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We have tested our hypothesis suggesting (i) that for the reliable determination and counting of muscle spindles (Msp) at the light microscopy level in extraocular muscles (EOM), analysis of the spindle specific myosin heavy chain (MyHC) immunoreactivity of intrafusal fibers, especially after staining with anti-slow-tonic MyHC antibodies, is the most convenient tool, (ii) that the number of Msp determined by the slow-tonic MyHC immunoreactivity of intrafusal fibers in EOM is much lower than that based on histological examination and (iii) that the previously reported numbers of Msp based on histological examination of EOM could be overestimated. In order to determine the number and distribution of Msp and to analyze the MyHC isoform immunoreactivity of intrafusal fibers in porcine EOM, paraffin sections of three 9-month-old pig medial (MR) and lateral rectus (LR), levator palpebrae (LP) and retractor bulbi (RB) muscles were stained histologically or using specific monoclonal antibodies (mAbs) against MyHC isoforms. Msp in recti and LP muscles studied by immunocytochemistry contained nuclear bag (NB) fiber(s) reacting with mAbs against slow-tonic, slow-twitch, alpha-cardiac and neonatal MyHCs, but not with the mAb against fast-twitch MyHC, which, on the contrary, stained nuclear chain (NC) fibers. Based on determination of spindle specific slow-tonic MyHC isoform immunoreactivity we have found 72 Msp in the MR and 68 Msp in the LR and 12 Msp in LP muscles, which was only 62, 55 and 32% of the Msp total counts according to histological examination, respectively. In the RB muscle, we have even found only 15 spindle-like-structures composed of encapsulated thin muscle fibers, which possessed only a reaction with anti-fast-twitch MyHC mAb, but lacked slow-tonic, slow-twitch or alpha-cardiac MyHCs immunoreactivity. Our analysis of porcine EOM confirmed the above suggestions, demonstrating, for the first time in the pig, the presence of "false Msp" mimicking encapsulated muscle fibers on histological sections that lack spindle specific MyHC immunoreactivity. In analogy with other muscles we suggest that "false Msp" are not innervated by sensory axons and therefore do not contribute to the physiological sensation of the muscle length changes. Our results thus show that the reliable identification of functionally effective Msp in EOM must involve immunohistochemical analysis of spindle specific MyHC isoforms of intrafusal fibers, as "false" spindles appearing on histologically stained sections as encapsulated muscle fibers could be regarded as "true" Msp and thus increase the spindle number counts in earlier studies.

The capacity of muscle spindles in adult human extraocular muscles (EOM) to provide effective proprioception was questioned on the grounds of their peculiar morphology. Their appearance could be attributable to ageing and to test this possibility examples of infant muscle spindles have been examined. Forty encapsulated structures from five extraocular muscles removed post mortem from 4 infant patients aged 6 days, 5, 23 and 30 months were examined by means of light microscopy using serial transverse sections. Seven of them were identified as false spindles. The remaining 33 structures, identified as spindles, contained a total of 175 intrafusal fibres varying from 2 to 12 (mean: 6) in each. 130 of these fibres (74.3%) were of nuclear chain type. Unequivocal evidence of bag fibres was not found. Spindles lacked or had a limited equatorial expansion, and the inner capsule was incomplete and irregularly shaped. 45 (25.7%) of the intrafusal fibres had extrafusal features with large diameters, peripherally placed nuclei, no equatorial modification and without associated sensory nerve terminals. Serial sections revealed that a majority of the nuclear chain fibres were interrupted, fragmented or terminated abruptly, and most spindles contained at least one incomplete fibre. These observations show that the atypical features observed in adult human EOM spindles are also present in infants and are therefore not attributable to ageing., (Copyright 2001 S. Karger AG, Basel)

Background: Increasing importance is attributed to the proprioceptive innervation of extraocular muscles for the development of binocular vision. In the literature, data on proprioceptors in human extraocular muscles are rare and inconsistent. Therefore a detailed morphological analysis of spindles in human extraocular muscles appeared indicated., Material and Methods: Complete serial sections of all extraocular muscles of 3 human individuals, 67, 72, and 83 years of age, were alternately impregnated with silver, stained following Mowry and immunohistochemically stained for S100 or PGP 9.5. Distal parts of extraocular muscles from multiorgan donors, 2, 17, and 34 years of age, were prepared for electron microscopy., Results: Spindles contained 1 to 18 intrafusal muscle fibers, 55% of which were nuclear chain fibers, 2% nuclear bag fibers, and 43% fibers exhibiting the same morphological features as extrafusal fibers. The latter had been previously described as "anomalous fibers" (Ruskell). This intrafusal type exclusively occurs in human extraocular muscle spindles. Ultrastructural analysis revealed morphologically normal sensory terminals on all 3 types of intrafusal fibers. Findings in a two-year-old individual were similar to those in aged individuals., Conclusions: Spindles in human extraocular muscles are specifically structured. Their special morphological features are also present during the development of binocular vision. Spindles are supposed to play a role in the (fine) control of eye movements.

The motor and proprioceptive innervation of the retractor bulbi muscle of the lamb has been studied. The motor innovation of the muscles supplied only by the abducens never. The proprioceptive nervous fibers coming from the muscle spindles of this extraocular muscle enter the brain stem through the ophthalmic branch of the trigeminal nerve and have their perikarya in the semilunar ganglion.