Proprioception in the extraocular muscles of mammals and man.

This article summarizes the authors' previous studies on proprioceptors in extraocular muscles (EOMs) of mammals and man. They report on muscle spindles in the EOMs of man, Golgi tendon organs in the EOMs of even-toed ungulates, and palisade endings in the EOMs of the cat. Muscle spindles: Muscle spindles are present in the EOMs of some mammals and in the EOMs of man. Compared with muscle spindles in other skeletal muscles, those in human EOMs exhibit structural differences. These structural differences may indicate a special function. Golgi tendon organs: Golgi tendon organs are absent in human EOMs. Golgi tendon organs exhibiting a specific morphology are present in the EOMs of even-toed ungulates. Their high number and rich innervation indicate functional importance. Palisade endings: Palisade endings are nervous end organs confined to the EOMs of mammals and man. It is assumed that these organs have a proprioceptive function. The authors show that palisade endings are immunoreactive for antibodies against choline acetyltransferase. Neuromuscular contacts, if present in palisade endings, are alpha -bungarotoxin positive as well. Taken together, these results show that palisade endings exhibit molecular characteristics of effector organs.

Palisade endings in extraocular muscles of the monkey are immunoreactive for choline acetyltransferase and vesicular acetylcholine transporter.

PURPOSE: To analyze palisade endings in extraocular muscles (EOMs) of a primate species and to examine our previous findings in cat that palisade endings are putative effector organs. METHODS: Eleven monkeys (Macaca fascicularis) of both sexes, between 4 and 6 years of age were analyzed. Whole EOM myotendons were immunostained with four combinations of triple-fluorescent labeling and examined by confocal laser scanning microscopy. Labeling included antibodies against choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAChT), neurofilament, and synaptophysin. Muscle fibers were counterstained with phalloidin. RESULTS: Palisade endings were observed in all monkey EOMs. Nerve fibers extended from the muscle into the tendon and looped back to divide into a terminal arborization (palisade ending) around a single muscle fiber tip. In approximately 30% of the cases, nerve fibers supplying palisade endings often established motor terminals outside the palisade complex. Nerve fibers forming palisade endings were ChAT-neurofilament positive. Axonal branches of palisade endings were ChAT-neurofilament positive as well. All palisade nerve terminals exhibited ChAT-synaptophysin immunoreactivity. Within the palisade complex, palisade nerve terminals exhibited VAChT immunoreactivity. All palisade nerve terminals were VAChT-synaptophysin immunoreactive. CONCLUSIONS: The results confirm that in the monkey, palisade endings contain acetylcholine and are therefore most likely effector organs. Palisade endings are also present in human EOMs and because of their location at the myotendinous junction, these organs are of crucial interest for strabismus surgery.

Molecular characteristics suggest an effector function of palisade endings in extraocular muscles.

PURPOSE: To analyze palisade endings in cat extraocular muscles (EOMs) and to clarify whether these EOM-specific organs are sensory or motor. METHODS: Twelve cats aged between 1 and 16 years were analyzed. Whole EOM tendons were immunostained using four different combinations of triple fluorescence labeling. Triple labeling included antibodies against choline acetyltransferase (ChAT), neurofilament, synaptophysin, and alpha-bungarotoxin. Preparations were examined by confocal laser scanning microscopy. ChAT-labeled EOMs were also analyzed by immunoelectron microscopy. Three-dimensional reconstructions were made of palisade endings. RESULTS: Palisade endings were found in the distal and proximal myotendinous regions of cat EOMs. These endings arose from thin nerve fibers coming from the muscle and extending into the tendon. There, the nerve fibers turned back 180 degrees to divide into terminal branches around the muscle fiber tips. Terminal branches established numerous contacts with the tendon attached to the muscle fiber tip and only a few contacts with the muscle fiber. Often, nerve fibers forming palisade endings on muscle fiber tips were observed to establish multiple motor contacts on muscle fibers outside palisade endings. Three-dimensional reconstructions depicted the complex morphology of the palisade endings. All nerve fibers supplying palisade endings stained positively for ChAT and neurofilament. All nerve terminals in palisade endings were ChAT and synaptophysin positive. Only neuromuscular contacts in palisade endings were positive for alpha-bungarotoxin, as well. CONCLUSIONS: This study provides evidence that palisade endings in cat EOMs have effector function. The findings may be of significance for strabismus surgery because palisade endings are also found in human EOMs.

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 alpha-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 alpha-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 alpha-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.