New England College of Optometry - Scope Yearbook (Boston, MA)

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THE SCOPE Co1rtiooQNuolea1r Pathways for Eye Movements PAUL S. CLINE The study of the ocular innervational pathways have always held the in- terest of the investigator of neuro-anatomy, not only on account of the intimate relationship existing between the organ of sight and brain through its embryo- logical, anatomical and functional connections, but also because of the fact, that in disorders of the nervous system, ocular symptoms, principally as disturbances of movement or coordination, are in the great many of the cases, present. Since the memorable experiments of Vogt, anatomists have written vol- umes on the localization of function of the brain cortex. A motor area in the precentral gyrus, a somesthetic region in the parietal lobe, a visual center in the calcarine region, etc., have revealed themselves to the indomitable perse- verance of men of the calibre of Schaefer and Luciani, Sherrington and Horsley, Bernheimer and Dejerine, Tilney and Riley and so forth. It must be remembered that a cortical center for ocular movements, best thought of as a physiological rather than an anatomical entity, serves for the purpose of initiating the efferent or motor impulse to the nuclei controlling the innervation of a set of conjugate muscles associated obviously in a particular con- jugate movement, and does not preside over a single nerve or a single muscle. To the present day, three sites have been uncovered in the cerebral cortex, which are engaged in ocular gyrations. One exists in the frontal lobe, another in the occipital lobe and iinally one is located in the temporal lobes of the cerebrum. The oculogyratory area in the frontal lobe occupies a small area in the second and third frontal convolutions of the both hemispheres. This region is purely for the execution of eye-movements dependent upon the volition of the individual. Stimulation of this area results in a conjugate movement of the eyes and the head to the opposite side, vertical ocular movements. convergence and clilatationof the pupil. The frontal centers in each lobe are thereforeantagonistic, The cortical center in the occipital lobe covers quite an area of cortex, Stimulation of the angular gyrus of one side results in a conjugate movement to the opposite side, convergence, pupillary dilatation and vertical movements. Schaefer. Horsley, Bernheimer and others have shown that stimulation of the an- terior part of the angular gyrus, resulted in binocular elevation, while a similar stimulation in the posterior part of this convolution resulted in depression ofthe both eyes. The above results cannot only be produced by stimulating the angular gyrus, but also by exciting a large area in the lateral surface of the occipital lobe, and even its medial surface. The occipital center is purely a reflex center, and serves for the completion of the attention reflex arc, having for its afferent links, the optic impulses traveling to the calcarine area. The center in the temporal lobe is situated in the posterior part of the su- perior temporal gyrus 4Foersterj, while even some advocate a center located in the parietal lobe. This temporal region serves for the execution of ocular rota- tions dependent upon acoustic stimuli, and is thought to be purely a reflex center. If a center exists in the parietal lobe it probably serves for ocular con- jugate innervation in response to a somesthetic or perhaps equilibratory stimulus. Stimulation of the temporal center results in the same movements as is ob- tained by stimulating the occipital centers. l3l

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THE SCOPE - FUNCTIONAL DIFFERENCES In the course of investigation the following results were obtained: a. a lesser stimulation of the frontal center was required to effect a cer- tain conjugate movement than if the corresponding occipital or temporal cen- ters were stimulated. b. a similar intensity of stimulus resulted in quicker eye movements when the frontal area was stimulated, than if either of the other two was sti- mulated. c. destruction of the frontal centers resulted in an abolition of the voli- tional pathways for conjugate eye movements, but Hxation of gaze was possible due to the mediation of the efferent impulses originating in the occipital lobe. d. fixation depends upon the occipital lobe, while direction depends upon the frontal lobe. e. destruction of both calcarine areas, abolished convergence. CORTICO-NUCLEAR PATHWAYS l,Suprasegmental Pathwaysb lUpper Motor Neuronj The nuclei of the eye muscles are connected with the frontal cortex not by the regular pyramidal system, but by two systems of fibers of what Dejerinc terms the aberrant pyramidal system. known as the subthalamic and peduncular components of the aberrant pyramidal tracts. The axones of the cortical cells pass downward through the corona ra- diata in the pyramidal system, pass through the genu of the internal capsule into the mesencephalon or mid-brain. Here the fibers destined for the oculomotor and trochlear nuclei detach themselves from the remainder of the aberrant pyramidal system and pass to the nuclei of the opposite side, while the fibers destined for the abducens nucleus pass down further in the stem to the upper pontine region and pass to the opposite abducens nucleus. The subthalamic group is principally concerned with vertical movements and is associated with the muscles of the upper eyelid and the frontalis. It leaves the aberrant pyramidal system in the subthalamus, and passes backward to reach the superior colliculus. Here the fibers are redistributed and sent to the nuclei in- nervating the muscles concerned in vertical movements. The peduncular group is concerned with lateral movements of the eyes, and leaves the aberrant pyramidal system in the cerebral peduncle as two sets of fibers, known as the pes lemnisci profundus, and the pes lemniscis superficialis. The pes lemnisci profundus is situated in the basis mesencephali dorso- medial to the occipito-parieto-temporo-ponto-cerebellar tract, and is the deep layer of the aberrant pyramidal system which blends with the median lemniscus or Hllet. The pes lemnisci superficialis is situated in the basis of the mid-brain dor- sal to the fronto-ponto-cerebellar tract. These two bundles descend for a short distance in the medial fillet and proceed dordally to the tegmentum to the center for lateral movements which is believed to be the nuclus of Fuse. which is part of the triangular nucleus. From here fibers are relayed to the nuclei concerned with lateral movements. Convergence movements have been produced by stimulating an area in the frontal lobe, and also by exciting a part of the lateral convexity of the oc- cipital lobe. The aberrant pyramidal system also sends branches to the spinal accessorv nerve as well as to the upper segments of the spinal cord for the associated move- ments of the head and eyes. l4l