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The vestibulo-ocular reflex (VOR), sometimes known as but not completely synonymous with〔http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1031870/pdf/jnnpsyc00551-0015.pdf〕 the oculocephalic reflex (which itself is colloquially known as the "doll's head reflex" and is used to assess the health of coma patients, along with VOR)〔 is a reflex eye movement that elicits eye movement by stimulating the vestibular system. This reflex functions to stabilize images on the retinas (in yoked vision) during head movement by producing eye movements in the direction opposite to head movement, thus preserving the image on the center of the visual field(s). For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movement is present all the time, the VOR is very important for stabilizing vision: patients whose VOR is impaired find it difficult to read using print, because they cannot stabilize the eyes during small head tremors, and also because damage to the VOR can cause vestibular nystagmus.〔http://www.dizziness-and-balance.com/practice/nystagmus/vestibular.html〕 The VOR does not depend on visual input. It can be elicited by caloric (hot or cold) stimulation of the inner ear, and works even in total darkness or when the eyes are closed. However, in the presence of light, the fixation reflex is also added to the movement.〔"Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 179 Encyclopaedia Britannica, 1987〕 In other animals, the gravity organs and eyes are strictly connected. A fish, for instance, moves its eyes by reflex when its tail is moved. Humans have semicircular canals, neck muscle "stretch" receptors, and the utricle (gravity organ). Though the semicircular canals cause most of the reflexes which are responsive to acceleration, the maintaining of balance is mediated by the stretch of neck muscles and the pull of gravity on the utricle (otolith organ) of the inner ear.〔 The VOR has both rotational and translational aspects. When the head rotates about any axis (horizontal, vertical, or torsional) distant visual images are stabilized by rotating the eyes about the same axis, but in the opposite direction. When the head translates, for example during walking, the visual fixation point is maintained by rotating gaze direction in the opposite direction, by an amount that depends on distance. ==Circuit== The VOR is ultimately driven by signals from the vestibular apparatus in the inner ear. The semicircular canals detect head rotation and drive the rotational VOR, whereas the otoliths detect head translation and drive the translational VOR. The main "direct path" neural circuit for the horizontal rotational VOR is fairly simple. It starts in the vestibular system, where semicircular canals get activated by head rotation and send their impulses via the vestibular nerve (cranial nerve VIII) through Scarpa's ganglion and end in the vestibular nuclei in the brainstem. From these nuclei, fibers cross to the contralateral cranial nerve VI nucleus (abducens nucleus). There they synapse with 2 additional pathways. One pathway projects directly to the lateral rectus of eye via the abducens nerve. Another nerve tract projects from the abducens nucleus by the medial longitudinal fasciculus to the contralateral oculomotor nucleus, which contains motorneurons that drive eye muscle activity, specifically activating the medial rectus muscle of the eye through the oculomotor nerve. Another pathway (not in picture) directly projects from the vestibular nucleus through the ascending tract of Dieters to the ipsilateral medial rectus motoneuron. In addition there are inhibitory vestibular pathways to the ipsilateral abducens nucleus. However no direct vestibular neuron to medial rectus motoneuron pathway exists. Similar pathways exist for the vertical and torsional components of the VOR. In addition to these direct pathways, which drive the velocity of eye rotation, there is an indirect pathway that builds up the position signal needed to prevent the eye from rolling back to center when the head stops moving. This pathway is particularly important when the head is moving slowly, because here position signals dominate over velocity signals. David A. Robinson discovered that the eye muscles require this dual velocity-position drive, and also proposed that it must arise in the brain by mathematically integrating the velocity signal and then sending the resulting position signal to the motoneurons. Robinson was correct: the 'neural integrator' for horizontal eye position was found in the nucleus prepositus hypoglossi in the medulla, and the neural integrator for vertical and torsional eye positions was found in the interstitial nucleus of Cajal in the midbrain. The same neural integrators also generate eye position for other conjugate eye movements such as saccades and smooth pursuit. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vestibulo–ocular reflex」の詳細全文を読む スポンサード リンク
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