 Okay, so now we're faced with another mechanical stimulus, and we talked a little bit about this in the beginning. Sound waves are mechanical disruptions of air molecules, and we probably should do a little tiny bit of review of ear anatomy just to be comfortable with this. So essentially the pinna is the outer part of your ear that you can see, and you have the external auditory canal basically the pinna takes the sound waves and channels them into the external auditory canal, but all that's being channeled in here literally are waves, and you can feel it if you turn up the volume on your computer right now, and I start yelling at you, like you can feel the vibrations of the sound, and when it gets too crazy, you're like, dude, just stop, just keep going because this is physio and we have work to do. When those sound waves hit the tympanic membrane, which is your eardrum, that's this thing right here, tympanic membrane, it causes the tympanic membrane to wiggle, and then you have a series of three little inner ear bones, middle ear bones, I do it every time, this is your outer ear, this is your middle ear. The middle ear is filled with air and the tympanic membrane vibrates thanks to the sound waves and it wiggles the bones of the middle ear, and the wiggling of those bones basically amplifies the stimulus, amplifies the wiggle 22 times. So if you didn't have those bones in there and you just relied on your sound waves to wiggle your cochlea, and this is your ultimate receptor organ, this is the cochlea, you would hear, you would have to talk 22 times louder to be heard at the same level, which, you know, go ahead and give me a challenge, I'm happy to try to talk 22 times louder, it might be entertaining. The cochlea is found in the inner ear and it's filled with fluid. So when the inner middle ear bones wiggle because of the sound waves, they are contacting another, like a tympanic membrane, but it's called the oval window and that's, it's just another membrane. So basically, whoa, that says oval window, the tympanic membrane and the oval window both wiggle, the oval window is going to wiggle 22 times bigger than the tympanic membrane wiggled from the original sound waves and the oval window wiggling makes waves in the fluid. Okay, so now you've got waves inside this snail-shaped structure and of course the waves, like I hope by this point you're thinking, what are those waves do? How is a wave translated into an action potential? Well, since you asked, I guess only just because you asked, let's go look. Dude, where'd it go? This way, but of course. This is my sensory receptor cell. This is called a hair cell and hair cells are embedded in the wall of the cochlea. This is the fluid inside the cochlea and you can imagine that this is cochlea wall, like the snail shell of the cochlea and these hair cells. Now, look at this, this is a hair cell, right there. This is a neuron and my hair cell is essentially going to, I love this, this is fantastic. You have to take a deep breath because it's a little bit weird. When the wave comes, here comes the wave and waves like move things. So the wave wiggles the hair on this hair cell and the wiggling hair, take a deep breath because this is so weird. The wiggling hair, there's my stimulus, the result is that calcium channels open and that causes an action potential. That causes depolarization and an action potential is created and a message is sent. Now, does anybody go like, dude, bull crap? Because there's no way that opening potassium channels causes an action potential. We know from our other experiences with action potentials that no, it actually would inhibit the creation of an action potential. So here's the first example of, dude, there are exceptions to every rule and this exception is possible because the makeup of the fluids is different. The resting potential of a hair cell is different. The extracellular fluid in a hair cell is different than the extracellular fluid surrounding most neurons. So we set up a different scenario where now if we open potassium channels, that's what's going to generate the action potential. The action potential gets generated, the message gets sent through the cranial nerve, 8, awesome, vestibulococlear and the message gets sent to your brain and you hear sound. Also in your ear, you have the semicircular canals, another fantastic mechanism. Don't you wish we had like a year to spend just on sensory mechanisms? Because it really is interesting to compare all these different ways that stimulus gets turned into an action potential. The last thing we're going to talk about is sight because sight is pretty amazingly complicated.