Welcome back. We're going to continue our consideration of the special senses. Talking about the hearing and vestibular system that are going to be found in the ear. We're going to start off with the auditory system that is going to detect sounds. And it's going to be detecting sound waves which are going to be compression and expansion of the air molecules in the forms of waves. So it's going to be waves that are expanded air and compressed air that are alternating to form waves. And it's going to be the amplitude of the wave, the height of the wave, that's going to determine volume of the sound. And then it's going to be frequency of the waves that is going to determine the frequency or the pitch, of the sound. And so that's how we interpret these sound waves, into the noises that we actually hear. So 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 gotta get involved in the anatomy of the ear. That's obviously going to be critically important into how we hear. Where the sounds are going to be focused onto the tympanic membrane, or the eardrum. By the auditory canal and then we're going to have a set of three bones. The malleus, incus, and stapes that are going to transduce the vibrations of the tympanic membrane. To vibrations in the fluid of the cochlea, which is going to be in the inner ear. So here is a zoomed in portion of the middle ear and the inner ear. The middle ear being where we have the three bones that are going to rock next to one another er, when the tympanic membrane vibrates from the sounds. That's going to cause the, malleus, incus, and stapes to rock and cause, the vibrations at the oval window of the cochlea. So this is a zoomed in picture of the cochlea, where we going to have two different fluid pads, and we'll see about this in the next figure as well. Where here's the oval window, where we have it being vibrated form the topanic membrane, transduced by the bones, and then causing the oval window to be vibrated. And I forgot to mention but this, these bones are so important for amplifying the vibrations. So that's what there 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 and require much more energy to cause the same amount of vibration of a fluid. Compared to vibration of the air. And so that's the roll of these three bones, is to amplify the vibrations from the air so that it can cause significant vibrations in a fluid, in the fluid of the cochlea. So that's what's going to happen when this oval window is vibrated strong enough To transduce the vibrations from the sound waves and air, to vibrations in a fluid. And the fluid vibrations are going to first enter this top compartment, the scala vestibuli. And then, those are going to travel down the cochlea, so then if you unrolled it, it would just be a single tube, so 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 sc, scala tympani. And we'll see how that vibration coming through the scala tympani is what's going to cause the vibration that's going to activate the basilar membrane to vibrate. So you can see here are the two fluide compartments and sitting between it is what's called the organ of corti. And that's what we are going to see in the next diagram. So here we have a zoomed in view of the organ of corti. Where at the top we have the scale of the stibuli and 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 scale of vestibuli, those will travel all the way down the cochlea, make a u-turn and come back the scala timponi and right above the scala tympani is the basilar membrane. And that, on top of that, sits hair cells that have these hairs that are embedded in the tectorial membrane. So the tectorial membrane is going to stationary, and you've got hairs that are embedded in it. Sitting on the basilar membrane which is going to vibrate. And so as that happens that is going to move these hair cell, the hairs on the hair cells and bending of those hairs is going to cause depolarization or hyperpolarization, depending on which direction. And so based on which portion of the basilar membrane along the length of the cochlea that's vibrating most, that's going to determine the frequency or the pitch of the sound. So once the hair cell is going to be activated then that's going to activate an afferent neuron which are going to join to form the cochlea, cochlea nerve, which is going then send its to the rest of the brain for processing. And as I just said the region of the basilar membrane that's going to vibrate the most is going to correlate with the frequency of the sound. And then 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 certain, 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 will tell the brain, of 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. So changes in the angle of your head as well as linear acceleration, changes in the horizontal or vertical plane as well. And we're going to have two different organs. One, are going to be the semicircular canals which are going to respond to changes in head rotation, and otolith organs which are going to be able to detect the tilt of the head as well as vertical or horizontal movement. With there being two different otolith organs, the saccule and the utricle. So again, let's understand the anatomy. Where again, this is going to be in the inner ear adjacent to the cochlea. Where we are going to have three semicircular canals, which are going to be tubes with fluid in them. And it, it's convenient that there's three and they're in, at 90 degree angles from one another, which allows us to detect, ang- 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. And if you're doing some sort of combination of those, then there will be multiple of the two canals that are activated. Also on this diagram, it shows the otolith ograns, the utricle and saccule. That are these little compartments kind of, between the semicircular canals and the cochlea. So we said the semicircular canals are going to be these tubes that are full of fluid. And then sitting in the loop is going to be the cupe, cupula, which is going to be these hair cells they have extensions that are in the fluid. And so 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. And 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, and causing 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. And so this, in this way, with the fluid remaining stationary and the canal tilting, that's what's signals to that apparatus attached to the hair cells. And lets you know that your head is tilting. Let's move to the otolith organs, which are going to again involve hair cells, so this is a common theme, or idea of cells with extensions like hairs. And these are going to, as we've said, detect changes in linear acceleration or in the position in the head, as in if it's tilted. And at the ends of these tips of these hair, there's going to be gel. They're going to be sitting in this layer of gel and then, on top of that, are going to be otoliths, which are going to be calcium carbonate crystals that are in this gel that are, is found at the tips of the stereocilia. And since they're car, calcium carbonate, they're very heavy. And so 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 stereocilia, which will be the signal that there' s been movement or change of position. So the urticle 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 where we're going up and down. This is animation showing you the movement of the head or the body, and the response by the otolith organs. So here you see the purcells with their stereosyillius sticking up into this blue. Here shown as blue gel with these green otoliths of calcium carbonate crystals that are going to pull on those stereocilia and illicit a response. So here when we tilt the head backwards. Then you can see that because gravity is going to act on those otoliths since they're so heavy. That is going to cause them to pull on those stereocilia. Whereas when we tilt forward, then the otoliths are going to pull be pointed in the other direction pulling on those stereo cilia. 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. And so it's like they stay in place while the rest of the body goes forward. And that's what causes the tugging. So it's actually very similar to what happens when you tilt your head backwards. And then when you de, decelerate, it's going to be the opposite. The otoliths are going to 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 going to cause very similar. Changes in the otoliths. Where forward acceleration similar to tilting your head backwards and deceleration is going to be similar, causing a similar affect, as tilting your head forwards. So, finishing this section up, we've talked about the auditory system. How we are going to be able to detect complex sounds by breaking them into their basic sound frequencies and then being able to determine the volume of each of those basic frequencies. And this is going to be obviously converted into action potentials, and then sent to the brain for interpretation. And then the, the vestibular system is going to aid us in maintaining our balance. In part by knowing where our body is in space. And we're going to be able to detect position and motion of the head, and they're going to be found with these sensory hair-like cells, the're going to be found. In the semicircular canals, as well as, in the otolith or, organs, the utricle and the saccule.
