Number, distribution, and morphologic particularities of encapsulated proprioceptors in pig extraocular muscles

To analyze qualitatively and quantitatively the total complement of encapsulated proprioceptors (Golgi tendon organs [GTOs] and neuromuscular spindles) in pig extraocular muscles (EOMs).

EOMs of four pigs of different ages were prepared for light microscopic histochemical and immunohistochemical analysis and for transmission electron microscopy.

GTOs and muscle spindles were numerous in pig EOMs. GTOs were found to be distributed in aponeurotic expansions of the distal and proximal EOM tendons, being more numerous in the distal aponeurosis than in the proximal aponeurosis. The total number of GTOs was higher in the recti EOMs (100-128) than in the oblique EOMs (45-61). Spindles were distributed over the entire muscle length. In each EOM the number of muscle spindles (142-333) exceeded those of GTOs. The morphology of the GTOs was variable. In addition to collagen bundles, approximately one third of the GTOs contained intracapsular muscle fibers that resembled the multiply innervated fiber type. Intracapsular muscle fibers entered the poles of the GTOs and either terminated inside the receptors in collagen bundles or exited the GTOs at the opposite poles. Nerve terminals were numerous in each GTO and established intimate contacts with collagen fibrils.

Most structural particularities formerly observed in GTOs of rhesus monkey and sheep EOMs are also present in GTOs of pig EOMs. The high number of GTOs with their typical nerve terminals indicates functional importance. During muscle activity, afferent signals from GTOs and muscle spindles may provide sufficient information about eye position.