NUMBER AND DISTRIBUTION OF NEUROMUSCULAR SPINDLES IN HUMAN EXTRAOCULAR-MUSCLES

Purpose. To examine the number and distribution of muscle spindles in all extraocular muscles (EOMs) in humans. Methods. Thirty-six EOMs wer e obtained after death from three persons 67, 72, and 83 years of age. Serial sections were made throughout the length of these muscles. Con secutive sections were stained with different methods. To discriminate true spindles from false spindles, light microscopic criteria were de fined and were subject to ultrastructural investigation. A distal port ion of a single EOM was gained from a multiorgan donor 17 years of age , processed for electron microscopy, and analyzed. Results. Spindles w ere observed in all muscles studied,with the medial rectus exhibiting a mean of 18.83 spindles (standard deviation), the lateral rectus 19.3 1.9, the superior rectus 15.82.5, the inferior rectus 34.04.4, the sup erior oblique 27.38.2, and the inferior oblique 4.31.8 per muscle. For each different human EOM, a typical distribution of spindles was obse rved in the persons examined. The ultrastructural investigation reveal ed sensory endings in structures primarily identified as spindles. Con clusions. By comparing 1 g of tissue, spindles are found to be at leas t as frequent in human EOM as in skeletal muscles known to have a high density of spindles. This fact and the peculiar distribution of spind les in human EOMs suggest that spindles are functionally important pro prioceptors in EOM.