Structural and Molecular Features of Human Extraocular Muscle Spindles Suggest a Unique Role in Proprioception

Purpose: Muscle spindles in extraocular muscles (EOMs) of humans exhibit atypical features, putting their function into question. We tested the presence of molecules, previously described in skeletal muscle spindles, within the human EOM spindles. We also investigated the anatomic trajectory of proprioceptive fibers within the EOM motor nerves. Methods: The EOMs and the EOM nerves were obtained from three individuals (mean age of 84.7 ± 13.6). We also used data from two separate studies, including the EOMs of a 2-year-old child and pig EOMs. For the analyses, we integrated multi-color immunofluorescence, confocal 3D reconstructions, and transmission electron microscopy (TEM). Results: Human EOM spindles contain nuclear chain fibers and intrafusal fibers that resemble extrafusal fibers. Only nuclear chain fibers express myosin light chain 2. The spindle capsule expresses glucose transporter 1. Sensory nerve terminals of the muscle spindles contain clear vesicles, which is confirmed through synaptophysin expression. These terminals also express vesicular glutamate transporter, indicating storage of the neurotransmitter glutamate. Furthermore, sensory nerve terminals express syntaxin, complexin, and synaptobrevin, proteins involved in neurotransmitter release. Sensory (proprioceptive) fibers are not present at the exit site of the EOM motor nerve from the brainstem. They merge with the EOM motor nerves at the superior orbital fissure. Conclusions: Our study has uncovered glutamatergic machinery in human EOM spindles, analogous to that of skeletal muscle spindles. These findings do not support the hypothesis that human EOM spindles are vestigial. Instead, they suggest that spindles are functional, providing feedback about the eye's position.