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Current time:0:00Total duration:11:37

Video transcript

I'm gonna be totally honest with you I don't really spend a lot of time thinking about my bodily functions for the most part maybe maybe sometimes but in the next few episodes I'm gonna be talking about all of the organ systems that make our lives possible even occasionally Pleasant and to start it all off I'm going to go straight to Mission Control the nervous system pretty much every single animal except for some really simple ones have nervous systems which is great because it's what lets things do things like have behaviors it makes you the sentient living thing that you are the whole set up here you brain your nerves your spinal cord everything's made up of specialized cells that you don't find anywhere else in the body most of those are neurons which you've seen them before they look kind of like a tree with the roots in the trunk and the branches neurons bundled together to form nerves pathways that transmit electrochemical signals from one part of your body to another so when you bite into bees pizza oh all of them live proof in the videos receptor neurons in my taste buds recognize I'm eating something salty and fatty and awesome and they care that information a long nerve pathway to my brain and then my brain can be like yeah pizza and then it can respond by sending back information through different nerve pathways that say you should eat more of that pizza and you despite what my brain is telling me I'm going to try to not eat any more of that pizza you wouldn't think that it's terribly complicated to like know that Pizza tastes good and to tell someone to eat more pizza but it turns out that our brains and our nervous systems are crazy complicated your nervous system basically has a big old bureaucracy of neurons and it's divided into two main departments the central nervous system and the peripheral nervous system central and peripheral the central nervous system basically your brain in your spinal cord is responsible for analyzing and interpreting all of those data that your peripheral nervous system all of the nerves outside of your brain and spine collects and sends its way once the central nervous system makes a decision about data it sends a signal back through to the peripheral nervous system saying do this thing which the peripheral nervous system then does so these systems contain two different types of neurons afferent and efferent afferent and efferent are biological terms and they're horribly confusing and I apologize on behalf of the entire institution of biology for them afferent systems carry things to a central point and effort systems carry things away from a central point so afferent neurons carry information to the brain and spinal cord for analysis the peripheral nervous system afferent neurons are called sensory neurons they're activated by external stimuli like the complex and glorious flavor of pizza and then they convert those data into a signal for the central nervous system to process the central nervous system has afferent neurons too and there they bring information into special parts of the brain like the part of the brain that goes salty efferent neurons carry information out of the center in the peripheral nervous system they're called motor neurons because many of them carry information from the brain of the spinal cord to muscles to make us move but they also go to pretty much every other organ in your body thus making them like work and do stuff to keep you alive in the central system efferent neurons carry information from special parts of the brain to other parts of the brain or spinal cord of course if it ended there it would be way too simple and no good bureaucracy has just two departments so the peripheral nervous system is actually made up of two different systems with two very different jobs the somatic nervous system and the autonomic nervous system the somatic system controls all the stuff you think about doing like all the information coming through your senses and the movement of your body that it makes when you want it to make movements so here's something interesting since we're totally in love with our brains as sort of the center of all being of ourselves we think that all the information about everything going on in our bodies goes to our brains for some kind of decision not so sometimes like when we touch a hot stove the afferent neurons carry the signal hot to the central nervous system but that information doesn't even ever get to the brain the spinal cord actually makes that decision before it gets to the brain sends a message directly back to the muscle saying get your hand off freaking stove this bit of fancy nerve work lets the spinal cord make decisions rather than the brain and it's called the reflex loop so the other branch of the peripheral nervous system the autonomic system carries signals from the central nervous system that drive all of the things that your body does without thinking about them your heartbeat your digestion breathing saliva production all of your organ functions but we're not done yet here we need to go deeper the autonomic nervous system has two divisions of its own the sympathetic and the parasympathetic and the jobs that these two perform aren't just different they're completely opposite and frankly they're always vying for control of the body in some kind of nervous system cage match the sympathetic division is responsible for like freaking out you've probably heard this talked about as the fight-or-flight response in other words stress but stress isn't all bad it's what saves our lives when we're being chased by saber-toothed Tigers right the sympathetic system prepares our body for action by increasing the heart rate in the blood pressure enhancing our sense of smell dilating the pupils activating our adrenal cortex to make adrenaline shutting down blood supply to our digestive and reproductive system so that there will be more blood available for our lungs and our muscles when we have to like run and even though you're not in a constant state of panic at least I hope not I kind of am that system is running all the time every day but right next to it is the parasympathetic division working hard to make sure it would take it nice and easy dials down the heart rate and the blood pressure constricts our lungs makes our nose run increases blood flow to our reproductive junk our mouths produce saliva encourage us to poop and pee that's basically what we have to thank for taking a nap sitting