WEBVTT - How Do Your Nerves Tell Your Muscles to Function? 0:00:01.890 --> 0:00:07.890 Welcome to BrainStuff, a production of iHeartRadio. Hey BrainStuff, Lauren 0:00:07.910 --> 0:00:13.850 Vogelbaum here. Living bodies are really weird, in a marvelous way. 0:00:14.390 --> 0:00:19.810 We are all electrochemically driven meat. You might consider yourself 0:00:20.010 --> 0:00:23.230 to exist in your thoughts, in the electrical signals in 0:00:23.250 --> 0:00:27.410 your brain, but Of course, it's impossible to do anything 0:00:27.450 --> 0:00:30.729 without your muscles, including the muscles that keep your brain 0:00:30.850 --> 0:00:35.870 fed with blood and oxygen to make thinking possible. Absolutely 0:00:36.070 --> 0:00:39.089 everything that you conceive of with your brain must be 0:00:39.220 --> 0:00:43.559 transmitted through your nerves and then expressed as muscular motion. 0:00:44.800 --> 0:00:47.740 Consider all of the ways you have to express ideas. 0:00:48.360 --> 0:00:51.930 You have to either use the muscles of your larynx, mouth, 0:00:52.050 --> 0:00:55.290 and tongue to speak words, or use the muscles of 0:00:55.330 --> 0:00:59.350 your fingers to write words or form sign language, or 0:00:59.630 --> 0:01:04.180 use your skeletal muscles to gesticulate or dance or build something, 0:01:04.700 --> 0:01:08.220 just to name a few methods of expression. The way 0:01:08.260 --> 0:01:11.400 that your brain communicates with the muscles in your mouth 0:01:11.520 --> 0:01:18.270 or fingers or heart or groove thing is through electrochemical stimulation. Today, 0:01:18.550 --> 0:01:21.950 let's talk about that communication and how your nervous system 0:01:22.090 --> 0:01:25.470 works together with your muscles to get stuff done, from 0:01:25.560 --> 0:01:28.720 reflexive actions like pulling your hand away from a hot pan, 0:01:29.140 --> 0:01:34.060 to voluntary actions like singing or dancing, to involuntary actions 0:01:34.240 --> 0:01:38.840 like digesting food and keeping your heart going. You can 0:01:38.900 --> 0:01:42.709 think of your body as being any sufficiently complex organization 0:01:42.730 --> 0:01:47.050 that you want, like a medieval citadel, or a crime syndicate, 0:01:47.390 --> 0:01:52.310 or Starfleet. There's a headquarters where decisions are made, a 0:01:52.790 --> 0:01:56.910 central place that both collects information from the periphery and 0:01:56.990 --> 0:02:01.830 sends out orders to accomplish tasks. That HQ is the 0:02:01.890 --> 0:02:05.650 central nervous system, consisting of your brain and spinal cord, 0:02:05.890 --> 0:02:09.100 plus the nerves that connect them, and the cranial nerves 0:02:09.139 --> 0:02:12.620 that connect your head's sensory organs to your brain. The 0:02:12.660 --> 0:02:15.970 central nervous system is in constant communication with what's called 0:02:15.990 --> 0:02:19.430 the peripheral nervous system, which consists of all of the 0:02:19.530 --> 0:02:23.190 other sensory nerves and the motor nerves that stretch throughout 0:02:23.230 --> 0:02:26.940 your body. The sensory nerves gather data about the world 0:02:26.980 --> 0:02:29.600 around you and send it back to HQ, and the 0:02:29.639 --> 0:02:34.620 motor nerves relay the central nervous system's orders to your muscles. Today, 0:02:34.639 --> 0:02:38.870 we're talking mostly about those motor nerves. Now, nerve cells, 0:02:39.030 --> 0:02:43.790 or neurons, are specialized cells that gather and transmit electrochemical 0:02:43.850 --> 0:02:47.600 signals over long distances, up to several feet or a 0:02:47.620 --> 0:02:52.320 couple meters. A neuron consists of three parts, the cell 0:02:52.360 --> 0:02:57.010 body or soma, the variably long transmission bit called the axon, 0:02:57.470 --> 0:03:02.070 and the nerve endings or dendrites. A motor neuron's cell 0:03:02.110 --> 0:03:05.230 body contains the same parts as all of your other cells, 0:03:05.710 --> 0:03:09.389 like a nucleus with DNA and ribosomes to build proteins 0:03:09.610 --> 0:03:14.900 and mitochondria to make energy. The axon is a long, thin, 0:03:15.260 --> 0:03:20.480 cable-like projection that can carry nerve impulses. And the dendrites 0:03:20.580 --> 0:03:23.000 come off the far end of the motor neuron's axon. 0:03:23.500 --> 0:03:28.130 They're small, branch-like projections that can pass messages to other cells, 0:03:28.790 --> 0:03:33.040 namely to muscle cells. Okay, when you think of muscles, 0:03:33.260 --> 0:03:36.110 you might first think about the muscles that you can see, 0:03:36.410 --> 0:03:39.490 like the biceps and your arms. But there are three 0:03:39.670 --> 0:03:44.240 unique kinds of muscle in any mammal's body. Skeletal, smooth, 0:03:44.560 --> 0:03:48.340 and cardiac. Skeletal muscle is the type of muscle that 0:03:48.360 --> 0:03:51.560 we can see and feel. When you work out to 0:03:51.620 --> 0:03:56.750 increase muscle mass, skeletal muscle is what's being exercised. These 0:03:56.810 --> 0:04:00.190 muscles are controlled by a department of your peripheral nervous 0:04:00.230 --> 0:04:05.620 system called your voluntary or somatic nervous system, meaning that 0:04:05.720 --> 0:04:09.020 you think about doing some action and your somatic nerves 0:04:09.280 --> 