Proprioception and palisade endings in extraocular eye muscles

Palisade endings occur only in extraocular muscles, and their function is unknown. They form a cuff of nerve terminals around the tips of muscle fibers. We describe here the advantages of using antibodies to a synaptosomal-associated protein (SNAP-25) to study properties of palisade endings in man, monkey, and rat. The stain can be combined readily with other immunofluorescence procedures, and results suggest that the synapses of palisade endings do not bind a-bungarotoxin (i.e., are not motor), nor do they contain substance P These double-labeling data support the hypothesis that palisade endings are non-nociceptive sensory receptors, and could serve a proprioceptive function. With SNAP-25 immunolabeling, palisade endings were identified in the rat for the first time. Thus, palisade endings appear to be present in all vertebrate extraocular muscles studied to date. Their apparent universality, which contrasts with the more variable manifestation of extraocular muscle spindles and Golgi tendon organs, would be expected if proprioceptive feedback is necessary to the function of the ocular motor system, and if palisade endings are the critical proprioceptive structure.