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.
