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Neuro: 2:00 - 3:00
Scribe: Laura Adams
Monday, February 23, 2009
Proof: Brittney Wise
Dr. Gamlin
Vestibular System
Page 1 of 4
I. Vestibular System [S1]:
a. Or, why don’t we fall in the dark!
b. Start with the peripheral vestibular system, both those vestibular organs that allow us to sense angular
acceleration and linear acceleration.
c. Then their central projections and some basic roles in balance, locomotion, and how we keep track of where we
are when we move around in darkness.
II. Picture of the Ear [S2]
a. The vestibular system is made up of 3 semicircular canals and the otolithe organs.
III. Vestibular organs sense head motion [S3]
a. This is focused in on the vestibular organs. The 3 semicircular canals, which are labeled horizontal posterior and
superior and then the 2 otolithe organs, the utricle and the sacculus.
b. The semicircular canals will sense head rotations. That means if I turn my head left and right, up and down.
c. Then the otoliths sense linear acceleration, the strongest linear acceleration, is one you are sensing right now
which is accelerating you to the center of the earth 9.8 m/sec 2, this is gravity. This is a constant linear
acceleration. Other linear accelerations would be pulling from a stop sign and your head is flung to the back of
your chair. Or a ride at Six Flags where they drop you.
IV. 3D Framework [S4]
a. Terminology for rotations that are based on nautical notations. Roll, pitch, and yaw.
b. Like in a boat: Pitch – up and down bow to stern, Yaw – left to right.
V. Vestibular outputs very rapidly influence eye, head, and postural reflexes [S5]
a. One of the advantages of the vestibular system is that the sensory system responds very quickly. It can
produce changes in eye movements in about 6 ms (about 10 times faster than visual information can control eye
movements.
b. In addition we have vestibulo-colic responses innervating the neck musculature so that despite my body
movements my head stays in the right position.
c. Vestibulospinal signals for eliciting postural changes in response to vestibular signals. (So if I start falling, I will
make appropriate postural changes to right myself.)
VI. Scanning electron microscope of hair cell from the bullfrog inner ear [S6]
a. Sensory organ is the hair cell, and it has a number of stereocilia (short stubby) and one kinocilium. There is a
stack of the stereocilium and then a kinocilium.
VII. Hair cell and supporting cells[S7]
a. This is a cross-section through a hair cell. Remember the kinocilium would be next to the longest of the
stereocilium. The hair cell releases neurotransmitter. More when it is depolarized and less when it is
hyperpolarized. It contacts the vestibular afferent fiber, which the cell bodies and in Scarpa’s ganglion and then
travel along the 8th nerve to the vestibulo nuclei.
VIII. [S8]
a. The stereocilia are connected by these very small filamentous connections called tip link. It is at this point
where the sensory transduction occurs, whereby the stereocilia are deflected the tip links are pulled and they
open channels (mechanico-receptive channels) in the ends of the stereocilia and they operate in a specific way.
IX. Mechanism of Hair cells [S9]
a. This shows the 3 conditions:
b. The first thing you should know is that in the vestibulo system the resting condition is a situation whereby a hair
cell is depolarized at about 60mV. It is releasing a steady stream of neurotransmitter (which is manly
glutamate). That is causing the postsynaptic afferent fiber to be depolarized somewhat and generally it is
generating a resting rate of about 90 action potentials per second. So at rest both the semicircular canals and
the otolithe system the resting state is an afferent discharge which is around 90 spikes per second.
c. If for some reason the stereocilia are deflexed toward the kinocilium, the tip links pull on these channels and K
(K is unique here) will flow into the cell causing it to depolarize, Ca flows in because of the depolarization. This
causes an increase in neurotransmitter release. This depolarized the postsynaptic afferent fiber and causes an
increase in vestibule afferent discharge.
d. On the other hand if the stereocilium move away from the kinocilium, these channel actually close and the hair
cell hyperpolarized, release much less neurotransmitter and the net result is vestibule afferent discharge rate
decrease.
X. Linear acceleration [S10]
a. These are detected in the sacculus and utricle. The way these bundles of stereocilium and kinocilium are
arranged, they have a tuning sensitivity. They are only sensitive to movement in one direction (primarily).
b. If you look at the population of hair cells in the sacculus and the utricle you see that first there is a midline
running down called the Striola which separates to halves of each of these otolithe organs.
