are almost certainly motor, whereas terminals contacting tendon fibrils are arguably sensory. Thus, IMCs might be best described as "propriocept-effectors".