Myotendinous nerve endings in human infant and adult extraocular muscles

Myotendinous nerve endings in extraocular muscles of some mammals consist exclusively of palisade nerve endings incorporated in myotendinous cylinders. There is evidence for a similar form in man, some doubt remains. The objectives of the present study were to examine the structure and distribution of nerve endings in extraocular muscles of infant and adult human material. Muscles from five infants and six adults aged 3 days to 90 years were prepared for light and electron microscopy. Nerve endings were sparse in a 4-year-old and none were present in the muscles of younger donors. They were present in all adult samples. One group of nerve endings branched from single recurrent nerve fibers and were distributed in the encapsulated tendon of single Felderstruktur muscle fibers. Terminals were varicose and shared certain characteristics of known sensory endings and were similar to those of myotendinous cylinders except that none formed neuromuscular junctions. In other myotendinous complexes capsules were fragmented and nerve endings were dispersed in tendon common to two or more muscle fibers. In the myotendon of two adult donors, a further group of endings issuing from non-recurrent nerves were unencapsulated and distributed randomly in tendon. The frequency of nerve endings varied across myotendon and in some instances, most marked in one case, large areas lacked nerve endings. Golgi tendon organs were not present. The terminals having features characteristic of sensory endings suggest a proprioceptive function, which is apparently unavailable in infancy. In mature muscles, the irregular distribution and variety of terminal form cannot be equated with those found in extraocular muscles of animals. We suggest that these features reflect an aberrant development and conclude that their capacity to fulfil an effective proprioceptive role is open to question.