Neuro: 2:00 - 3:00
Scribe: Laura Adams
Monday, February 23, 2009
Proof: Brittney Wise
Dr. Gamlin
Vestibular System
Page 2 of 4
i. So look at the way the sacculus and utricle are oriented. See what is labeled superior and inferior,
posterior and anterior. The sacculus and utricle are slightly different. Each of these organs can sense
anterior-posterior movement and superior-inferior movement. This is based on the tuning of the hair cells in
different areas of the organ.
c. In the superior part of the sacculus, the hair cells are most suited to detect anterior-posterior linear acceleration
and as you move around the sacculus the posterior part is most suited to detect superior-inferior linear
accelerations. So you have a 90 degree range of linear acceleration sensitivity here.
d. The utricle is arranged differently. (Sacculus arranged more vertical; utricle is slightly more horizontally
arranged.) Again, the anterior portion is tuned for medial-lateral movements (moving head left and right).
e. So between the sacculus and the utricle there are hair cells that are tuned for up, down, left, right, and back and
forth. (All the major axis of linear acceleration.) There is a specific map within the otolithe organs so you can
interpret the direction of linear acceleration by seeing which population of these hair cells is being activated at a
given time.
XI. Scanning Electron Microscope of calcium carbonate crystals (from a cat, smaller in humans) [S11]
a. How do you detect this linear acceleration? It turns out you have small rocks in your head, or actually your inner
ear.
b. Your inner ear (the otolithe organs) contains what are called otoconia. They are very small calcium carbonate
crystals. In humans they are 3-30 microns long.
XII. [S12]
a. The crystals sit on a gelatinous membrane into which the hair cells insert the stereocilia and the kinocilia.
b. Basically it is in these that they have got this stereocilia sitting in a layer of jello, with these calcium carbonate
crystals on top that gravity acts on. So when gravity works on these otoconia that will deflect the jello layer and
that will move the stereocilia that are embedded in the jello layer. The otoconia are there to mechanical
amplification for linear acceleration purposes. They basically are small rocks which are very sensitive to linear
acceleration.
XIII. Forces acting on head [S13]
a. When your head is totally upright, there is no linear acceleration acting on the otoconia, or the gelatin layer, and
the stereocilia are straight up. So the vestibulo afferent fibers are discharging at a resting rate of about 90.
b. If you tilt your head back or forward the stereocilia will be bent either toward or away from the kinocilia. The
reason they tilt is because gravity is acting on the otoconia and pulling them. The reason the utricle generates a
signal for head tilts is because gravity acts on the otoconia.
c. The other way you can get these otoconia to cause the utricle to move in a tilting direction, is by accelerating out
of a stop sign or breaking quickly in your car. No tilt, but you could accelerate away – in which case in the
utricle the stereocilia will be bend back. The brain potentially could mistake this stimulation due to acceleration
away from a stop sign for head tilt. So that is an ambiguous signally system.
d. In general the nervous system understands a sustained tilt. If you accelerate briefly (like walking down the
stairs) you don’t think your head is tilting.
e. Theme park rides actually stimulate your vestibular system in ways that contradict what your brain is thinking.
You often get sustained acceleration and you feel like you are tilting one way or the other.
XIV.
[S14]
a. This is a vestibular afferent fiber from the utricle; it has a resting rate of about 40 (a little lower than we said
before). And what you notice is that when they tilt, as long as there is a continuous tilt, the cell fires at 100
spikes per second, and when the tilt goes in the other way it decreases. Some the same fiber tells you about tilt
in one direction by increasing and decreasing for tilt in the other direction. This is just on one side of the head.
The utricle on the other side would be behaving as a complementary response. Some you have utricles on both
sides to give you information.
XV.
Angular acceleration [S15]
a. This is a little more complicated. The semicircular canals are these thin narrow semicircular ducts, which are
fluid filled and at the end of each one is a swelling called an ampulla. Within the ampulla there is a cupula (like
of it as a sail) sitting there which is able to catch the fluid as it travels through the semicircular canal. Imbedded
in this cupula are the hair cells. They are inserting their stereocilia into the cupula, and as the head rotates fluid
pushes on this cupula, and that moves the stereocilia. It uses a slightly different system to move the stereocilia
away or toward the kinocilia.
XVI.
