hey everyone we're going to start by uh just finishing up talking about uh nervous systems we talked very specifically about neurons and how they communicate we're just going to spend a little bit of time before moving on to sensory systems to talk about the sort of overall organization of a typical nervous system a typical nervous system in most but not all animals contains two major branches what we call the central nervous system and the peripheral nervous system the central nervous system typically includes all of the sort of interneurons um that aren't directly tied to the sensory apparatus for detecting things that are going on in the environment and they're not directly tied to the muscles themselves it's basically everything that's in between all of these interneurons in a human those would be your brain in your spinal cord also in a lizard um our brain contains lots and lots of interesting neurons that allow us to have really really flexible behavior and also our spinal cord contains neurons not all of them but most of them having to do with integrating information invertebrates bilateral ones at least also have a brain as part of their central nervous system but instead of a spinal cord they have a segmented what we call nerve cord those little clusters of cell bodies in it and then neurons axons that come branching off of them now that's the central nervous system so basically it's our brain and spinal cord or similar things in a in an insect or another invertebrate peripheral nervous system basically counts our sensory neurons and our motor neurons and as we'll see some other neurons that lie outside of our brain and spinal cord our peripheral nervous system can be broken up into many many different parts the two first levels of organization break up the peripheral nervous system into the somatic and soma means body peripheral nervous system so the somatic nervous system uh it's basically all of the nerves that go out to your muscles that you control uh voluntarily that you can you can think about moving your skeletal muscles uh and all of the nerves carrying sensory information uh from the outside world that you have conscious perception of so it's kind of like uh the outgoing and incoming signals that you are aware of and can control so that's the somatic peripheral nervous system contrasting with that is what we call the autonomic nervous system it's a bunch of things that are going on underneath the surface that you don't have conscious control over scientists like to use mnemonics for these kinds of things little ways of trying to remember them and so for the autonomic nervous system scientists often refer to the four f's that it helps regulate fighting fleeing feeding and mating now within the autonomic nervous system we can break it down into two subcomponents uh one called the sympathetic autonomic nervous system and one called the parasympathetic autonomic nervous system these have typically opposing effects on the body the sympathetic nervous system has its own rhyming mnemonic sympathetic is often referred to as the fight or flight part of the autonomic nervous system uh it's the thing that makes your heart race uh your breathing go more rapid gets you ready to fight or flee uh dilates your pupils uh the parasympathetic the rhyme there is rest and digest uh it kind of has the opposing actions of the fight or flight if you eat a big meal lots of your blood goes to your gi tract um you don't become quite as active you are more likely to be resting these two things are are typically sort of both active at a at a low level um it's not like one turns on and then the other turns off um they're always kind of both going a little bit they're kind of usually in balance but certainly during really extreme cases like a real honest fight-or-flight situation that particular part of the autonomic nervous system will be particularly active okay and the last thing we'll just talk about is there are lots of different kinds of nervous systems we've been you know speaking mostly about bilateral organisms like humans uh even squids have a have a typical nervous system like most invertebrates with a brain and a nerve cord and then peripheral nerves coming off of it animals with radial symmetry often have more interesting or different kinds of organizations for example a starfish has a neural ring instead of a brain and something like an anemone or a hydra has just a big set of branching nerves called a nerve net so there are other strategies that allow those animals to do interesting things okay i'm going to move on now to talk about sensory systems remember uh the nervous system uh allows for rapid communication of information uh with the main goal of of it is being allowing the animal to behave in a appropriate way in response to whatever situation it is in and we'll talk later about how these neural signals are causing behavior by activating muscles we're going to start now by talking about the input the sensory systems how does an animal know what's going on in its world so that it may behave appropriately so we'll talk about sensory systems uh the job of sensory systems is to do something called transduction transduction is essentially turning some kind of energy that is going on in the outside world or impinging upon the animal and turning it into the language of the nervous system and ultimately it's going to be like turning stimulus energies into changes in membrane potential right which is the language of the nervous system post-synaptic potentials um action potentials things like that uh and also you know chemical release when appropriate changes in in chemical signaling those are of course the two languages of the the nervous system electrical changes and chemical signaling transduction takes for example light that we see with our eyes and turns it into translates it into signals that our nervous system can deal with the cells in our body that do transduction we call receptors receptors are specialized cells that react by changing their voltage in response to some kind of stimulus a stimulus is is is just something that impinges upon the animal that causes these kinds of of reactions uh like a noise or a flash of light or something so a receptor is just a cell that's specialized for doing this transduction and when it has been