Muscle spindles and Golgi tendon organs in bovine calf extraocular muscle studied by means of double-fluorescent labeling, electron microscopy, and three-dimensional reconstruction

In the present study muscle spindles (MSps) and Golgi tendon organs (GTOs) in bovine extraocular muscles (EOMs) were analyzed in detail. The innervation pattern of these proprioceptors was investigated with transmission electron microscope and confocal laser scanning microscope after double-fluorescent labeling. Three-dimensional (3D) reconstructions were performed of GTOs. Muscle spindles. MSps are numerous, each containing two nuclear bag fibers and up to eight nuclear chain fibers. In the equatorial region and paraequatorial region thin axons enwrapping the intrafusal muscle fibers form numerous nerve contacts on the muscle fiber surface. Double staining of such nerve terminals with synaptophysin and α-bungarotoxin and their fine structural features confirm their sensory nature. In the encapsulated part of the polar region neuromuscular contacts have structural features of motor nerve terminals and stain positively with α-bungarotoxin. Golgi tendon organs. GTOs are numerous in bovine EOMs. Each GTO contains collagen bundles but frequently also intracapsular muscle fibers. Intracapsular muscle fibers either terminate inside the GTO in collagen bundles or pass through the proprioceptor. GTOs are richly supplied with sensory nerve terminals which intermingle with the collagen bundles. Nerve terminals on intracapsular muscle fibers exhibit fine structural characteristics of motor nerve terminals and are α-bungarotoxin positive. The 3D images of GTOs show the detailed spatial arrangement of the GTO tissue components. These new insights in the complex and specific morphology of MSps and GTOs in bovine EOMs indicate that we deal with highly developed proprioceptors. These are supposed to provide important information for EOM innervation.