in front of the TV going to the bathroom and getting it on so consider yourself lucky you got both the stress response and the chilled the heck out response working side by side because together they create a balance or a homeostasis now that's what the nervous system does next we have to talk about how it does it the neurons that make up our nervous systems make it possible for our bodies to have their very own little electric systems so to understand how they work you have to understand their anatomy like I've said before a typical neuron has branches like a tree these are called dendrites and they receive information from other neurons neurons also have a single axon the trunk of the tree which is branched at the end and transmit signals to other neurons the axon is also covered in a fatty material called myelin which acts as insulation but the myelin sheath isn't continuous there are these little bits of exposed neuron along the axon which have the sweetest names in this whole episode they're called the nodes of ranvier it seems like an excellent working title for the 8th Harry Potter novel Harry Potter in the nodes of ranvier anyway these nodes allow signals to hop from node node which lets the signal travel down a nerve faster this node hopping by the way has a name it's called saltatory conduction conduction because it's electrical conduction in saltatory because saltatory means leaping finally the place where an axons branches come in contact with the next cells dendrite is called a synapse and that's where neurotransmitters pass information from one neuron to the next now think back to or just go watch the episode that we did on cell membranes where we talked about how materials travel down concentration gradients well in much the same way all of the neurons in your body have a membrane potential a difference in voltage or electrical charge between the inside and the outside of the membrane you might also remember that this buildup of voltage is handled in part by sexy little protein called the sodium potassium pump basically the bump creates a voltage differential like charging a battery by moving three positively charged sodium ions out for every two potassium ions it lets in creating a net negative charge inside the cell relative to the outside when a neuron is inactive this is called its resting potential and voltage is about negative 70 millivolts but in addition to the pumps neurons also have ion channels these are proteins that straddle the membrane but they're a lot simpler and don't need ATP to power them each cell can have more than 300 different kinds of ion channels each tailored to accept a specific ion now don't zoom out here because all of this stuff has got to come into play when a neuron becomes active this happens when an input or stimulus creates a change in the neuron that eventually reaches the axon creating what's called an action potential a brief event where the electrical potential of cell rapidly rises and falls when an action potential begins like when a molecule of sugar touches one of my sweet taste buds some ion channels open and let those positive sodium ions rush in so that the inside starts to become less negative with enough stimulus the internal chart of the neuron reaches a certain threshold which triggers more sodium channels to respond and open the floodgates till that even more ions in that's happening on one tiny little area of the neuron but this change in voltage creeps over to the next bunch of sodium channels which are also sensitive to voltage and so they open that exchange triggers the next batch and the next batch and so on down the line so this signal of changing voltage travels down the neurons membrane like a wave but remember the myelin sheath insulates most of the neuron and just leave those little nodes exposed so instead of being a steady wave the wave jumps from node to node speeding up the travel time of action potential down a neuron that's your saltatory conduction at work when the way it reaches the end of the on it triggers a release of neurotransmitters from the neuron through exocytosis and those neurotransmitters then flow it across the synapse to the next neuron where they trigger another action potential over there and by this time so many sodium ions have gotten inside the first neuron that the difference between the outside and the inside is actually reversed the inside is positive and the outside is negative and it seems like neurons hate that more than pretty much anything else so it fixes itself the sodium channels close and the potassium channels open up the positive potassium ions rush down both the concentration and electrochemical gradients to get the heck out of the cell that brings the charge inside the cell back to end a negative on the inside and positive on the outside notice though that now the sodium is on the inside of the cell and the potassium is on the outside then in the opposite places of where they started so the sodium potassium pumps get back to work burn up some ATP to pump the sodium back out and the potassium back in and foof things are now back at the resting potential again so that my friends is how action potential allows neurons to communicate signals down a whole chain of neurons from the outer reaches of the peripheral nervous system all the way up the spinal cord into the brain and then back out again so let's zoom out look at the broad view here I'm gonna take a bite of this pizza all my taste buds have neurons in them each of my taste buds contains between 50 to 100 specialized taste receptor neurons chemicals from this beautiful pizza dissolve in the saliva and then stimulate the dendrites on the afferent neurons this generates bunch of action potentials that travel from the afferent neurons and my tongue all the way to my brain which is like my goodness I think that's pizza let's have another bites the brain then sends messages through the efferent nerve pathways to do all sorts of things one chew which involves constricting the muscles in my jaw over and over again to lower my head down to catch another bite which involves moving all kinds of neck muscles three swallowing which involves constricting the muscles in my throat and esophagus for opening my mouth again to receive another bite that signal is also going to my jaw and that's not even to mention what's going to go on with the digestion of this bad boy driven by the autonomic nervous system but digestion is still a couple of episodes now hopefully the we'll be more pizza
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