activated right when it responds it responds with what we call a receptor potential which is just a change in the membrane potential sometimes it's up sometimes it's down that's a lot like an excitatory or inhibitory postsynaptic potential that you'd find in a dendrite of a neuron so it's typically relatively small decays as it goes although we'll see what happens to it later but they change their membrane potential in response to stimuli that's how you go from light existing in the universe coming from the sun for example uh to us being able to see it it has to be translated into changes in voltage now humans have the ability to transduce basically three different types of stimulus energy into neural signals that we can interpret as things that are going on and those are mechanical stimuli things that are moving so like if something is touching you that's a mechanical stimulus we also have um electromagnetic uh in humans this would be just visible light there are other animals that can detect electrical fields there are animals that can detect ultraviolet radiation infrared radiation out just outside of the bounds of what we call visible light but that's one one category of stimuli we can transduce and finally chemical transduction oops a chemical doesn't have an h there chemical transduction is are things like smell and taste right chemicals bind to receptors on our uh on our cells and activate them by changing their voltage typically okay we're going to start by going through the major sensory systems that humans have and we're going to start with hearing and so which of those three stimulus energies is sound is it mechanical electromagnetic or chemical well hearing is hearing sound is essentially vibrations of molecules uh that are done in waves um generated by the vibration of typically like solid objects that send the water that you're in or the air that that you're in uh vibrating here's a cartoon picture of it and so sound in the air is essentially just increases and decreases in the air pressure which is how close the molecules are together and they kind of travel as a wave you notice that if i circle this these red dots these red molecules those red molecules don't really move travel with the waves the waves travel sort of through them individual molecules just vibrate in place but the pressure waves move all right now because they are waves uh sound stimuli have two major components uh they have both a frequency and an amplitude the frequency is how fast the amplitude goes up and down um at low frequencies it takes longer for the wave to go up and then down and then up and down to our perceptions frequency is indicative of tone lower toned sounds have lower frequencies and higher tone sounds have higher frequencies and so that's how we determine pitch is essentially by the frequency of the of the sound waves amplitude is essentially how high these waves are it's exactly how much pressure differential there is between the peak and the trough uh of the air or the water that the the waves are going through and that's essentially the volume of the sound the higher the pressure differential the louder the sound is now if we look at the range of sounds that we can detect as a human we see that we have the ability to detect sounds as low as 20 hertz and a hertz just means um cycles per second uh so 20 hertz would be a wave that goes up and down 20 times a second that's about as low uh a uh a tone as we can hear um below that we start like feeling it in our skins as like vibration um we can hear up to around 20 000 hertz uh so notice this is on a log scale we can hear many many orders of magnitude of a range of frequencies but above 20 000 hertz we can't really hear it dogs of course can hear much higher right a dog whistle we can't hear but a dog can and bats for example can hear well out over a hundred thousand in fact they can make sounds that are that high and use those sounds to echolocate they can use the echo of their voice uh to find their prey in the dark now on this graph we have not just the range of hearing that we can detect in terms of its frequency but we can see that at every frequency we have a sound amplitude that is sort of what we call the threshold right this lower line here indicates the threshold of hearing the lower it is the lower the amplitude of sound we can hear so if we look at this we hear sound best from like 500 hertz to about a thousand hertz uh it turns out that this is fairly useful for us that's kind of where a lot of our speech is so we hear really well there to hear really low sounds the sound has to be really loud before we can detect it um and similar out here at the higher end we can hear really really loud high-pitched sounds because our threshold is is really high but if it goes beyond 20 000 or so we can't hear it anymore okay i'm going to now just talk about how hearing works most vertebrates have most mammals have some sort of external part of their hearing apparatus sometimes called the oracle or the pinna it's basically like a funnel that helps funnels oops funnel sound waves uh in through in us we have this ear canal it's a sort of hollow tube that leads into our inner ear at the end of our ear canal we have this thin membrane you will probably know it as the eardrum it is also called the tympanic membrane uh tympanic because it's like a timpani which is a big kettle drum and it kind of acts like a timpani uh it is stretched really tight and when the air vibrates uh the eardrum the tympanic membrane vibrates in response to the air the eardrum vibrates and in turn it vibrates three tiny little bones that we have in each of our inner ears sometimes called together the ossicles they are the malleus incus and stapes or hammer anvil and stirrup based on what they look like to old-timey anatomists their job is to sort of take this floppy vibration of the air and vibration of the eardrum that responds really well to the air and focus the vibrations through this little stapes here in a much more small place so they kind of like are focusing the the vibrations onto this organ here called the cochlea actually goes all the way out here where the stapes meets the cochlea is a little membrane called the oval window and it gets set vibrating in response to the vibration of the eardrum and the ossicles