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  • One of the things that makes the sense of touch so amazing is how it's actually multiple

  • senses wrapped into one. If I take any given object around the studio here, I can detect

  • the object's temperature and I can run my fingers over it and pick up the shape. With

  • a little more sensation, I can pick up the finer details like texture and grit, and how

  • heavy it is or whether it's movingOmg is it tacos? It's tacosThe fact that

  • I could figure that out just with my hands is kind of weird! The same tissue that senses

  • hot and cold also senses pressure, texture, and even pain signalsAnd I could detect

  • all of that totally independently from my other senses. If I had my eyes open and looked

  • lovingly at the tacos, I'd get new information about them like color but also solidify information

  • like shapeAll of this sensory information is possible because I have a nervous system,

  • one of our most intricate bodily systems that also handles motor control among other things.

  • And this system is fast, with some nervous impulses traveling up to 120 meters a second,

  • or over two hundred and sixty miles an hour. That's literally faster than a racecar.

  • The ultra fast but diverse type of signaling is thanks in part to a long, spindly cell

  • called the neuron that makes up the nervous systemSo today we'll talk about how the

  • nervous system can transmit information so quicklytaking a closer look at the neuron

  • and how scientists are getting closer to reconstructing the sense of touch in the lab. A few videos

  • ago, we spent time talking about how different messages get sent around your body, with an

  • emphasis on how cells communicate. We mentioned close range chemical messaging like paracrine

  • signaling, or the slow, long distance messaging of the endocrine system. Notice

  • that we didn't talk about how the nervous system communicates messages. That's because

  • everything about the nervous system is so complex that we're dedicating this entire

  • episode to it. The big picture idea here is that the nervous system is built to take messages

  • from point A to point B extremely quickly. Point A could mean your brain and point B

  • is your muscles, but the nerves can go the other way too. You can touch something with

  • your fingertips and send signals of temperature, pressure, or vibration back to your brain,

  • or you can step on a LEGO and send pain signals. Whichever direction your nervous system is

  • firing, it's built for speed. So it's laid out in an organized way with two main

  • componentsWe'll start with the brain and spinal cord which make up your central

  • nervous system. We're dedicating the next video in the series to the brain, but for

  • now, let's say that it wants to send a nervous signal to your arm to tell it to move. That

  • impulse travels down your brain stem and into your spinal cord which, despite what those

  • nervous system diagrams tend to show, ends at the top of your lower back, not down at

  • your pelvis. Eventually that impulse will get to the level of the anatomy it innervates,

  • or supplies with nerves. In practice, we'd say that the femoral nerve innervates the

  • quadriceps muscles. I used to think of innervation asplugging into a muscle or organ,

  • but as we'll see later, there really is no plugYou see a USB on this thing? Didn't

  • think so. Now, on the spinal cord you'll notice two little nerve roots. The one in

  • front, or the ventral root, sends down motor messages from from the brain while the dorsal

  • root in the back sends up sensory information to the brain. There's nothing special about

  • dorsal or ventral by the way, those terms just mean back and front respectively. But

  • yes, it's the same dorsal as in dorsal finThere you go, a little animal anatomy connection

  • for ya. After passing through the spinal roots, now we've entered a new part of the nervous

  • system called the peripheral nervous system. These are all the spindly neurons that branch

  • off into the complex network of nerves we're used to seeing on diagrams. There are also

  • some nerves in the peripheral nervous system that branch directly off the brain that hook

  • up to the eyes, ears, and the rest of the face called the cranial nerves. Once we get

  • to the peripheral nervous system, our nerves are heading to their destination tissue, which

  • is where we start seeing some differencesYou can have fibers that send a motor signal

  • from the brain to the muscles or glands, and you can have sensory signals coming from the

  • skin, eyes or any other organ. Those nerve roots in the peripheral nervous system

  • branch off into multiple individual neurons, the cells that transmit impulses around the

  • body. And that's important to remember, neurons are cells. These special cells are

  • often called the electrical wires of the nervous system but they're so much more than that

