THE ELEMENTARY VESTIBULO-OCULAR REFLEX ARC

Abstract

A method was developed for artificial stimulation of semicircular ducts of-dogs and cats. Fine glass cannulas filled with normal saline are introduced under the control of a stereoscopic microscope into the ducts with the aid of a micromanipulator. Through these cannulas endolymph movements are created by changing the pressure in the cannula. The superior and lateral ducts were stimulated in this way on the otherwise intact labyrinth; the posterior duct was stimulated by the same method also after extirpation of the utriculus, sacculus, and the membranous ampullas of the superior and lateral ducts. The activity of the extraocular muscles during stimulation was recorded simultaneously on a kymograph. The receptors of the vertical semicircular ducts are more responsive to endolymph currents of the direction "from the ampulla," the receptors of the horizontal ducts to those "toward the ampulla." From stimulation of the different semicircular ducts it appears that there is a predominant functional correlation between any crista and 2 of the extraocular muscles. The crista of the superior semicircular duct has a. predominant connection with the ipsilateral superior rectus and the contralateral inferior obliquus; the crista of the posterior duct with the ipsilateral superior obliquus and the contralateral inferior rectus; the crista of the horizontal duct with the ipsilateral medial rectus and the contralateral lateral rectus. This functional correlation is in accordance with previous anatomical findings concerning the pattern of the 3-neuron-reflex-arc between labyrinth and extraocular muscles. From this agreement it may be inferred that the predominant correlation between any crista and 2 extraocular muscles is established by the elementary vestibulo-ocular reflex-arc, in which 3 neurons are involved. This assumption is supported by the results of expts. with transection of the posterior longitudinal fasciculus and the pons with the exception of the former tract. Besides the elementary 3-neuron-arc there exist numerous arcs, most of them closed through the reticular formation, by way of which the impulses from any labyrinthine receptor may be brought into connection with any one of the extraocular muscles. The function of the reticular formation seems to be essential, especially for elaboration of inhibitory responses.