they focus this vibration down onto this much smaller area called the oval window and then the oval window sets up vibrations in this fluid called perilymph inside this organ called the cochlea if we look at the cochlea here we see that it kind of looks like a snail it is a long tube but it is coiled up inside our inner ear this is kind of a cartoon of it where it is unwound that long tube is fluid filled here's a cross section through it uh it is filled with a couple different things one called paralymph where the vibrations are sort of traveling importantly running along the middle of this tube is a strip of specialized cells that also include the receptors that are going to allow us to hear the whole collection of cells is called the organ of corti right here and it just runs as a long strip along the entire length of the cochlea so this red and yellow thing here is kind of the organ of corti running right down the middle of it now the structure of the organ of corti is what allows us to detect sound there are a couple of important parts let's look at it from the side there is a floppy membrane along the bottom called the basilar membrane basilar means base that has a bunch of cells in it we'll talk about those cells there's also a roof called the tectorial membrane and tech kind of means like top or roof that sits kind of above all of these cells now when sound starts to vibrate the fluid inside the cochlea it is going to cause the basilar membrane to vibrate as well it's like a big sort of floppy plastic ruler or something right it's going to vibrate it's actually much much more floppy than that it's going to vibrate up and down that basilar membrane is going to vibrate up and down now the tectorial membrane on the other hand is relatively rigid and so what ends up getting what ends up happening is that the cells that are attached to the basilar membrane kind of get pushed up against the tectorial membrane now how does that allow us to hear sound it has to do with the receptors the receptors are these cells here they are called hair cells here's a better picture of one it's more accurate importantly why are they called hair cells they're called hair cells because they have cilia that you remember from cell biology these cilia sticking out the top right they are kind of columnar in shape they are not just cilia they are not just a cilium a hair cell is a regular cell with a nucleus that just has a bunch of cilia on them and kind of this staircase thing staircased arrangement um so students often get confused and think a hair cell is this just made of a psyllium a hair cell is this whole thing uh cell with cilia that kind of give it hair okay now how does that allow us to hear well it turns out that when the basilar membrane bounces up and down those cilia as those hair cells come close to or even sometimes in contact with the pictorial membrane those cilia bend and so you end up with the cilia bending back and forth here's a really cool electron micrograph of the cilia in the staircased arrangement now how does bending cilia cause this cell to respond i'm going to let you think about it remember that the response of a receptor is to change its membrane potential now thinking back to neurons how does a neuron change its membrane potential what's the only way that a neuron knows how to change its membrane potential how does a neuron cause its voltage to go either up or down the answer is to open or close ion channels and that's what we're going to see happens here when those cilia bend it turns out that they are all sort of attached to each other by what are called tip links they're little tiny protein filaments you can actually see them in this drawing here these little protein filaments are attached the protein filaments themselves are attached to what we call stretch-gated channels they are ion channels that are essentially pulled open this is a cartoon they there doesn't look exactly like this but by these tip links and so when the cilia bend they've been pushed this way these links are pulled and they pull the channels open so when the cilia bend that way the channels open then they bend that way and the channels close and so what happens is that you have a period of time where ions can flow in you notice they are flowing into the cell and you have a period of time where the ions are not flowing in and ultimately when the ions come in they are going to change the membrane potential go back up to here and we can see that when the cilia move channels open now you might think if we want the membrane potential to go up when these cilia bend that we're going to open sodium channels that would make a lot of sense and it's how a neuron would do it i apologize for biology being the way it is but these hair cells their outer part of them where the the cilia live they actually live in an interesting fluid called endolymph that has high potassium and low sodium so uh like most things in biology there's always exceptions so in your ear whoops you are going to open up potassium channels by activating these tip links and bending the cilia and when those channels open they allow potassium to come in now potassium is positively charged and so membrane potential goes up when those ions come in it actually turns out that as they vibrate back and forth the membrane potential also vibrates goes up and down channels are open when they're pushed one way they're closed when it pushes the other way but that's the signal that is the transduction of vibrations in the air they have now become a change a cyclical change in this case of membrane potential in your your in your hair cells in your cochlea now finally those hair cells need to communicate to the nervous system that they heard something right that they were activated well that is the job of these nerve fibers that are going to send information to your brain how do you think that the hair cell is going to communicate to these nerve fibers that are going to carry information to your brain if you are thinking that the hair cell is going to release neurotransmitter onto them you are right when the voltage goes up in this in this cell here when the membrane potential is increased it turns out that it is going to activate down here at the release zone a channel that we have seen before these are going to