Extraocular muscle proprioception and the descending trigeminal nucleus

Abstract

Scope of the present investigation was to look for the termination within the brain stem of the central process of the perikarya contained in the semilunar ganglion which provide the extraocular muscle spindles. Unitary responses to stretching single eye muscles were recorded by means of tungsten microelectrodes from the pars oralis of the ipsilateral descending trigeminal tract and nucleus in the lamb. The responses were characterized by a sudden increase in discharge rate of the units followed by a modest adaptation; the firing ceased as soon as the stretch was released. Single-pulse electrical stimulation of the cellular pool of the medial dorsolateral part of the semilunar ganglion which innervate the eye muscle proprioceptors elicited evoked potentials in the same ipsilateral pontine sites responsive to stretching the extraocular muscles. The latency of such evoked potentials was very short: 0.42–0.95 msec. We conclude that the afferent impulses from the eye muscles project to the descending trigeminal nucleus. Thus the central process of the semilunar ganglion cells concerned with the eye muscle proprioception enters the brain stem through the trigeminal sensory root, runs along the ipsilateral descending trigeminal tract, and terminates within the homonymous nucleus.