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Sensory System

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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The sensory system is the part of the nervous system that recognizes and processes sensory information. This system includes sensory receptors, neural pathways and parts of the brain. The sensory system is comprised of vision, hearing, somatic sensation, taste and olfaction.

The sensors is the way that we perceive the world it's how our nervous system detects changes in the environment I do want to emphasize this whole idea about changes because we can't actually detect things that don't change very well for example if the temperature stays the same you usually don't notice that its only when it gets hotter or colder that you notice it. You can do a fun little test if you, you may have done this even in the elementary school take 3 bowls of water, make one bowl of water filled with ice alright? Ice water, right in front of you have a bowl of regular tap water just room temperature water the last bowl have it filled with very warm water. If you put your hand into the warm water your left hand you'll say "hot" because it's detecting a change in temperature. If you put your right hand into the ice water you'll go "aah cold" so your right hand would be reporting cold your left hand will be reporting hot then take both hands and put them in the same temperature water the one that had been in hot water would say "huh cool" the one that had been in cold will say "hot" that's because you're detecting changes not just "I'm reporting a temperature of approximately 23 degrees Celsius" alright?

Now there's these specialized cells called sense receptors and they are categorized in many different ways but the most basic way to categorize them is how they function what kind of stimulus do they detect. There's a big loop called mechanoreceptors and those are the ones that detect mechanical or physical change. These are involved a lot in touch but they are involved in some things that you don't even think about like hearing alright? That's caused by changes in the little hair cells inside of your inner ears think of the cochlear. Thermoreceptors you might be able to guess are involved in detecting temperature changes. Chemoreceptors you might be able to guess on that one hopefully chemo means chemicals alright those are involved in things like taste and smell. Photoreceptors the root word photo means light so those detecting light those are involved in your eyes for example. Nociceptors is a kind of detector or sensory receptor that people rarely think about but they're very important those are the ones that are involved in detecting damage i.e. pain and these are one of the few receptors that we don't have the ability to easily ignore. With chemoreceptors if you smell yourself pretty soon your brain stops being able to pay attention to that that's why you rarely can smell yourself or even your own bad breath but pain receptors take a needle stick it in your arm, you'll know it's there two minutes later you'll know its there you'll still know its there why don't we adapt very well to nociceptors signals that's because it means that you're being damaged, it's not it goes away I don't really need to pay attention to it. Pain I need to know it's there I need to stop doing that damage pull the pen out for oh that's why you don't adapt very well to pain.

Now some of the special senses include balance which scientists will sometimes call equilibrium and the major organ of balance is something called the inner ear which has two kinds of structures involved in dynamic equilibrium which uses something called the semicircular canals those detect rotation of your head, versus static equilibrium which detects changes in your static balance i.e. suddenly accelerating forward or backwards or going up or down.

If we take a quick look at this diagram over here you can see this is the inner ear it's embedded within your skull if you take your finger and put it in your ear and then shave really hard and crunch with some bones you'll have to finally hit the inner although the your nociceptors will hope fully stop you from doing that. Here you can see the semicircle of canals in there as the name implies kind of circular they are filled with fluid and at the base of each ring you have a little cup which has a flap and they're filled with liquid and if you suddenly rotate your head like this, the semicircular canal that's oriented like that will suddenly shift and the fluid will stay still pushing on the flap. The mechanoreceptor there will pick up the signal and send off to the brain warning I'm rotating in this axis there's another one that set up to rotate or detect rotation like this and another one set up to detect rotation like that. Now and that's your very very good at doing your gymnastics you're very rarely go exactly like this or exactly like this instead you'll be doing some combination and so all of these will be reporting to your brain they would and your brain will assemble that information to give you a good idea of what the heck is your body doing if you want everyone to mess with this a good fun illusion of this you may have done this in some kind of obstacle race or course, take a baseball bat put it on the ground put your head on the baseball bat so that your head is about maybe a yard above the ground, and then go round the baseball bat as fast as you can five times fast then as soon as you've run five times faster around the baseball bat stand up and run and you'll discover that the world seem to be falling over why? Because you got one of those semicircular canals you finally got the fluid moving with you and so when you stand up all over a sudden it continues moving but that flap gets pushed and your brain is whoa the world's falling it's really entertaining when it happens to other people.

Over here we have utricle and saccule which are much like the semicircular canals they're filled with something and that pushes on mechanoreceptors only here its typically a gel like membrane and when you suddenly accelerate forwards it pushes those hairs one way or when you accelerate backwards and same thing with the circular which detects up down kind of acceleration.