be voltage-gated calcium channels and they work in the same way that they do in the axon terminal when the voltage goes up they open when they open calcium comes in and what does calcium do it causes the vesicles to dock and release their neurotransmitters so the change in membrane potential brought about by the flow of potassium in brought about by the opening of the channels because the cilia are moving the cilia moved because the basilar membrane was deflected up and down the basal membrane was deflected up and down by the movement of the fluid which was set into motion by the vibrating of your bones which was set into motion by the vibration of your eardrum all that ends with the release of neurotransmitters from your hair cells onto these nerve cells here that are going to carry information up to your brain and then it's beyond the scope of the class what your brain does with it but now your brain has the information about about the sound and that is how the transduction of sound works uh it starts with a vibration of air vibrates this membrane vibrates these bones vibrates the oval window membrane vibrates the water vibrates the basilar membrane which wiggles the stereocilia back and forth which causes potassium to come in opens these potassium channels which causes potassium to come in because there's a crazy high amount of potassium outside which is weird that raises the voltage of the cell which opens voltage-gated calcium channels and that causes the release of neurotransmitters which activates these neurons that is it that was a summary of it that's talked about that one a lot okay now the basilar membrane has some interesting properties um near the um base of it um so if it's not unwound the base of it is here by the oval window um the membrane itself uh is much thicker uh and sort of smaller in in width than it is at what we call the apex and that would be the end of it if if you uncoil the whole thing uh the apex is is here at the very end at the apex um the basilar membrane is much wider and thinner and floppier now why do you have those things well if you play a stringed musical instrument you will know that the tighter something is uh the higher frequency it vibrates at and the lower um notes come from uh strings that are looser right and so the apex is floppier and looser and so when sounds come in they are going to vibrate at much much lower frequencies when lower frequency sounds come in so your lowest frequency sounds that you can hear 20 hertz or so are going to really sort of activate the floppier thinner apex and your high frequency sounds 20 000 10 000 hertz are going to preferentially activate the cells and the basilar membrane closer to the base this is how our nervous system detects sounds of different pitches we have what is called a tonatopic map tono means tone or pitch topic means place a tonatopic map means that along the length of the basilar membrane sounds of similar pitch activate nearby areas and nearby cells on the basilar membrane specifically higher frequency sounds activate cells and the basilar on the basilar membrane at the base medium frequency sounds activates the basilar membrane vibrates the basilar membrane and therefore the cells on it in the middle and lower frequency sounds uh activate the apex and so we can tell based on which of these sets of of receptors are active what frequency of sound that we're hearing oops you can see that right here here's just a picture of the basilar membrane un unwrapped and this is kind of the measured deflection that we see based on a low frequency sound which kind of has the highest peak kind of about here uh a medium frequency sound has its highest peak here and then a really high frequency sound vibrates sort of preferentially nearer to the base all right it's not exact it's not like they all activate the basilar membrane right to some degree in different places but higher frequencies closer to the base now what's really cool about this is that this regular organization of the basilar membrane allows us to in many cases give hearing to people who are deaf if you are deaf because your hair cells don't work but the underlying neurons are okay so if your hair cells don't work for whatever reason and there are a number of causes for that but these neurons down here work you can stimulate these neurons electrically with an electrode and so what you can do is you can put in like a bunch of electrodes on a wire like like little beads that go the whole length of the cochlea and attach them to a microphone that picks up sound and a computer processor as the sound comes in uh the computer processor turns those sounds uh into like what you see on like an old-time stereo with the bars that tell you like how much bass there is and how much right bass and how much treble is going on right it breaks these these sound waves into their frequencies and depending on whether there's lots of bass right if there's lots of bass you're going to be activating the ones electrodes at the apex if there's a lot lots of trouble you'll be activating the ones on the base and this allows you to stimulate the neurons in the cochlea which literally gives hearing to people who are deaf uh it's an amazing technology um people with these cochlear implants can understand speech they don't hear exactly what a full hearing person would hear but it is amazing that they could they can hear and have conversations uh using this technology based on what we know about the layout of the cochlear membrane all right now that's kind of how our ear works um most ears in the animal kingdom work in a similar way frogs have ears um just sort of on the side of their heads without a big sort of ear flaps or or anything but that little area behind their eye vibrates uh and then vibrates uh hair cells ultimately uh insects for example have little places where they have membranes this is a generic insect and each of these red spots is one place that scientists have discovered an ear on some insect no insect has ears all over it but these are just many of the different places that you find ear-like organs uh on insects and they're the same way they have a thin membrane that vibrates in response to sound ultimately that's gonna vibrate um hair cells uh in all animals uh and give rise to changes in neural signaling