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 were obtained after death from three persons 67, 72, and 83 years of age. Serial sections were made throughout the length of these muscles. Consecutive sections were stained with different methods. To discriminate true spindles from false spindles, light microscopic criteria were defined and were subject to ultrastructural investigation. A distal portion of a single EOM was gained from a multiorgan donor 17 years of age, processed for electron microscopy, and analyzed. RESULTS Spindles were observed in all muscles studied, with the medial rectus exhibiting a mean of 18.8 spindles +/- 3.0 (+/- standard deviation), the lateral rectus 19.3 +/- 1.9, the superior rectus 15.8 +/- 2.5, the inferior rectus 34.0 +/- 4.4, , the superior oblique 27.3 +/- 8.2, and the inferior oblique 4.3 +/- 1.8 per muscle [corrected]. For each different human EOM, a typical distribution of spindles was observed in the persons examined. The ultrastructural investigation revealed sensory endings in structures primarily identified as spindles. CONCLUSIONS By comparing 1 g of tissue, spindles are found to be at least as frequent in human EOM as in skeletal muscles known to have a high density of spindles. This fact and the peculiar distribution of spindles in human EOMs suggest that spindles are functionally important proprioceptors in EOM.