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.
INTERNUCLEAR ORGANIZATION: GAZE CENTERS
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.
To achieve horizontal gaze, motor neurons innervating the lateral rectus and the contralateral medial rectus subnucleus need to receive equal and simultaneous activation.
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).
Key brainstem nuclei involved in eye movement control and neighboring structures.
(Modified with permission from Kline LB: Neuro-ophthalmology Review Manual, 7th edition. Thorofare, NJ SLACK, 2013.)
Horizontal gaze circuits.
This rendering of the brainstem circuitry involved in eye movements is drawn from the examiner's view of a patient. The drawing is not to scale. The example shown is right gaze in a normal subject: Right gaze is initiated by the right paramedian pontine reticular formation, activating the right cranial nerve (CN) VI nucleus. The right lateral rectus is activated via CN VI. The left medial rectus subnucleus is activated by an internuclear pathway originating from cells within the CN VI nucleus, whose axons cross to the left medial longitudinal fasciculus ...