Functional Organization of Semicircular canals [S16]
a. At rest the cupula is not deflected, and there is no fluid moving in the semicircular canal (Diagram A). If you
start to rotate your head (counterclockwise in Diagram B) the fluid doesn’t quite keep up (think of moving a glass
of water). So the semicircular canal moves one way, the fluid moves the other, and it produces a weak force on
the cupula. It is enough however to displace the cupula, and if it is the way to move the stereocilia toward the
Neuro: 2:00 - 3:00
Scribe: Laura Adams
Monday, February 23, 2009
Proof: Brittney Wise
Dr. Gamlin
Vestibular System
Page 3 of 4
kinocilium then the afferent discharge rate will increase. On the other hand if it moves the stereocilium away
from the kinocilium then the afferent discharge rate will decrease.
b. The discharge rate is proportional to how rapidly the fluid is moving and how much force is placed on the
cupula.
c. The semicircular canals can tell you about rotations in their axis. These are circular structures which mean they
are only really sensitive to rotations along the axis of the semicircular canal, not at right angles to it. So for
example the horizontal canals are sensitive to horizontal rotations (although it is actually tilted up 30 degrees).
Rotations at 90 degrees to the semicircular canals with move the fluid at right angles and will have no effect.
d. So the anatomical organization of the semicircular canals insures that they are tuned for rotations primarily in
one axis.
e. The horizontal canals are tuned for rotations 30 degrees up. The left and right horizontal canals behave as an
agonist/antagonist pair or as a complementary pair. When fluid moves in the right canal in one direction it tends
to move in the left canal in the opposite direction. So they are functionally paired.
f. The pairing of the posterior and anterior canals is a little different. It turns out they do form functional pairs.
i. The right anterior canal’s functional pair is the left posterior canal (RALP)
ii. Anterior on the left is in the same plane as the right posterior. (LARP)
g. In general with functional pairs, if fluid is moving one way in one it is going to move the other way in the other
canal.
h. This is relevant with balance control and eye movement control. The examples we will discuss with eye
movement will just have to do with the horizontal canals (because they are easier!)
XVII. [S17]
a. Here is another diagram just laying it out more clearly. This shows the plane of the utricle, the horizontal canal,
the sacculus, and the semicircular canal.
b. Note: be sure to look over these diagrams to understand the orientation. He did a lot of moving around to show
us in class.
XVIII. Horizontal Ducts [S18]
a. It is easy to explain how this works in the horizontal ducts. Remember if the head turns in one direction the fluid
tends to move in the opposite direction.
b. Let’s see if we can figure out what will happen if we turn our head to the left.
i. The head turns to the left but fluid is going to turn in the opposite way. In the left horizontal section of the
semicircular canal it will move in the preferred axis of the hair cells and actually cause an increase in the
firing rate. Whereas on the opposite side the stereocilia will move away from the kinocilia and cause a
decrease in firing.
c. Remember the decrease or increase in firing is caused from the way the hair cells are arranged.
XIX.
Vestibular nerves signal head velocity [S19]
a. The semicircular canals are sensitive to angular acceleration, but because of the characteristics of the canals,
when people actually record from the afferent fibers and they look at the firing rate of the afferent fibers, and
they see how the firing rate modulates for the changes in head position they find that the signal is related mainly
to head velocity. So the signal that reaches the brain is mainly related to how fast the head is turning not how
much its accelerating.
b. The vestibular system signals head velocity, that doesn’t mean the vestibular apparatus is sensitive to that, it
just means the optimal stimulus for the vestibular apparatus is head acceleration.
XX.
Vestibulo-ocular reflex [S20]
a. What we want to do is work out how a vestibular signal from the horizontal canals, which is coming in through
Scarpa’s ganglion, is used to control eye movements.
b. The vestibule-ocular reflex requires that when you move your head to the left your eyes go right.
c. Illustration: If you stick your finger out in front of you and focus on it, then shake your head (pretty fast) and you
should be able to stay focused on your finger. Now stop moving your head and shake your finger. You cannot
stay focused on it!
d. Vestibulo-ocular reflex has about a 6ms latency.
e. If you don’t have these hair cells what would it be like? A certain ototoxic drug destroyed a man’s hair cells, and
he wrote that he could not even read a book in bed because his heart beat, and if he was walking his heal strike
(which normally sends vibrations to your vestibule-ocular reflex) would cause his vision to blur so he had to hold
on to lamp posts to recognize familiar faces. VOR is really important!
