1. Voluntary Movement of the Eyes
The eyeballs move when the extraocular striated muscles contract. Medial movement is mediated by the medial rectus muscle, innervated by somatic motor axons traveling in cranial nerve III (the oculomotor nerve). Lateral movement is mediated by the lateral rectus muscle, innervated by axons which travel in cranial VI (the abducent nerve). Upward movement is mediated by the superior rectus and inferior obliques muscles (both innervated via cranial III) and downward movement is mediated by the inferior rectus muscle (innervated via cranial III) and by the superior oblique muscle (innervated via cranial IV, the tro-chlear nerve). The brain stem location of the various groups of motoneurons is shown in Fig. 2.
The eyeballs normally move in a mutually coordinated manner. When, for example, the individual voluntarily looks to the right, the lateral rectus muscle of the right eyeball contracts at the same time as the medial rectus muscle of the left eyeball. This horizontal conjugate eye movement depends on coordinated activity in the right abducent nerve (cranial VI) and the relevant fibers in the left oculomotor nerve (cranial III). This coordination is achieved via activity descending from the left cerebral cortex to controlling pre-motor and interneurons in the brain stem, as outlined in Fig. 3. Vertical eye movements are coordinated by premotor neurons in the upper midbrain.
2. Reflex Movements of the Eyes by the Vestibuloocular Reflex
In a dead person, the eyes stay still in relation to the head so that when the head is held and turned to the left, the eyes move together with the head. In an alive but unconscious person with an intact brain stem, when the examining doctor turns the person's head to the left, the functioning vestibuloocular reflex turns the eyes to the right so that they continue to look straight ahead. This compensatory gyroscope-like adjustment of the eye position is known as a dolls-eye movement. It also occurs in the conscious individual, facilitating fixation of the gaze when the head is moving.
Head movements are detected by the afferent neurons innervating the canal system of the vestibu-lar-labyrinthine apparatus (Fig. 3). For example, if an individual is looking at an object straight in front of him or her and the head is suddenly turned to the left, the left horizontal canal primary sensory neurons will increase their discharge rate. Axons of these neurons enter the brain stem via cranial nerve VIII and excite second-order afferent neurons in the vestibular nuclei of the brain stem (Figs. 2c and 2d). These second-order vestibular neurons project to the contralateral abdu-cens motoneurons (innervation of the right lateral rectus muscle) and to the contralateral abducens interneurons, whose axons cross the midline and ascend in the medial longitudinal fasciculus to the oculomotoneurons (cranial III) that innervate the medial rectus muscle. Thus, turning the head to
the left reflexly leads to contraction of the right lateral rectus and the left medial rectus muscles, turning the eyes to the right.
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