Okay. So, now, let's focus in on the ampulla of
a semicircular canal. So, this is a bulbous expansion at the
base of the semicircular canal. So, this is where we find our sensory
epithelium, which is called the crista. And overlying that crista is a gelatinous
mass through which protrude the stereocilia of the hair cells.
So, this gelatinous mass here is called a cupula.
And so, we see the bundles of hair cells that are projecting into this cupula.
Now again, there's no axis of symmetry that bisects the sensory epithelium as we
saw in our otoliths so there is simply one axis for depolarization that's
present in a crista of a semicircular canal.
Now, this cupula creates a barrier to the flow of endolymph.
And so what this means is that if we accelerate our head around an axis of
rotation it's something analogous to, let's say, rotating a cup of water.
So, the glass would be the membrane, and the fixed components, and this cupula is
indeed fixed here at its base along the crista as well as on the, a more distal
part of the ampulla. So, if we were to turn that glass of
water, the glass turns, right, but the water has some lag because of the force
of inertia. The same thing is going to happen here in
the semicircular canals. We turn our head, the force of inertia is
going to cause a deflection of this cupula.
So, let's have a look at that. So, here's the semicircular canal at
rest. And then, if we were to rotate it so we
are actually accelerating our sensory epithelium with a turning of the head.
the lag or the inertial force causes endolymph to essentially flow in the
opposite direction. Really, it's the fixed structures that
are moving, the endolymph is staying more or less in the same place that causes
displacement of this cupula. And as you could see, this displacement
is going to deflect the hair cells in this crista towards the longest
stereocilium that's going to lead to depolarization of all of the hair cells
together in this one crista. Now, we're turning our heads to activate
this particular ampulla of a semicircular canal, it has a functional pair on the
opposite side of the head. The turning of the head in one direction
is going to activate the crista on the side of the turn and it will deactivate
or hyperpolarize the hair cells on the opposite side of the head in the
corresponding member of the pair. So, let's see what this looks like from a
physiological perspective. So now, we are recording the discharge
rate of an axon that receives a synaptic input from a hair cell in the crista of a
semicircular canal. And we see that as we accelerate, that is
as we begin to turn our head, we see that there is now a transient response where
there's a temporary increase over the course of a couple of seconds in the
firing rate of this 8th cranial nerve afferent.
But as we attain constant velocity, so you can imagine if you start to turn
around in circle, circles, I'm not going to be able to do this very long,
because I'm going to get dizzy real quick.
But in the first few seconds of rotation, my left horizontal canal began to
increase its firing rate. But as I was maintaining that rotation,
essentially, the movement of the endolymph caught up with the movement of
the fixed structures, and so, the deflection of the cupula now relaxed back
down to its neutral position. And so the velocity of rotation was
maintained but the deflection of the hair cells relaxed after several seconds of
rotation. So, that is why this response in the hair
cells and in the 8th nerve axons that innervate the semicircular canals is
phasic, okay? It's only transitory.
Now, as we decelerate, we essentially are producing the opposite biomechanical
effect, that is we are reducing the rate of rotation, and that's causing the
endolymph to flow against the decelerating crista or the decelerating
cupula. And so, that's going to now deflect the
hair cells in the opposite direction and that will produce hyperpolarization and a
reduction in the release of transmitter on the 8th nerve aferrent, and therefore
a reduction in firing rate. And it will again take a few seconds to
equilibrate the flow of endolymph and the flow of the fixed structures.
So, this is why the semicircular canals only operate with a phasic response,
rather than the sustained response that we saw in the otolithic membrane.
I would encourage you to think through this again on your own.
If you don't quite understand why would the otoliths give rise to, give rise to a
sustained response where the semicircular canals ephasic response you might want to
go back and think through this again to make sure that makes sense to you.
It basically is explained by the biomechanics of sensory transduction.
And lastly, I want to to talk about the function of these pairs of organs on
either side of the head. I alluded to this just a moment ago.
but this is very important for understanding central processing in the
vestibular system and this is what we'll come to in the next tutorial.