  • they're living, metabolizing cellsWe can see that firsthand when we zoom into the

  • cellular level. We see familiar cell structures like a nucleus with DNA, or mitochondria and

  • other organelles inside the cell bodyBut we also see plenty of specialized structures

  • that all help neurons send their message across the body like dendrites, axons, the myelin

  • sheath, and axon terminals. Let's start at the dendrites, those tentacle-looking things

  • branching off of the cell body. These dendrites collect chemical signals from other cells,

  • which eventually sends an electrical impulse  down the axon, this long branch hereAxons

  • are sometimes coated in these fatty cells that form the myelin sheath around an axon

  • which makes the nervous impulse travel along the nerve faster. Once the impulse makes it

  • to the end of the axon, it stimulates these tiny branches, the axon terminal, which releases

  • some chemicals into the synapse, the junction between it and the neuron it attaches to.

  • But as we talked about in that cell communication video, our bodies speak a language of chemicals.

  • So let's get into what that language actually is. Within the axon terminals are little containers

  • that carry neurotransmitters, chemicals that signal the neurons to do different things.

  • Neurotransmitters are made from all kinds of different chemicals and amino acids. Currently,

  • we've identified at least dozens of neurotransmittersFor instance, you might've

  • heard the neurotransmitters serotonin or dopamine, referred to as the happiness chemicals since

  • they get attention for regulating moodBoth of those neurotransmitters do other jobs too

  • though. Like dopamine can help regulate blood vessel dilation and serotonin has a role in

  • the gut plus a lot more. You've also got some like acetylcholine which are important

  • for the autonomic nervous system, or GABA, an extremely common inhibitory neurotransmitter.

  • The point is, these chemicals themselves are diverse and have lots of messages they can

  • send. And since each of these neurotransmitters is so unique, they fit into unique receptors

  • once they float across the synapseThat means that, yes, synapses take advantage of

  • paracrine signaling. Excellent connection to one of our past episodes, you get a gold

  • star. One of the other things that makes neurons so useful is the sheer diversity of tissues

  • they can interface with. The way your eyes detect wavelengths of light is different than

  • how your tongue tastes the salsa on your tacos. The point is, they all attach to neurons.

  • We even have variation within the same senseYour sense of touch alone has multiple kinds of

  • touch receptors, including different ones for touch and others for temperature. Understanding

  • how these touch receptors work can help us solve some big challenges in medical technology.

  • New research published earlier this year in Scientific Reports worked on restoring the

  • sense of touch to folks with forearm amputations with some slick roboticsWhen someone receives

  • an amputation like this, one of the consequences is a loss of sense of touch given that their

  • hand was removed. Their overall nervous system still works, but their quality of life would

  • be greatly improved with the ability to feel different things. Prosthetic hands have come

  • a long way and in the past, they've even successfully passed on sensory cues from the

  • prosthetic hand to the user's brain, which is awesome! But of course, this technology

  • still needs improvements before it can perfectly mimic the sense of touch. In particular, we

  • need it to create a more life-like connection to the peripheral nerves than we currently

  • have. So a team of researchers in Italy came up with a technique called morphological neural

  • computation that could improve prosthetic hand design. Their design uses a robotic fingertip

  • to feel sensory information, then converts that into an electrical pattern that replicates

  • what the mechanical receptors in the fingers would have felt, then relay that signal

  • to electrodes implanted in the patient's arm stump. Using this new type of stimulation,

  • patients were able to consistently detect very fine detailAfter a trip through the

  • nervous system, that information made it back to the brain. But what does the brain actually

  • do with that information once the neurons carry nervous impulses to it? Well, in the

  • next video, we'll take a look at what messages get spoken in what parts of the brain and

  • how. Thanks for watching this episode of Seeker Human, I'm Patrick Kelly.

  • I feel like there's cheese hanging out of my face.

One of the things that makes the sense of touch so amazing is how it's actually multiple

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体内に隠された電気の世界 (The Hidden Electrical World Inside Your Body)

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    林宜悉 に公開 2021 年 01 月 14 日
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