The Basic Brainstem Circuit for the VOR when the head turns to the right.

Direct path of the VOR is a short tri-synaptic reflex
consisting of 1) vestibular n.(vn), 2) motoneurons (in the VI and III nuclei), and 3) muscles (lateral rectus, lr, and medial rectus, mr).

The direct path, by itself, is not enough. Why?

During the head rotation there is a phasic response seen in vestibular afferents (sense how fast you rotate), which rotates the eyes to the left.

After the head rotation this phasic activity stops.
This presents a problem: the eye would drift back to center because the muscles need a tonic input to keep the eye rotated.

Copyright © 1996 Tutis Vilis and Douglas Tweed
University of Western Ontario
London Ontario Canada
Updated October 28, 2005

Comments welcome. Email to tutis.vilis@schulich.uwo.ca