f. (He is working this out from one side.) We are familiar with the abducens nucleus. We are also familiar with the
ascending medial longitudinal fasciculus, which contains the fibers from the neurons within the abducens
internucleus that are not motor neurons. About 40 percent of the neurons in the abducens nucleus instead of
projecting out the 6th nerve, instead they cross in front of the abducens nucleus and project contralateral
mediorectus motor neurons. [we will hear more of this later] The abducens internuclear neurons send
Neuro: 2:00 - 3:00
Scribe: Laura Adams
Monday, February 23, 2009
Proof: Brittney Wise
Dr. Gamlin
Vestibular System
Page 4 of 4
information to the medial rectus motor neurons which then project out to the medial rectus. (This is a crossed
pathway.)
g. This pathway is essential in humans and primates to ensure that when the left lateral rectus on one side is
active (the left lateral rectus motor neurons are active), the right medial rectus are also activated. So when the
right lateral rectus motor neurons decrease activity and the right medial rectus will receive the same signal from
this abducens internuclear pathway.
h. So what about the vestibular system. The main thing you should note is that when the signal is coming in from
the vestibular apparatus synapses in the medial vestibular nucleus and from there the first step is a inhibitory
connection to the ipsilateral abducens and excitatory to the contralateral abducens.
i. For example if the left vestibular apparatus is activated (so the left horizontal canal is activated by turning left)
you want to move your eyes to the right. So I activate the right lateral rectus (by the excitatory pathway) and the
inhibitory pathway ensures that the left lateral rectus receives a decreased signal. So this connection from the
medial vestibular nucleus to the abducens takes care of the appropriate activity for the vestibule-ocular reflex in
the lateral rectus on both sides. And then the internuclear pathway to the medial rectus motor neurons on the
left and the right take care of the appropriate activity for the left and the right medial rectus motor neurons. The
net result is that when activity increases on the left due to leftward head movement the eyes moves right
through these neural connections.
j. That is the basic anatomy and you can see why the latency is so short. There are no more than 4 synapses!
k. The complementary connections were emitted here for clarity, but do remember that pathway as well.
l. So those vestibular signals can generate complementary eye movements.
XXI.
VOR gain is low at low frequencies [S21]
a. The other thing you should note is that the vestibulo-ocular reflex, where I move my head one way and my eyes
move the other, is only good for relativity high rates of head movement.
b. If you move your head very slowly, (move a glass of water slowly it does not slosh around), the vestibulo-ocular
reflex doesn’t do well. Actually the vestibular system in general doesn’t do well with slow head movements.
c. Fortunately the visual system which compensates for this inability of the vestibular system. For slow
movements the visual system is ideal, because it is slow.
XXII. Vestibular projections to the spinal cord [S22]
a. Let’s talk little more about vestibular projection, the ones that stop me from falling over when the lights go out.
b. The first thing to note is that this is a diagram of the vestibulospinal projections synapsing in the lateral
vestibular nucleus and showing another projection to the medial vestibular nucleus.
c. So you have the lateral vestibulospinal tract (which will be going to motor neurons controlling more distal
musculature and it is usually unilateral) and the medial vestibulospinal tract (which is to more axial musculature
involving posture and it is bilateral.)
d. Those are the projections that allow, through the appropriate vestibular signals to motor neurons, the control of
posture.
XXIII. Lateral Vestibulospinal Tract[S23]
a. A little more detail. Input from the otolithe organs to the lateral vestibular nucleus so that is involved with head
and body tilts. It has an effect on the extensor muscles which are the ones you require to extend so you don’t
fall over.
b. You don’t need to know about the decerebrate rigidity. (At least not in this lecture!)
XXIV. Medial Vestibulospinal Tract [S24]
a. An important projection is to the upper cervical levels where it regulates neck muscles in response to stimulation
of the semicircular canals, so if you feel your head rotating in one direction you can move your neck in the other
direction, or if you are moving forward you can keep your head up (like if you are tripping).
b. This is a system that is involved mainly to the cervical.
XXV. Vestibular signals are sent to cortex [S25]
a. In addition to these reflexes from the vestibular system that go to the spinal cord and control the vestibule-ocular
reflex, it is clear there are vestibular signals that relay in thalamus and project to the cortex. The 2 areas that we
know they project to are the somatosensory cortex (near the face) and posterior parietal cortex (region which is
involved with evaluating where you are with respect to the external world.)
b. If you blind fold someone and move them, they only signals that tell them they have moved are the vestibular
signals. So can move through the world and use those signals to keep track of where you are in space if you
don’t have any other cues.
[end 43 min]