Pierrot-Deseilligny C
Service de Neurologie 1, Assistance Publique-Hôpitaux de Paris, Hôpital de la Salpêtrière, Paris, France.
Ann N Y Acad Sci. 2009 May;1164:155-65. doi: 10.1111/j.1749-6632.2009.03864.x.
There is growing evidence that gravity markedly influences vertical eye position and movements. A new model for the organization of brainstem upgaze pathways is presented in this review. The crossing ventral tegmental tract (CVTT) could be the efferent tract of an "antigravitational" pathway terminating at the elevator muscle motoneurons in the third nerve nuclei and comprising, upstream, the superior vestibular nucleus and y-group, the flocculus, and the otoliths. This pathway functions in parallel to the medial longitudinal fasciculus pathways, which control vertical eye movements made to compensate for all vertical head movements and may also comprise the "gravitational" vestibular pathways, involved in the central reflection of the gravity effect. The CVTT could provide the upgaze system with the supplement of tonic activity required to counteract the gravity effect expressed in the gravitational pathway, being permanently modulated according to the static positions of the head (i.e., the instantaneous gravity vector) between a maximal activity in the upright position and a minimal activity in horizontal positions. Different types of arguments support this new model. The permanent influence of gravity on vertical eye position is strongly suggested by the vertical slow phases and nystagmus observed after rapid changes in hypo- or hypergravity. The chin-beating nystagmus, existing in normal subjects with their head in the upside-down position, suggests that gravity is not compensated for in the downgaze system. Upbeat nystagmus due to brainstem lesions, most likely affecting the CVTT circuitry, is improved when the head is in the horizontal position, suggesting that this circuitry is involved in the counteraction of gravity between the upright and horizontal positions of the head. In downbeat nystagmus due to floccular damage, in which a permanent hyperexcitation of the CVTT could exist, a marked influence of static positions of the head is also observed. Finally, the strongest argument supporting a marked role of gravity in vertical eye position is that the eye movement alterations observed in the main, typical physiological and pathological conditions are precisely those that would be expected from a direct effect of gravity on the eyeballs, with, moreover, no single alternative interpretation existing so far that could account for all these different types of findings.
越来越多的证据表明，重力对垂直眼位和眼球运动有显著影响。本文综述提出了一种脑干上视通路组织的新模型。交叉腹侧被盖束（CVTT）可能是一条“抗重力”通路的传出束，该通路终止于动眼神经核中的提上睑肌运动神经元，其上游包括前庭上核和y组、绒球和耳石。该通路与内侧纵束通路并行发挥作用，内侧纵束通路控制为补偿所有垂直头部运动而产生的垂直眼球运动，也可能包括“重力”前庭通路，参与重力效应的中枢反射。CVTT可以为上视系统提供抵消重力通路中所表达的重力效应所需的紧张性活动补充，根据头部的静态位置（即瞬时重力矢量）在直立位置的最大活动和水平位置的最小活动之间进行永久调节。不同类型的论据支持这一新模型。重力对垂直眼位的永久影响强烈体现在低重力或高重力快速变化后观察到的垂直慢相和眼球震颤。正常受试者头朝下时存在的叩颌性眼球震颤表明，下视系统中重力未得到补偿。脑干病变导致的上跳性眼球震颤，很可能影响CVTT回路，当头部处于水平位置时会有所改善，这表明该回路参与了头部直立和水平位置之间重力的对抗。在因绒球损伤导致的下跳性眼球震颤中，可能存在CVTT的永久性过度兴奋，也观察到头部静态位置的显著影响。最后，支持重力在垂直眼位中起显著作用的最有力论据是，在主要的、典型的生理和病理条件下观察到的眼球运动改变恰恰是重力对眼球直接作用所预期的，而且，到目前为止还没有单一的其他解释能够解释所有这些不同类型的发现。