Alright let's go back over here. Hearing also uses the ear now the ear is actually composed of the outer ear which helps funnel in the sound the middle ear which takes the vibrations of your eardrum and transfers that into vibrations of 3 bones the hammer, anvil and staple stirrup otherwise known as the malleus, incus and stapes bones and those bones help amplify those vibrations to cause vibration and yet again a fluid in the inner ear. In the inner ear there's this coiled up tube which is lined with bazillions of those mechanoreceptor hair cells and the vibrations caused by that middle ear bones causes vibrations somewhere along this long tube and those hair cells will detect it. That's why if you listen to you iPod for too long you're causing certain hair cells to be vibrating a lot and eventually you can cause damage to those and once you've got that damaged it's really hard to repair and it may cause permanent hearing loss, something to be thinking about.

Alright, now the eye does vision, you hopefully know that one. There's two big things going on during vision. First there is the focusing of the light then there's the actual detection of the light. The focusing is done by the cornea the clear outer part here, that's a fixed focus curved lens. Then you have inside of your eye you have the adjustable lens and that can adjust so that you're looking far away or close up but like I said that cornea is what does the first part of focusing and if you've ever heard of somebody getting lazor surgery on their eye, what their doctor is doing is that they're actually reshaping that cornea to adjust that person's permanent focus. The retina is in the back of the eye and if we take a look that we can see here, here is the front of the eye light comes in through the curved cornea goes through the lens which can adjust its shape based on whether you're looking at something far away or close up. Then this dark region here is the inner portion of the eye on the back you can see this faint yellow thin layer, that's the retina that's where the photoreceptors are. You've got over a 100 million photoreceptors there specialized cells that are detecting light some of them detect colors some of them detect specific colors some of them are just looking for motion light and dark stuff but all that data is collected and run through the back of your eye forming the optic nerve. Now you'll also see these red lines going through those are the arteries that feed the retina the re- retinal arteries and the little purple guys are the retinal veins. They actually sit on top of the retina which is weird when you think about it "why about it why don't I see it?" Well remember you detect changes. Do those retinal arteries change their position? No so you hardly if ever see them now the only times that you may notice those retinal arteries is if you're really bored get a small flashlight put it right below your eye not touching your eye that hurts and those nociceptors in your cornea will tell you stop touching your eye and just move that back and forth really fast what that does is having a close light source moving back and forth really fast makes the shadows of the retinal arteries start shifting a little bit aah suddenly there's a change and you'll start to see this weird shadowy cracks in your vision its kind of creepy to do. Also if you've ever seen those sparkling lights when you almost passed out or you stood up suddenly and your feeling lightheaded those sparkling lights often are due to sudden drops in the blood pressure and then as the blood pressure returns the retinal arteries start feeding the photoreceptors and they go - I'm back! And you report that, I'm back signals from your photoreceptors as aah light.

Alright, the last thing we're going to talk about is smell and taste now I group those together that's because they both basically are detecting the same thing chemicals that are entering into your pharynx the space here then within your nose and the oral pharynx is the your mouth. Now in the top of your nasal cavity you have something called the olfactory bulb which has dazillionz well not gazzilions but several several thousand chemoreceptors each able to detect different kinds of chemicals. Now they're embedded within mucus which helps dissolve the chemicals and bring them to the membrane receptors of the olfactory bulb your tongue is also covered with chemoreceptors clustered together in these things called taste buds here's the top of your tongue here's the bottom of your tongue and so again dissolving the chemicals in your saliva helps bring them to these receptors which then detect and send off to your brain. Now your taste buds are all not all that good at detecting taste there's only 5 basic tastes; sour, sweet, bitter, salty and then there is is other one called umami otherwise known as savory and if you have a curious how it tastes like, get some MSG, Monosodium glutamate, that's what they're detecting, so why do we taste lots of different things when we eat? Well that's because when you're chewing some of that odor some of the chemicals coming off of your food goes to the back of your mouth into your nasal cavity sending signals to your olfactory bulb and that's how you're able to tell the difference between the sweetness of say chocolate milk chocolate and the sweetness of say dark chocolate because there's this tiny variations in the smells now I might be able to say one's a little bit bitter or a little bit more sour than the others but the main function or the main component of taste is actually smell and again I said they have to be dissolved. If your tongue is dry you can't tell what a taste is you can try this out if you take your tongue just stick it out eeh and let it dry then put something on it like powdered up vitamin C tablets that's really entertaining I've done this to some of my students without them realizing they see what looks like powdered sugar they put on their tongue and their tongue can't detect the taste because their tongue is dry but then they let the saliva start soaking in and all of a sudden they start going aaah and scrapng stuff off their tongue. You could also try this with sugar if you want to be a little bit nice with yourself alright?

One last little trick that she can do with this to just demonstrate how interconnected these two things are take somebody and offer them while they're blindfolded a piece of apple while they hold while you hold a onion slice underneath their nose if they bite into this apple slice they'll think it's the onion because it's got the right texture but the smell that's going in their nose will report onion and that's the senses.