2010 NPB 12, Note 13, Page: 1
LECTURE 13: Balance and Hearing
(Note: Professor Gregg Recanzone will give the lecture)
VOR (Vestibulo-Ocular Response)
A different kind of eye movement is involuntary, or at least in the sense that you don't
have to think about it, and that relates to keeping the visual world stationary in spite of
the fact that you are continuously moving your head.
If you take a video recorder and
record what you are looking at while you walk, when you watch the tape you will see
that the images are very jumpy and bouncy.
Yet, when you walk you do not perceive
the visual world as jumping around, it is very stationary.
This is due to a reflex of the
brainstem: the vestibulo-ocular response (VOR).
A classic demonstration of the VOR is
to look at your finger, and then move your head back and forth while still looking at your
As you move your head, your eyes move in an equal and opposite direction,
keeping your eyes still in the world although your head is moving.
This keeps the image
of your finger stationary on your retina.
In order to do this, you could use your
knowledge that you are moving your head, and use the motor commands that move
your head to drive the eye movements in the opposite direction.
However, you are still
able to keep your eyes fixed on one object even though you move passively, like in a
As a passenger, you have no control of how your head is going to move in space,
yet keeping your eyes fixed on something outside the car, like a road sign, is very
That is because you have another sense, the vestibular sense or sense of
balance, that provides the information your brain needs to know how the head is
The vestibular apparatus, as it is called, is located in the temporal bone in a complex
that also houses the sensory endings for hearing.
It is believed that the receptors for
balance and hearing are exactly the same, so I'll describe how balance works, and we
will assume that it is essentially the same thing for hearing.
The balance/hearing apparatus is contained within the temporal bone, right opposite the
It is composed of the cochlea (hearing) the semi-circular canals, the saccule and
In each of these structures there are a series of receptor cells called hair
They are called hair cells because they have small processes out of the top that
look like hairs.
They all have a longest one, called the kinocilium, and many shorter
ones, called stereocilia.
In all the hair cells, there is a membrane separating the cilia
from the rest of the neuron, called the reticular lamina.
This effectively makes a barrier
dividing the outside of the neuron into two different compartments.
The outside of the
cell next to the hairs is called endolymph.
This has actually a very high concentration of
potassium (K+), much higher even than the inside of the cell.
The other compartment
has the same old ionic concentration that we have seen before.
What is important is