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II PHYSIOLOGY OF EYE MOVEMENTS        

III EXAMINATION: EYE MOVEMENTS & NYSTAGMUS

         

IV FIXATION INSTABILITY

     

V SUPRANUCLEAR to NUCLEAR 

 

VI VESTIBULAR SYSTEM  

 

VII CEREBELLAR EYE MOVEMENTS    

 

VIII CN PALSIES, VISUAL FIELDS, PUPIL & THE EYE

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The same motor neurons and extraocular muscles are active for all types of eye movements (e.g. saccades, pursuit and vergence). As a result, the systems generating these different movement types share a final common path1.

Midbrain centers:
The center for vertical saccades is the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), and the center for vertical gaze-holding function is the interstitial nucleus of Cajal (INC).
Clinically this means that an isolated vertical saccadic paresis or isolated vertical gaze deviation nystagmus would suggest a midbrain lesion. 
The INC projects to the ocular motoneurons, mainly via the posterior commissure2.
 

Pontine and pontomedullary centers:
Pons
The center for horizontal saccades is the paramedian pontine reticular formation (PPRF). 
Clinically, this means that isolated horizontal saccadic palsy indicates a pontine lesion, and a unilateral PPRF lesion will result in saccadic disturbances on the side of the lesion. 

Medulla
The medulla contains most of the vestibular nuclei. Other major medullary structures involved in the control of eye movements include the perihypoglossal nuclei, including the NPH, the paramedian nuclei and tracts, the inferior olivary nuclei, and the inferior cerebellar peduncles.

The center for horizontal gaze-holding function is the nucleus prepositus hypoglossi (NPH), which acts in concert with the vestibular nuclei of the medulla, and the vestibulocerebellum, together forming the “neural integrator.”

Disturbances of the medulla are best described in relation to a stroke of the dorsolateral medulla, a Wallenberg syndrome.

The connections of the horizontal semicircular canal to the medial vestibular nucleus in the medulla activate second order neurons in the vestibular nucleus, which project to the contralateral abducens nucleus.
The abducens nucleus projects to:
  -Ipsilateral lateral rectus (the 3rd neuron in the three neuron arc of the vestibulo-ocular reflex)
  -Contralateral medial rectus via the medial longitudinal fasciculus.

Vestibular nuclei
The vestibular nuclei receive information about the vestibular function of the opposite ear through an important commissure which carries excitatory and inhibitory signals from the vestibular systems, left and right. Multimodal integration of information at the level of the vestibular nuclei takes place allowing vision and proprioception to guide vestibular responses.

 

See: Vestibular Neuron & Vestibular Nuclei

Figure 1. The structures of the neural integrator

Cerebellar centers:
Cerebellar lesions are often accompanied by clinically easily identifiable oculomotor disturbances.
For example, defects of the flocculus/paraflocculus are characterized by saccadic pursuit, downbeat nystagmus, and impairment of the visual fixation suppression of the VOR.
Paraneoplastic cerebellar disorders often lead to opsoclonus, in addition to a range of oculomotor disturbances. 

 

Figure 2. Brainstem centres for gaze holding and saccade generation

Figure redrawn from Clinical Examination of the Ocular Motor and Cerebellar Ocular Motor System. Strupp, M. https://www.youtube.com/watch?v=meXAjVoQdCI

References

 

  1. Ramat S, Leigh RJ, Zee DS, Optican LM. What clinical disorders tell us about the neural control of saccadic eye movements. Brain. 2007;130(Pt 1):10-35.
  2. Kheradmand A, Colpak AI, Zee DS. Eye movements in vestibular disorders. Handb Clin Neurol. 2016;137:103-17.