загрузка...
 
Voluntary and Reflex Innervation of the Eye Muscles
Повернутись до змісту

Voluntary and Reflex Innervation of the Eye Muscles

The extremely precise cooperation among the individual ocular muscles that move the eye charge of ocular movements. Such intimate connections are provided by the medial longitudinal fasciculus, which runs bilaterally parallel to the midline from the tegmentum of the midbrain down to the cervical cord. It interconnects the nuclei of the motor nerves of the eye muscles. It also receives impulses from the cervical cord (serving anterior and posterior neck muscles), from the nuclei of the vestibular nerves, from the reticular formation controlling pontine and mesencephalic “centers of vision”, and from cerebral cortex and basal ganglia.

The eyes can be moved both voluntarily and by reflex. They can, however, move only jointly – in other words, in conjugation. In whatever direction the eyes move, all eye muscles participate in the action, in part by increasing muscular tension (agonists), in part by relaxation (antagonists).

When looking voluntarily at an object, the eyes perform very fast jerking and most precise movements (saccades). Most eye movements take place by reflex, however, and are not voluntary. Any object entering the field of vision attracts attention, and vision is involuntarily directed to it. As the object moves, the eye follows involuntarily in an attempt to keep the image of the object continuously in the area of most acute vision, the fovea of both maculae. If we voluntarily focus on an object of interest, the eye is automatically kept on it, even if we as well as the object of interest are moving. Thus, all voluntary eye movements are appended by involuntary reflex movements. This reflex (holding the image of the object of interest in the retinal area of most acute vision) is called the fixation reflex.

The afferent portion of this reflex runs from the retina over the visual pathway to the visual cortex (area 17). Then the impulses are transmitted to areas 18 and 19. The efferent fibers probably originate in these areas and temporarily join the optic radiation on their course to contralateral mesencephalic and pontine centers of oculomotion from there they join the corresponding nuclei of the ocular motor nerves. It is likely that some of the efferent fibers go directly to the ocular motor centers and the rest do so indirectly, making a detour over Brodmann's area 8.

Special components of the reticular formation contained in the anterior portion of the midbrain regulate individual directions of vision. E.g. the nucleus of the posterior commissure – downward movements, and the interstitial Cajal's nucleus and Darkschewitsch's nucleus – rotatory movements of the eyes.

Segments of the superior colliculi can also produce eye movements in certain directions. Upward movements are represented at the anterior margin of the superior colliculi. Destruction of this area produces paralysis of upward gaze (Parinaud's syndrome). Impulses originating at the convexity of the occipital lobes also travel to contralateral pontine centers for eye movement and produce conjugate lateral movements of the eyes. Voluntary eye movements are triggered in the frontal eye field of Brodmann's area 8, located in front of the precentral gyrus, and perhaps also in parts of areas 6 and 9. A conjugate movement of the eyes (deviation conjugu?) toward the opposite side (the patient looks away from the side of irritation) is the most frequent response to stimulation in this territory. The movement of the eyes is sometimes accompanied by a movement of the head toward the opposite side. Unilateral destruction of area 8 creates a predominance of the contralateral area, producing a conjugate movement toward the side of the lesion (the patient looks towards the paralyzing focus). This deviation abates with time. The situation is reversed with pontine lesions, because the corticopontine pathways cross. A pontine paralysis of gaze is seldom followed by full recovery.

It is not quite clear how the frontal eye fields are connected with the nuclei for ocular motion. The fibers accompany the corticonuclear tract on its way to the internal capsule and cerebral peduncle. They do not terminate directly in the nuclei of the cranial nerves, however. It appears that their impulses arrive at the nuclei via intercalated neurons in the reticular formation and via the medial longitudinal fasciculus.

All voluntary movements of the eyes are under the influence of reflex arcs. Some of these arcs belong to the optical and others to the acoustic, vestibular, and proprioceptive reflex arcs (originating in ventral and dorsal muscles of the neck and transmitted via the spinotectal tract and the medial longitudinal fasciculus).

After unilateral destruction of the frontal eye field, the eyes for a time cannot be turned voluntarily to the opposite side, but such movement is still possible by reflex action. The patient is able to pursue with his eyes an object that moves slowly through his field of vision even if he is not able to follow its direction voluntarily (pursuit reflex).

In contrast, if the occipital fields are destroyed, the reflex movements of the eyes are abolished. The patient can move his eyes voluntarily in every direction, but he cannot pursue an object. It immediately escapes from the area of highest acuity and must be searched for again by voluntary eye movements.



загрузка...