Instability of the eye in the dark and proprioception

Abstract

THE extraocular muscles of most mammals contain stretch receptors which are not necessarily in the form of muscle spindles. The cat, for example, has no muscle spindles, but the oculomotor muscles contain structures, with spiral endings, that are in parallel with the muscle fibres and respond to stretch¹. Most proprioceptive fibres run extraorbitally in the ophthalmic branch of the Vth nerve^2,3. The cell bodies of these fibres seem to be located in the mesencephalic root of the Vth nerve⁴. There is electrophysiological evidence of proprioceptive projections to the cerebellum^5,6, superior colliculus^7,8 and visual cortex⁹. The function of ocular proprioception is uncertain, although there is evidence that in man it may be an indicator of eye position in the dark¹⁰. In the cat it seems to have a role in maintaining the binocularity of single neurones in the visual cortex^11,12. Because most proprioceptive fibres run into the ophthalmic branch of the Vth nerve, we have investigated eye movements in the alert animal before and after section of this branch. We report here that, in the absence of visual and vestibular stimuli, the eye ipsilateral to the section becomes unstable and performs slow pendular oscillations. Asymmetries are observed in the horizontal vestibular nystagmus recorded in total darkness from the two eyes after unilateral section.