Welcome back. We're going to continue our consideration of the special senses, talking about the hearing and vestibular system that are found in the ear. We're going to start off with the auditory system that detects sounds. It detects sound waves which are the compression and expansion of air molecules in the forms of waves. Sound is going to be waves of expanded air and compressed air that alternate to form waves. And the amplitude of the wave, the height of the wave that will determine the volume of the sound. And it is the frequency of the waves that is determine the frequency or the pitch of the sound. So that's how we interpret these sound waves into the noises that we actually hear. The auditory system is going to be able to detect complex sounds by breaking them into their basic sound frequencies and then they're obviously going to be converted into action potentials. And, volume is going to be relayed by the frequency of action potentials. So the louder a sound is, the more frequent the action potentials are. And we'll talk about how pitch, or the frequency of the sound is going to be determined, as well. So now we've got to get involved in the anatomy of the ear, that's obviously gonna be critically important in to how we hear. The sounds are going to be focused onto the tympanic membrane, or the ear drum by the auditory canal. Then we're going to have a set of three bones, the malleus, incus and stapes that transduce the vibrations of the tympanic membrane to vibrations in the fluid of the cochlea which is in the inner ear. Here's a zoomed-in portion of the middle ear and the inner ear. The middle ear is where we have the three bones that are going to rock next to one another when the tympanic membrane vibrates from the sounds. Sound waves cause the malleus, incus and stapes to rock which cause vibrations at the oval window of the cochlea. This is a zoomed-in picture of the cochlea. Here we're going to have two different fluid paths. We'll see this in the next figure as well. But here's the oval window which is being vibrated from the tympanic membrane, transduced by the bones and then causing the oval window to vibrate. I forgot to mention, but these bones are so important for amplifying the vibrations. That's what their crucial role is. because out here in the outer ear we have vibrations of air, but then we need to cause vibrations in fluid. Which is going to be much less efficient. This requires much more energy to cause the same amount of vibration in a fluid compared to vibration of the air. So, that's the role of these three bones, is to amplify the vibrations from the air, so that it can cause significant vibrations in the fluid of the cochlea. So that's what's going to happen when this oval window is vibrated. It's going to be strong enough to transduce the vibrations from the sound waves in air to vibrations in a fluid. The fluid vibrations are going to first enter this top compartment, the scala vestibuli. And then, those are gonna travel down the cochlea. If you unrolled it, the cochlea it is just a single tube. The vibrations will go down the scala vestibuli, and then come back around towards the second fluid pathway, which is in the compartment called the scala tympani. We'll see how that vibration coming through the scala tympani is what's going to cause the vibration that activates the basilar membrane to vibrate. So you can see here are the two fluid compartments. And sitting between it is what's called the organ of corti. That's what we're going to see in this next diagram. Here we have a zoomed-in view of the organ of corti where at the top we have the scala vestibuli. Remember that is the fluid that's right behind the oval window. So when the oval window gets vibrated, then that will cause ripples of fluid to travel down the scala vestibuli. Those will travel all the way down the cochlea, make a u-turn, and come back in the scala tympani. Right above the scala tympani is the basilar membrane. On top of that sits hair cells that have these hairs embedded in the tectorial membrane. The tectorial membrane is stationary. You've got hairs that are embedded in it sitting on the basilar membrane, which is going to vibrate. As that happens, it is going to move these hairs on the hair cells. Bending of those hairs is going to cause depolarization or hyperpolarization, depending on which direction they bend. And so, based on which portion of the basilar membrane along the length of the cochlea that's vibrating the most, that's going to determine the frequency or the pitch of the sound. Once the hair cell is activated then that's going to activate an afferent neuron. THis in turn joins to form the cochlear nerve, which is going to then send the signal to the rest of the brain for processing. As I just said, the region of the basilar membrane that vibrates the most is going to correlate with the frequency of the sound. And the louder the sound, the greater the vibration and the greater frequency of action potentials. So it gets into the idea again that the brain is going to know that signals coming from this particular neuron that feeds from a specific portion of the basilar membrane means that there's a certain frequency of sound and then the frequency of action potentials tells the brain the loudness of that frequency of sound. We're going to move to the vestibular system, which is going to be able to detect different types of motions of the head or the body. They're