Vestibulo-ocular reflex

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The vestibulo-ocular reflex (VOR) or oculovestibular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movements are present all the time, the VOR is very important for stabilizing vision: patients whose VOR is impaired find it difficult to read using print, because they cannot stabilize the eyes during small head tremors. The VOR reflex does not depend on visual input and works even in total darkness or when the eyes are closed.

Gain

The "gain" of the VOR is defined as the change in the eye angle divided by the change in the head angle during the head turn. If the gain of the VOR is wrong (different than 1)—for example, if eye muscles are weak, or if a person puts on a new pair of eyeglasses—then head movements result in image motion on the retina, resulting in blurred vision. Under such conditions, motor learning adjusts the gain of the VOR to produce more accurate eye motion. This is what is referred to as VOR adaptation.

Ethanol consumption can disrupt the VOR, reducing dynamic visual acuity.^[1]

Circuit

The main neural circuit for the horizontal VOR is fairly simple. Vestibular nuclei in the brainstem receive signals related to head movement from the Scarpa's ganglion located on CN VIII, or the vestibular nerve. From this Vestibular nuclei excitatory fibers cross to the contralateral CN VI nerve nucleus. There they synapse with 2 additional pathways. One projects directly to the lateral rectus of eye. Another nerve tract projects from the CN VI nucleus by the abducens internuclear interneurons or abducens interneurons to the oculomotor nuclei, which contain motorneurons that drive eye muscle activity, specifically activating the medial rectus muscles of the eye. Another pathway directly projects from the vestibular nucleus through the ascending tract of Dieters to the ipsilateral medial rectus motoneurons. In addition there are inhibitory vestibular pathways to the ipsilateral CN VI nucleus. However no direct vestibular neuron medial rectus motoneuron pathway exists. ^[1]

Role of cerebellum

The cerebellum is essential for motor learning to correct the VOR in order to ensure accurate eye movements. Motor learning in the VOR is in many ways analogous to classical eyeblink conditioning, since the circuits are homologous and the molecular mechanisms are similar.

See also

• Caloric reflex test
• Semicircular canals
• Vestibular system
• Pursuit movement

External links

• Motor Learning in the VOR in Mice at edboyden.org
• Review on VOR adaptation via slides at Johns Hopkins University
• MeSH Vestibulo-Ocular+Reflex
• ent/482 at eMedicine - "Vestibuloocular Reflex Testing"

References

v • d • e Nervous system physiology: neurophysiology - reflex
Cranial nerve	midbrain: Pupillary light reflex - Accommodation reflex pons/medulla: Jaw jerk reflex - Corneal reflex - Caloric reflex test/Vestibulo-ocular reflex - Gag reflex
Tendon reflexes	upper limb: Biceps reflex - Brachioradialis reflex - Extensor digitorum reflex - Triceps reflex lower limb: Patellar reflex - Ankle jerk reflex - Plantar reflex
Primitive reflexes	Galant - Grasp - Moro - Rooting - Stepping - Sucking - Tonic neck
Other	Baroreflex - Acoustic reflex - H-reflex - Oculocardiac reflex - Stretch reflex - Startle reaction - Optokinetic - Withdrawal reflex - Crossed extensor reflex

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Acknowledgement and Attribution Regarding Sources of Content

Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .