Mesodiencephalic representation of the eye muscle proprioception

Abstract

Unitary responses to moderate manual stretch of individual extraocular muscles were found in some mesodiencephalic areas of the lamb. Such responses were of the type induced by muscle spindle excitation and were recorded in the mesencephalon from the ipsilateral medial lemniscus, tegmentum and tectum of the superior and inferior colliculus; in the thalamus they were essentially localized in the ipsilateral posteroventrolateral nuclei. The mesodiencephalic sites influenced by stretch of extrinsic eye muscles could be also activated by single-shock electrical stimulation of the ipsilateral points of the pars oralis of the descending trigeminal nucleus, of the homonymous tract and of the main sensory trigeminal nucleus in which the eye muscle proprioception is represented. Such evoked potentials exhibited a very short latency (0.33–1 msec); thus, no synapse or only one synapse was intercalated between the stimulated and recorded sites. In this way it seems reasonable to assume that the axons of the second-order neurons of the extraocular muscle proprioception located in the pars oralis of the descending trigeminal nucleus and in the main sensory trigeminal nucleus project through the medial lemniscus on the tectum and tegmentum of the mesencephalon and on the posteroventrolateral nuclei of the thalamus.