Hello, everyone. welcome to a set of tutorials on the
structure and function of the vestibular system.
In this first tutorial, I'd like to speak to you about peripheral mechanisms of
vestibular processing, and in the second tutorial, we'll talk about the central
processes that operate within the brainstem.
on up through the brain and even down into the spinal cord that implement motor
commands based on feedback coming from the vestibular system.
And then, we'll also talk something about how vestibular input is contributing to
our broader sense of proprioception from which our body schema is created in the
brain. Now, I'll refer you to our core concepts
in the field of neuroscience as well as our learning objectives that are found at
the top of your tutorial notes. essentially our core concepts for this
session are that, once again, we're going to be confronted with the complexity of
the brain and you'll come to even more fully appreciate, I think, the fact that
the brain is the body's most complex organ.
And we will be talking about circuitry that is genetically determined, as well
as structure and form. And I think what you should appreciate is
how beautifully do form and function go together when we're talking about the
functions of the vestibular system. And I hope you appreciated that when we
talked about the cochlea as well when we considered audition but we'll carry that
theme onward as we consider the structure and function of the vestibular system,
okay? And our learning objectives are that I
want you to be able to describe the, describe the anatomy of the vestibular
labyrinth, this is what we call the peripheral apparatus that's deep in our
temporal bone. And I want you to be able to describe the
biomechanics of sensory transduction in both components of the vestibular
labyrinth that would be the biophysics of hair cell transduction in what we'll call
the otolithic organs, and then in our semicircular canals.
But let's begin with an overview of vestibular function.
So, you should recognize that the vestibular labyrinth is an extension of
the inner ear that is designed to sense the motions that arise from head
movements as well as the inertial effects that are due to gravity.
So, what do we mean here? Well, as your head is presumably in a
fairly stable position right now, there are signals that are due to the force of
gravity tugging on our bodies. And these signals are being processed by
the vestibular system with respect to a special set of organs that we find within
the vestibular labyrinth. So, we're going to talk about that, But,
of course our heads are often in motion mine tends to bob around quite a bit as I
speak. I know that.
But our heads are in motion and those motions produce accelerations.
sometimes linear accelerations, as we move in one plane of motion or another,
we'll see those planes in just a minute, or rotational accelerations.
So, these are different kinds of ways that we could move our head.
those linear accelerations in the static positions of our head, these are sensed
by the otolithic organs, while the rotational accelerations are being sensed
by the semicircular canals. And what we'll discover in each component
of the vestibular labyrinth are hair cells that do the job of sensory
transduction, very similar to the hair cells we described in the cochlea when we
talked about audition. So, the good news for you is that the
mechanisms of sensory transduction are quite similar in the vestibular system
and the auditory system. But we'll talk about the biomechanics of
these two components of the vestibular labyrinth.
And you'll see how the biomechanics really help to explain the sensitivity of
these hair cells, even though the sensory transduction is the same.
Okay. Well, let's continue with our overview
then. So, vestibular signals are generated in
those Otholitic organs in the semicircular canals.
And then these signals are relayed to integrated centers in the brainstem as
well as in the cerebellum. And these signals travel along the 8th
cranial nerve in the vestibular component of that 8th nerve.
Well, in the central stations, these signals are used to adjust postural
reflexes as well as eye movements. So, in our next tutorial, we'll talk
about the anatomy of how those kinds of motor actions are engaged by vestibular
signals. Of course, these, these vestibular
signals also can impact our sense of well-being, our sense of place, our sense
of position, and our sense of movement through the environment that is mapped
out largely via auditory and visual, and visual signals.
So, these vestibular signals then are going to reach into the parietal lobe
where our spatial awareness is constructed in cognition.
So, in the parietal lobe, we have our normal sense of orientation in
three-dimensional space, this is where the sense is constructed.
And should there be some kind of pathology, this is where feelings of
dizziness would come from. there are brainstem circuits that are
going to be important for that as well. And we'll touch about that in the next
tutorial. So, what I do want you to understand
though, is what is the fundamental, it means by which the vestibular signals
provide us with a normal sense of position and equilibrium.
And what are the conditions that might give rise to abnormal sensations and how
might those be expressed in our bodies. Okay.
Well, let's move on then and consider the anatomy of the peripheral components of
the vestibular systems. But before we explain the anatomy, it's
useful to talk about how we move our heads.
So, as I illustrated just a moment ago, we move our heads both in linear planes,
as well as around axes of rotation. So just to orient you to this figure.
So, this illustrates the basic planes through which we move and, of course, we
move through all the planes are intermediate between them.
And we move in, in rotations around axes that are intermediate between these
cardinal axes here. so, we can rotate our heads around the
x-axis. So, this is a, a, a rolling type of
action, okay? Think of a, of an airplane sort of
rolling off to one side. we can change our rotation around the
y-axis, as if there was an axis going from ear to ear.
This is pitch. This is nodding our head, forward and
then backward, okay, and then we can turn out head from side to side.
So, this is a rotation around the z-axis. This is called yaw.
So, if you're flying in that plane and the plane does this rather disturbing
turn left or right that is movement in yaw, okay?
So, those are rotational movements or rotational accelerations.
There can also be linear accelerations in the forward and backward direction in the
horizontal plane or to the left right direction in that same horizontal plane.
