 Hey everybody, Dr. O here. In this video we're going to cover hearing. So hearing is going to be occurring in the cochlea, which is in the inner ear. Really big picture. Your brain needs to know the frequency of the sounds you're hearing to determine the pitch. And it needs to know the intensity of the sound you're hearing to determine the volume. So we're going to see how the cochlea plays a role in telling the brain both the pitch or frequency of the sound you're hearing and then the intensity, which is going to be measured in decibels. Just go ahead and go through the steps here. So it starts with step one, it says sound waves represents alternating areas of high and low pressure. So these pressure waves enter the ear. They're directed towards the ear canal by the oracle or pinna of our external ear. And those sound waves are going to travel through our ear canal or external acoustic canal until they reach the tympanic membrane or the eardrum. So that's going to be step one. Step two, the tympanic membrane vibrates in response to those sound waves. So it takes these disturbances in the air that strike this eardrum or tympanic membrane and it's going to vibrate itself. So this vibration is going to lead to step three. So it says vibrations are amplified across the ossicles. So the movement of the tympanic membrane is going to cause the malius, incus and stapes, your inner ear bone, the middle ear bone, sorry, or your auditory ossicles to vibrate. This is going to allow soft sounds to be amplified by these levers. Within the same time, if a sound is really loud but not too rapid, we have what's called the tensor tympani and the stapedius muscles that can reflexively contract and minimize the vibration. So your auditory ossicles, we always talk about how they amplify sound, but they also quiet noises that are too loud as long as they have time to respond. So far, the sound waves have entered the ear and that's told us directional sensitivity where the sound came from, traveled through our ear canal and struck our tympanic membrane. Which as it vibrates, it moves these three levers, the malius, incus and stapes. So now we're going to be at the actual inner ear. So step four says vibrations against the oval window set up a standing wave in the fluid inside of here. And as this wave travels from the oval window to what's called the round window, the cochlea is going to get the information it needs to here. So step four, the stapes muscle is now moved the oval window and sent this wave traveling through it, kind of like if you were to push water in a pool. So the location of where the maximum amount of distortion occurs here is actually how we determine the frequency or pitch of a sound. So let's go ahead and look at that a little bit here. So here we see in the cochlea that we measure frequencies in hertz or cycles per second. Every day we're exposed to most everyday sounds are going to fall between like 250 and 6,000 hertz, but every day we're exposed to high frequency sounds and low frequency sounds. Good examples of a low frequency sound would be a dog barking, maybe a lawnmower going outside the sound of thunder. Those are common examples. High frequency, think about like a kid like squealing when they're being giggling when they're being tickled, birds chirping, whistles, those are examples of high frequency sounds. So as you can see here, the beginning of the cochlea is where the highest frequency sounds are and they get lower as you go. So a high frequency sound is only going to stimulate the cochlea there at the beginning and your brain's going to know it's a high frequency sound. Low frequency sounds are going to travel down the cochlea until they reach their destination. So a 20 hertz sound would have to travel all the way down the cochlea. So the human ear can hear any of these frequencies from 20 to 2,000. But we already said most of them fall kind of there in the middle. So this is how your brain, your brain can tell the pitch or the type of sound you're listening to by which part of the cochlea was stimulated. This is also as important because this is why high frequency sounds are the ones that start, we start to lose as we get older or if our ears are damaged because anatomically they're always going to be impacted by sounds. They're the first to go. So if you're exposed to loud sounds, listen to a lot of loud music when you're younger, et cetera, generally the high frequency sounds go away first. You can find apps on the phone where you can test different frequencies. And I know that my younger students can hear frequencies that I can't hear at all, very high pitch sounds. And then there are frequencies that I can hear that my dad can't hear because as we get older, we lose more and more of these high frequency sounds. All right, so that's kind of how we use frequency to determine what kind of sound we're listening to by its pitch. Now let's talk a little about intensity as well. So the last step here, pressure bends the membrane of the cochlear duct at a point of maximum vibration for a given frequency. So we've already said that the frequency of the sound is determined by where in the cochlea you get the maximum amount of distortion and stimulation causing hair cells in the basilar membrane to vibrate. But the key here then is the power or amplitude of a sound is going to be how many hair cells are triggered in that area. So you can have quiet high frequency sounds and loud high frequency sounds. They're going to stimulate the same part of the cochlea, but a loud sound is going to stimulate more hair cells than a quiet sound. So where on the cochlea the stimulation occurs tells your brain the frequency or pitch of the sound, how much of this cochlear membrane is disturbed by a sound will tell you about amplitude. And that's going to be measured in decibels, which I'll show you in a second. So however much is triggered, the cochlea is going to be connected to the cochlear branch of the vestibulocochlear nerve, cranial nerve eight. That's going to send information to cochlear nuclei in your medulla abangata and then on up to the temporal lobe, which is where your auditory cortex would be. So speaking of intensity, so sound energy is going to be measured in decibels. So here's kind of just a chart here. Normal noise, just average noise, soft music. The average noise in your house is probably going to be around 40 decibels. Here we see just a normal quiet conversation is going to be around 60 decibels. Something like a chainsaw or a leaf blower out in the yard is going to be a little over 100 decibels. Here you see like a rock concert being 120 decibels. So sporting events, rock concerts, they generally can fall in that 120 decibel range. A gunshot would reach 140 decibels, which is very harmful to the ears and painful. So just to review again, I've said several times, but the brain is receiving information of the pitch of the sound by which part of the cochlea is being stimulated and it's being told how loud the sound is by how much of that area of the cochlea is being stimulated. So that is hearing kind of a complex process, but pretty awesome. All right, I hope this helps. Have a wonderful day, be blessed.