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Six systems coordinate and stabilize eye movements. The systems are termed supranuclear because they are higher in the chain of command than the ocular motor nuclei. Internuclear pathways connect the ocular motor nuclei to coordinate conjugate movement of yoke muscles and provide a common pathway for supranuclear systems. Disorders of supranuclear or internuclear pathways can cause conjugate gaze palsies or ocular misalignment. Supranuclear disorders can also cause nystagmus or nystagmus-like oscillations, which are unwanted eye movements that can degrade vision or cause oscillopsia.
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INTERNUCLEAR ORGANIZATION: GAZE CENTERS
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Gaze centers are premotor nuclei that organize and relay supranuclear commands to the appropriate individual motor nuclei of yoked muscles to move the two eyes together in the same direction. Separate systems exist for horizontal and vertical eye movements.
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To achieve horizontal gaze, motor neurons innervating the lateral rectus and the contralateral medial rectus subnucleus need to receive equal and simultaneous activation.
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The paramedian pontine reticular formation (PPRF) is the horizontal gaze center. This nucleus is adjacent to the abducens (cranial nerve [CN] VI) nucleus in the pons (Figure 10–1). The PPRF activates the abducens nucleus in response to supranuclear gaze commands. As discussed in Chapter 9, the abducens nucleus contains two sets of neurons: (1) motor neurons whose axons innervate the ipsilateral lateral rectus muscle, and (2) internuclear neurons with axons that decussate to the contralateral medial longitudinal fasciculus (MLF) and travel to the medial rectus subnucleus (Figure 10–2). Thus, activation of the PPRF in turn activates the two populations of cells in the CN VI nucleus and produces conjugate gaze (to the ipsilateral side).
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