The orientation of the left and
right semi-circular canals in the
head is such that any movement
always induces an antagonistic
response in both canals.
Horizontal head movements in the
yaw plane are an example. During
rightward head rotation, the
endolymph in the lateral semi-circular
canals on both sides lags
behind, bending the cupula of the
right SCC towards the vestibulum
(ampullo- or utriculopetal) and
simultaneously deflecting the
cupula of the left SCC away from
the vestibulum (ampullo- or
utriculofugal). A key difference is
the polarisation of the hair cells.
Indeed, since the the hair cells in
the right and left canals are
implanted in opposing directions
(in a mirror image fashion), the
deflection on the “leading” right
side induces the movement of the
stereocilia towards the kinocilium,
whereas the movement of the
stereocilia is away from the kinocilium
in the opposing, “following”
ear. As a result of this “pushpull
principle”, the activity of
right lateral SCC primary afferent
neurons increases, and, at the
same time, the activity of left
lateral SCC primary neurons
decreases with respect to the normal
resting discharge rate.
The activity of the lateral SCC
primary afferent neurons is modulated
by horizontal head rotation.
The firing rate increases in the
leading ear (the ear towards the
movement is directed) and
decreases in the following ear.
This is the push-pull principle of
the VOR.
The right medial vestibular
nucleus in the brainstem receives
an increased input from the right
lateral SCC primary neurons (no
crossing). This excites the activity
of type I secondary vestibular neurons.
These excitatory neurons
drive the leftward compensatory
eye movements of the VOR, to
ensure gaze stabilisation.
However, commissural disinhibition
from the left lateral SCC primary
neurons also contributes to
the excitation of the type 1 neurons.
Both excitation of the right
SCC and disinhibition of the left
SCC are therefore needed for an
optimal VOR.