Sensory control of extraocular muscles.

The role of sensory receptors in eye muscles is not well understood, but there is physiological and clinical evidence for the presence of proprioceptive signals in many areas of the central nervous system. It is unclear which structures generate these sensory signals, and which central neural pathways are involved. Three different types of receptors are associated with eye muscles: (1) muscle spindles, (2) palisade endings, and (3) Golgi tendon organs, but their occurrence varies wildly between species. A review of their organization shows that each receptor is mainly confined to a morphologically separate layer of the eye muscle. The palisade endings - which are unique to eye muscles, are associated with the global layer; and they have been found in all mammals studied so far. Their function is unknown. The muscle spindles, if they are present in a species, lie in the orbital layer, or at its junction to the global layer. Golgi tendon organs appear to be unique to artiodactyls (i.e., sheep and goats, etc.); they lie in an outer distal marginal layer of the eye muscle, called the "peripheral patch layer" in sheep. The specific association between palisade endings and the multiply innervated type of muscle fibers of the global layer has led to the hypothesis that together they may act as a sensory receptor, and provide a source of central proprioceptive signals. But other interpretations of the morphological evidence do not support this role.