There can also be movement up or down, right, like elevator motion.
So, these are all linear accelerations. And we see those illustrated here as
movements along these principle axes that are illustrated.
Now these six basic directions or planes of motion are going to be sensed by
different components of the far vestibular labyrinth .
So, I want you to, at the end of this tutorial, be able to explain, which
elements of the vestibular labyrinth are sensitive to each of these kinds of
motions, okay? Well, in order to get there, we need to
look at the anatomy of the vestibular system.
And what we find is that the vestibular system truly is a labyrinth.
It is a set of interconnected canals that arise form the same embryological
precursor called the otic placode that grew out the cochlea.
So, it's not surprising then that our cochlea should be right adjacent to our
vestibular labyrinth. So, what we find in the vestibular
labyrinth is a series of canals. Now these canals are filled with
endolymph. So you'll recall from our discussions of
the cochlea, endolymph is that fluid that is enriched in potassium ions.
So, this means that it is more like the solutions that we typically find within
cells rather than our typical extracellular fluids.
So, the canals are filled with endolymph, which is high in potassium and low in
sodium. And then surrounding the vestibular
membranes is where we find perilymph, which is more like the normal extra
cellular fluids of the body that is low in potassium and high in sodium.
And just as endolymph was critical for sensory transduction in the cochleas, so
will it be critical for sensory transduction here in the vestibular
system. So, here's another view, of our
vestibular labyrinth, together with the cochlea.
So, we have our, our labyrinth over here to the left-hand side and our cochlea
here towards the right. And if we just look at the labyrinth now,
we see that there are really two classes of sensory structures, associated with
each side of the head with each vestibular labyrinth.
we have a set of otolithic organs, and these are the utricle and the saccule.
So, together, these two organs are similar in their histology and their
function. But what differentiates them, as we'll
see, is the orientation of their sensory elements.
Now, in a similar way, our second class of sensory structures are these three
semicircular canals. So, we have a superior canal, we have a
posterior canal, and a horizontal canal. Now, these three canals have very similar
mechanisms of biomechanics and sensory transduction.
What differentiates them is their orientation within the temporal bone and
that orientation allows them to be sensitive to different kinds of head
movement. Now, within each of these sensory
structures, the semicircular canals and the otolith organs, we find hair cells.
So again, these hair cells are going to be similar to what we described in the
cochlea, and they provide for the mechanisms of sensory transduction.
These hair cells are innervated by the peripheral processes of ganglion cells
that are, are, present in a ganglion called Scarpa's ganglion.
So, we see that here so there is a set of, nerve cells that sit in Scarpa's
ganglia that innervate the otolith organs and then, a separate population that
supply the central sensory regions in our three semicircular canals.
So, the peripheral process of these cells receives a synaptic input from the hair
cell and the central process then is conveyed along the axons of the
vestibular division of the 8th nerve. So, let's talk about sensory transduction
now and these vestibular sensory structures.
here's a view of a sensory epithelium, this is a view of what we might find in
our otolith organs. And what we see is a sensory epithelium
with hair cells and these hair cells are making synaptic connections with the
peripheral process of our 8th nerve fibers.
The hair cells grow a set of stereocilia and these stereocilia have a particular
axis to the length of the stereocilia, with the longest stereocilium on the side
of this very special structure called the kinocilium.
And the kinocilium may or may not be present in adult life it's okay if it's
not it often does degenerate. But what we're left with then is a set of
stereocilia that have some sort of directional access to their orientation
going from the shortest stereocilium towards the longest.
And when there is deflection of these stereocilia from shortest to longest,
there will be depolarization of the hair cells.
And conversely, if we were to have deflection in the opposite direction from
the longest hair cell towards the shortest, we would find
hyperpolarization, okay? So, let me remind you as to why that
would be. Again, it's very similar to what we
discussed for sensory transduction in the cochlea.
once again, we have stereocilia that, at their tips have these transduction
channels that are permeate to potassium ions.
And the gates on these channels are connected one to the next stereocilium by
this spring-like protein called a tip-link.
And as these stereocilia are deflected from shortest to longest, there is a
tension that's applied to this tip-link protein.
And that opens up the transduction gate, allowing potassium to rush into the tips
of the stereo, stereocillium. Now, you will remember that the tips of
these hair cells are bathed in endolymph, which has very high potassium
concentrations. so high that its even greater than what
we find in the cytoplasm. So, these very high potassium
concentrations allow potassium iron to enter through these potassium channels.
They depolarize the hair cell and that wave of depolarization now spreads down
where it reaches voltage-gated calcium channels.
These voltage-gated calcium channels open and calcium rushes into the cell from the
fluids that bathe the lateral and basal aspect of this hair cell.
And as calcium rises within this hair cell, then in a calcium-dependent manner,
there's excess cytosis of vesicles at the base of the hair cells, and the release
of neurotransmitter on to the afferent terminal.
This narrow transmitter is, is probably glutamate and it's probably binding to
receptors like amper receptors on the afferent ending of this 8th nerve fiber.
And if sufficient glutamates bound, there could be a generator potential developed
there. If sufficient glutamates bound, then this
afferent fiber can depolarize and generate an action potential and then a
signal will be conveyed along the 8th nerve to the brainstem.
