Role of cerebellar pathways in gaze holding

为了保持凝视，速度编码的运动指令必须被整合，才能由称为动眼神经积分器的神经电路产生适当的眼睛位置指令。小脑-脑干的相互作用是这种神经整合过程所必需的。参与这种相互作用的一种可能底物是旁正中神经束(PMT)的细胞群。已知PMT神经元投射到小叶，但对其放电行为知之甚少。在本研究中，我们在脑桥PMT区寻找与眼球运动有关的神经元放电，在OPN区背侧的MLF中，有一个集中的单位，呈现向上的爆发-紧张性放电模式(UP-BT神经元)。它们对电刺激和自然前庭刺激的反应表明，它们接受来自对侧前半规管的兴奋性输入。他们还接受了来自Cajal间质核的输入。通过注射GABA受体激动剂麝香酚使UP-BT细胞失活，持续产生下跳性眼球震颤。与单纯前庭眼球震颤不同的是，垂直凝视保持能力受损，表现为缓慢时相(凝视诱发眼球震颤)的指数时间过程。注射GABA受体拮抗剂荷包牡丹碱可引起乐观的眼球震颤，并伴有凝视保持失败。这些结果表明，PMT-小球-前庭核通路在维持稳定的凝视和前庭平衡方面起着重要作用。

For gaze maintenance, the velocity-encoded motor command must be integrated 'to produce an appropriate eye position command by a neural circuit called the oculomotor neural integrator. Cerebellar-brainstem interaction is necessary for this neural integration process. One possible substrate participating in this interaction is the cell groups of the paramedian tract (PMT). PMT neurons are known to project to the flocculus, but little is known about the firing behavior. In the present study, we searched the pontine PMT region for neurons discharging in relation to eye movements, In the MLF dorsal to the OPN area, there was a concentration of units that displayed burst-tonic firing pattern with upward on-direction (up-BT neurons). Their responses to electrical and natural vestibular stimulation indicated that they receive excitatory input from the contralateral anterior semicircular canal. They also received inputs from the interstitial nucleus of Cajal. Inactivation of up-BT cells by injection of muscimol, the GABA receptor agonist, consistently produced downbeat nystagmus. Unlike simple vestibular nystagmus, vertical gaze holding was impaired as indicated by an exponential time course of slow-phases (gaze-evoked nystagmus). Injection of bicuculline, the GABA receptor antagonist, caused upbeat nystagmus combined with gaze holding failure. These results thus suggests that the PMT-flocculus-vestibular nucleus pathway is important in maintaining both the..Stable gaze and vestibular balance