The purpose of today's presentation is to discuss the anatomy and physiology of the balance system. By way of background, there are three sensory systems that are involved in the maintenance of postural control imbalance. Vision, proprioception for which you get information from the bottom of your foot, there are sensory receptors on the bottom of your feet and also the angle of the ankle joint. Then the vestibular system which we'll talk about in more detail. The vestibular system is the first sensory system to develop. Specifically, it begins development in the first three weeks of the gestational period and it is complete by the 25th week of gestation. Then the system continues to develop over the course of pregnancy until it is completely finalized by the eighth or ninth month of pregnancy. Also, the vestibular nerve is the first cranial nerve to complete myelination. But balance function continues to mature over time. There are sequential development of motor milestones. Also as we move about our environment, we have experiential learning which helps fine tune the balance system. This is a busy slide that represents the entire inner ear with all of the organs labeled. For the purposes of today's talk, we're going to be talking about the vestibular system which is located on the right-hand side of this particular slide. So here is a cleaner version of the vestibular system. There is a bony labyrinth which is housed within the petrous portion of the temporal bone. Also a membranous labyrinth that sits within the bony labyrinth. While the bony labyrinth is filled with a fluid called perilymph, the membranous labyrinth is filled with endolymph. The end organs for sensory receptors are located within the membranous labyrinth. The parts of the vestibular system include semicircular canals. There are three pairs of semicircular canals. The superior which, is also referred to sometimes as the anterior canal, the lateral canal, which is also referred to as the horizontal canal and a posterior canal. You have one set of those on each side of your head. There are also two otolith organs, the utricle and the saccule. This arrow points to the superior anterior semicircular canal. This one points to the lateral or horizontal semicircular canal, and this one points to the posterior semicircular canal. Each semicircular canal has a swollen end to it called the ampulla and the sensory receptors are within that portion of the semicircular canal. Here is a cut through of the ampulla and you can see that there are cells called the crista that are labeled and embedded in the crista are hair cells. On top of those hair cells are cilia or hair bundles. You can see on the slide there's one long cilium on each hair cell and that's referred to as a kinocilium. The shorter cilia are collectively referred to as the stereocilia. Then you see the cupula is a gelatinous mass that's sits atop the hair cells and in which the hair bundles are embedded. There are also nerve fibers coming off of each hair cell that will leave the sensory end organ and become the vestibular nerve. So the stimulus for activity in the semicircular canals is fluid within the membranous duct causing the cupula to be deflected and it is that deflection that causes activation of the sensory cells. The direction of the flexion determines whether it will be an excitatory response or an inhibitory response. This just shows the approximate location of the two otolith organs which are the utricle and the saccule. This is a depiction of the otolith end organ. You see the macula and in the macula are embedded hair cells. As is the case with the semicircular canals, you see cilia sitting on top of the hair cells and those cilia are embedded in a gelatinous mass. On top of the gelatinous mass are otoconia, which are calcium carbonate crystals, which is the most important distinction between the semicircular canals and how they function and the otolith organs. So this is a depiction of the utricular macula in terms of its orientation. So it's oriented on the horizontal plane and you can see what happens with a force of gravity bending the cilia on top of the hair cells causing an excitatory response. The saccular macula are oriented in a vertical plane and it is also the case that gravity or movement in a linear direction will cause the otoconia to result in deflecting the cilia on top of the hair cells. The vestibular nerve is part of the eighth cranial nerve. The other part of it is the auditory nerve. The vestibular nerve itself has two branches, the superior vestibular nerve, which is responsible for innervating the superior and lateral or horizontal semicircular canal cristae and the utricular macula. Whereas, the inferior vestibular nerve is responsible for innervating the posterior semicircular canal cristae and the saccular macula. An interesting note is that our vestibular tests, until very recently, were only able to look at function of the horizontal or lateral semicircular canal. So we only obtained information about superior vestibular function. There are three important reflexes that are necessary for you to understand. The first is the vestibulo-ocular reflex which is the reflex that we're looking at most frequently during vestibular testing. The purpose of the VOR is to maintain objects of interest on the fovea, our most sensitive part of the retina. The result of the VOR is equal and opposite movement of the eyes relative to the head. So in other words, if I turn my head to the right, the VOR will generate eye movements to the left. The VOR is essential to maintenance of clear vision with head movement. The second vestibular reflex it's important to know about is the vestibulospinal reflex. The purpose of that reflex is to stabilize the body. The third reflex is a vestibulocollic reflex or VCR, which helps in stabilizing vision by contracting neck muscles which stabilize the head and the body during ambulation. All of this information is important to understand in order to appreciate the function of the vestibular system. There's still so much to know though about assessment of vestibular function and rehabilitation. Thanks for your attention.
