Thursday, December 27, 2018

Vestibulo ocular reflex

The vestibulo-ocular reflex is a reflex, where activation of the vestibular system causes eye movement. This reflex functions to stabilize images on the retinas during head movement by producing eye movements 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 movement is present all the time, VOR is necessary for stabilizing vision.

Circuit:

1)It starts in the vestibular system, where semicircular canals get activated by head rotation and send their impulses via the vestibular nerve and end in the vestibular nuclei in the brainstem.In addition the hair cells of opposite ear are inhibited because endolymph in that ear flows away from hair cells.

2)From these nuclei, fibers cross to the contralateral cranial nerve VI nucleus.

3a)There they synapse with 2 additional pathways. One pathway projects directly to the lateral rectus of the eye via the abducens nerve.
  b) Another nerve tract projects from the abducens nucleus by the medial longitudinal fasciculus to the contralateral oculomotor nucleus, which contains motorneurons specifically activating the medial rectus muscle of the eye through the oculomotor nerve. 

4)For instance, if the head is turned clockwise, then excitatory impulses are sent from the semicircular canal on the right side via the vestibular nerve to the right vestibular nuclei in the brainstem. From this nuclei excitatory fibres cross to the left abducens nucleus.There they project and stimulate the lateral rectus of the left eye via the abducens nerve. In addition, by the right medial longitudinal fasciculus, fibers cross and go to right oculomotor nuclei, they activate the medial rectus muscles on the right eye. As a result, both eyes will turn counter-clockwise.

-Srikar Sama

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