I want you to appreciate the fact that are otolith organs come in pairs.
There are two utricles in either side of the head.
There are two secules in either side of the head.
They, they tend to work together because they are arranged in the same plane.
The semicircular canals, however, are a little bit more complicated becasue
they're actually arranged orthogonally to one another within the same side of the
head. So, that means the functional pairs need
to be thought of as those canals that operate in the same plane, okay?
So, for the horizontal canals, it's pretty straightforward.
Both horizontal canals operate in the same plane.
But when it comes to the superior and the inferior canals, then we need to
recognize that the superior canal in one side of the head is actually operating in
the plane of the inferior canal in the opposite side of the head, and then vice
versa. My right superior canal will be a
functional pair with my left inferior canal.
So, this means that, in order to activate these functional pairs, I would really
need to rotate my head along the axis or within this plane orthogonal to the axis
of rotation defined by this orientation. So, this forward tilt to the right would
activate my right superior canal and deactivate my left inferior canal.
And tilt back would do the opposite, it would activate, the left inferior canal,
deactivate the right superior and, and so forth, okay?
So, if I rotated down into the left, I'm activating my left superior canal, rotate
my head back, and to the right, I'm activating my right inferior canal.
Now, you may be wondering just about the head nod kind of motion.
Well, we're going to activate both superior canals to some degree, not as
perfectly as if we turned directly in their axis of rotation.
But a forward tilt is going to produce phasic activation in both superior canals
and a backward tilt is going to produce phasic activation in both inferior
canals. Now the simplest of all to understand
would be simply the shaking of the head left and right along with the z axis of
rotation, okay? So, what would happen there is that as we
turn our head towards one side let's just say the head turns to the right, we would
activate the right horizontal canal. So, here is the ampulla of the right
horizontal canal. And because of the inertial effect the
endolymph is essentially a force against the crista along its axis of
depolarization. So, we have a nice increase in the firing
rate of the 8th cranial nerve on the right side of the head.
That very same right word, head turn, causes the endolymph to actually move In
the opposite direction that would be consistent with depolarization for the
left semicircular canal. Consequently, if the left semicircular
canal is firing away when we make that head turn, now it's going to decrease its
firing rate. And then after a few seconds, if we keep
rotating around then we would expect it to recover back to its natural position.
So, the key principle that I want you to take away from the slide is that we
activate the semi circular canal on the side of the turn.
So, if we turn to the left, we're activating the left semicircular canal,
the horizontal canal. If we turn to the right, we're activating
the right horizontal canal, and so forth. Forward turn activates the superior
canals, backward turn activates the inferior canals.
So, what I really want you to understand is that when we get to the brainstem and
talk about central processing, the brainstem is comparing the activity
that's coming from both vestibular nerves.
And if we turned our head to the left, you can imagine that in the horizontal
canal system, leftward head turn would elevate activity in the left vestibular
system and decrease activity in the right vestibular system.
Whereas, if we turned our head to the right, we'd have the opposite effect,
okay? So, what the brainstem is really doing is
comparing the afferent activity that's coming from these two 8th cranial nerves.
Now, imagine what would happen if you had a, a lesion, or some kind of injury, to
the hair cells or to the 8th cranial nerve, you may find that the activity on
the lesion side begins to drift downward. We call this unilateral vestibular
hypofunction. If I had hypofunction on the right side,
what's that going to make me feel like? Well, it's going to make me feel like I'm
spinning to the left and vice versa. If I had injury or damage to my left
vestibular labyrinth or left 8th nerve, there will be hypofunction on the left
and it's going to make me feel like my head is spinning to the right.
Well, that's a very unpleasant sensation as you might imagine, maybe some of you
have experienced it for yourself. It illustrates the power of this
vestibular system. And it's important that you understand
these peripheral biomechanics and the means for sensory transduction.
And this will allow us to now understand and discuss how the central processing
stations operate, and what are the consequences for the integration of these
vestibular signals. So, when I see you next time, we'll talk
about central mechanisms of vestibular processing.
