Innervated myotendinous cylinders in human extraocular muscles

PURPOSE. To analyze palisade endings and their end organs, the so-called innervated myotendinous cylinders (IMCs), of human extraocular muscle (EOM) in more detail and to clarify with the help of double-fluorescent labeling and electron microscopy whether terminals in IMCs are sensory, serving proprioception. METHODS. EOMs obtained from a donated cadaver (66 years) and distal parts of EOMs from multiorgan donors (35, 53 years) were processed for double-fluorescent 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 microscopy. 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 compartment. Ultrastructural analysis revealed that palisade endings form IMCs, which were associated exclusively with multiply innervated global layer muscle fibers. IMCs consisted of a muscle fiber tendon junction, tightly enclosed by fibrocytes, and a supplying axon with preterminals and terminals. Terminals contained mitochondria, few neurotubuli, few neurofilaments, and accumulations of clear vesicles of uniform size. A basal lamina always intervened between axolemma and tendon fibrils as well as between axolemma and muscle 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".