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

PURPOSE. To analyze qualitatively and quantitatively the total complement o f encapsulated proprioceptors (Golgi tendon organs [GTOs] and neuromuscular spindles) in pig extraocular muscles (EOMs). METHODS. EOMs of four pigs of different ages were prepared for light micros copic histochemical and immunohistochemical analysis and for transmission e lectron microscopy. RESULTS. GTOs and muscle spindles were numerous in pig EOMs, GTOs were foun d to be distributed in aponeurotic expansions of the distal and proximal EO M tendons, being more numerous in the distal aponeurosis than in the proxim al 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 additio n to Collagen bundles, approximately one third of the GTOs contained intrac apsular muscle fibers that resembled the multiply innervated fiber type. In tracapsular muscle fibers entered the poles of the GTOs and either terminat ed inside the receptors in collagen bundles or exited the GTOs at the oppos ite poles. Nerve terminals were numerous in each GTO and established intima te contacts with collagen fibrils. CONCLUSIONS. Most structural particularities formerly observed in GTOs of r hesus monkey and sheep EOMs are also present in GTOs of pig EOMs. The high number of GTOs with their typical nerve terminals indicates functional impo rtance. During muscle activity, afferent signals from GTOs and muscle spind les may provide sufficient information about eye position.