Role of cerebellar pathways in gaze holding

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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