going to be in the inner ear. They're going to be able to detect angular acceleration, changes in the angle of your head, as well as linear acceleration, changes in the horizontal or vertical plane as well. We have two different organs. One is the semi-circular canals, which respond to changes in head rotation. the other are the otolith organs, which detect the tilt of the head, as well as vertical or horizontal movement. There are two different otolith organs, the saccule and the utricle. So again, let's understand the anatomy. This is going to be in the inner ear adjacent to the cochlea, where we have three semi-circular canals. These are be tubes with fluid in them. It's convenient that there's three and they're at 90 degree angles from one another, which allows us to detect changes in the angle of the head along the three perpendicular axes. So one of the canals is going to be highly stimulated when we nod yes, versus one when we shake no, versus a third when we tip our head side to side. If you combine these motions, then there'll be multiple of two canals that are activated. Also on this diagram it shows the otolith organs, the utricle and the saccule. These are little compartments kind of between the semicircular canals and the cochlea. So we said, the semicircular canals are gonna be these tubes that are full of fluid. Then sitting in the loop is the cupula, which is has these hair cells that have extensions in the fluid. As our head rotates, then the canal or the tube is also going to rotate, but the fluid is actually going to stay relatively stationary. So, it's the movement of the tube or the canal with those hair cells, and the stationary aspect of the fluid that is going to cause tugging or pulling on these hair cells. This will cause the action potential eventually to signal that we're having movement in that direction. This is my demonstration of a semi-circular canal where if you tilt your head, the canal that's attached to the head is also going to tilt but the fluid which is in the canal will primarily remain stationary. In this way with the fluid remaining stationary and the canal tilting that's what signals to that apparatus attached to the hair cells, and lets you know that your head is tilted. Let's move to the otolith organs which are going to again involve hair cells. This is a common theme, the idea of cells with extensions like hairs. These are going to, as we've said, detect changes in linear acceleration or in the position of the head, as in, if it's tilted. At the ends of these tips of these hair, there's gonna be gel. They're gonna be sitting in this layer of gel and then on top of that are gonna be otoliths, which are calcium carbonate crystals that are in this gel that is found at the tips of the stereocilia. Since they're calcium carbonate, they're very heavy. That means that a change in the position of the utricle or saccule is going to cause those calcium carbonate crystals, the otoliths, to be pulled by gravity and then to pull on the tips of the stereocillia, which will be the signal that there's been movement or change in position. So the utricle based on its orientation in the body is going to detect movement in the horizontal plane. While the saccule because it's at 90 degrees is going to detect vertical movement, so movement going up and down. This is an animation showing you the movement of the head or the body and the response by the otolith organs. Here you see the hair cells with their stereocilia sticking up into this blue shown as blue gel with these green otoliths the calcium carbonate crystals that are going to pull on those stereocilia and elicit a response. So here when we tilt the head backwards, then you can see that, because gravity is gonna act on those otoliths, since they're so heavy. That is gonna cause them to pull on those stereocilia. Whereas, when we tilt forward. Then the otoliths are gonna be pointed in the other direction pulling on the stereocilia. And then when we have forward acceleration what's happening is that those otoliths, since they're so heavy and dense, they have greater inertia than the rest of the body. So it's like they stay in place while the rest of the body goes forward and that's what causes the tugging. It's actually very similar to what happens when you tilt your head backwards. When you decelerate, it's gonna be the opposite. The otoliths are gonna kind of stay in place, while the rest of the body moves backwards. So I'll let you watch it again. We're tilting and acceleration or deceleration are gonna cause very similar changes in the otoliths. Forward acceleration is similar to tilting your head backwards. Deceleration is going to be similar, causing a similar effect as tilting your head forwards. So, finishing this section up, we talked about the auditory system. How we are gonna be able to detect complex sounds by breaking them into their basic sound frequencies, and then, be able to determine the volume of each of those basic frequencies. This is going to be converted into action potentials and then sent to the brain for interpretation. Then the vestibular system is going to aide us in maintaining our balance, in part by knowing where our body is in space. And we're gonna be able to detect position and motion of the head. And they're going to be found with these sensory, hair-like cells that are found in the semi-circular canals, as well as in the otolith organs, the utricle and the saccule.