0:04:14.160 tell those muscles to contract. By contrast, your smooth and 0:04:14.220 --> 0:04:17.800 cardiac muscles are controlled by another department of your peripheral 0:04:17.839 --> 0:04:23.370 nervous system called your involuntary or autonomic nervous system, meaning 0:04:23.450 --> 0:04:27.010 that you don't have to think about contracting them. Smooth 0:04:27.060 --> 0:04:31.100 muscles are found in the digestive system, blood vessels, bladder, airways, 0:04:31.260 --> 0:04:35.440 and uterus. Cardiac muscle is specialized muscle found only in 0:04:35.480 --> 0:04:40.789 your heart. HQ controls these muscles' movements automatically through your 0:04:41.070 --> 0:04:46.570 autonomic nerves. Side note, there's actually some argument about whether 0:04:46.630 --> 0:04:49.270 the digestive system should be lumped in with the rest 0:04:49.290 --> 0:04:51.880 of the autonomic nervous system or whether it's really its 0:04:51.940 --> 0:04:55.400 own thing, but that's a complication for a different episode. 0:04:56.670 --> 0:05:01.470 We separate out the voluntary versus autonomic departments of the 0:05:01.550 --> 0:05:05.729 peripheral nervous system not just because they communicate with different 0:05:05.770 --> 0:05:09.250 kinds of muscles, but also because they do so through 0:05:09.310 --> 0:05:14.760 different neural pathways. Voluntary muscle movements start at the top, 0:05:15.480 --> 0:05:17.900 literally at the top of your head, in part of 0:05:17.940 --> 0:05:21.659 your brain called the primary motor cortex that runs like 0:05:21.700 --> 0:05:25.080 a headband across the top of your brain. It sends 0:05:25.120 --> 0:05:28.200 signals throughout your body in pathways from your brain to 0:05:28.240 --> 0:05:31.160 your spinal cord and then from your spinal cord out 0:05:31.200 --> 0:05:35.120 to your muscles. A lot of your autonomic nervous system 0:05:35.360 --> 0:05:38.480 operates through neural pathways that start lower down in your 0:05:38.520 --> 0:05:41.880 brainstem in a part of your brain called the medulla oblongata. 0:05:42.700 --> 0:05:45.860 It sends signals in pathways from your brain to a 0:05:46.040 --> 0:05:49.700 chain of nerve cell body clusters or ganglia that lie 0:05:49.800 --> 0:05:53.299 parallel to your spinal cord and then from those ganglia, 0:05:53.730 --> 0:05:57.989 out to your organs. In both cases, the neurons involved 0:05:58.029 --> 0:06:00.870 in the first or upper half of the pathway have 0:06:01.050 --> 0:06:05.200 pretty short axons. But the neurons that actually connect to 0:06:05.240 --> 0:06:09.279 your skeletal muscles or organ muscles have very long axons, 0:06:09.839 --> 0:06:12.679 so long that they come covered with a thin layer 0:06:12.740 --> 0:06:17.409 of myelin, like an insulated electrical wire. Myelin is made 0:06:17.450 --> 0:06:20.030 of fat, and it helps to speed the transmission of 0:06:20.070 --> 0:06:24.700 a nerve impulse down a long axon. Peripheral nerves axons 0:06:24.779 --> 0:06:27.850 get bundled into groups based on where they're coming from 0:06:27.900 --> 0:06:31.950 and going to. These bundles also contain tiny blood vessels 0:06:32.029 --> 0:06:35.750 to supply oxygen and remove waste, and are protected by 0:06:35.830 --> 0:06:41.390 various membranes. Each peripheral motor neuron may have many terminals 0:06:41.510 --> 0:06:44.390 at its far end, allowing it to connect to many 0:06:44.450 --> 0:06:49.570 muscle fibers. Big movements, like, say, moving your leg to walk, 0:06:50.200 --> 0:06:54.419 might involve one neuron connecting to a few thousand muscle fibers. 0:06:55.140 --> 0:06:58.340 But the more precise a movement needs to be, like 0:06:58.440 --> 0:07:01.430 moving your thumb to pick up a pencil, the fewer 0:07:01.490 --> 0:07:06.150 muscle fibers each neuron involved will connect to. But okay, 0:07:06.450 --> 0:07:10.370 by connect, I don't mean that your neurons actually physically 0:07:10.510 --> 0:07:13.770 connect to your muscle cells. It is not like how 0:07:13.850 --> 0:07:16.830 electricity can flow from a power plant to your home 0:07:16.870 --> 0:07:19.360 and then from your home to a lamp and make 0:07:19.390 --> 0:07:23.180 it light up. Or rather, your neurons communicate with your 0:07:23.220 --> 0:07:28.560 muscles chemically, across tiny gaps called neuromuscular junctions, or synapses. 0:07:29.960 --> 0:07:32.119 We talked a bit about this recently in our episode 0:07:32.140 --> 0:07:36.750 about myasthenia gravis. It's really interesting because it allows our 0:07:36.810 --> 0:07:40.650 nervous system to have very fine control over our muscles. 0:07:41.730 --> 0:07:44.390 Of course, our muscles can be triggered to contract by 0:07:44.450 --> 0:07:49.910 direct electrical stimulation— for example, internally through a pacemaker or 0:07:50.210 --> 0:07:55.430 externally through a TENS machine. But your body's electrochemical method 0:07:55.610 --> 0:07:59.739 is more directable. Let's say you want to wave your 0:07:59.770 --> 0:08:02.960 hand at a friend you see across the way. You 0:08:03.100 --> 0:08:06.080 think about waving, and your brain sends a series of 0:08:06.160 --> 0:08:09.520 electrical signals from the motor cortex to the spinal cord 0:08:09.820 --> 0:08:11.820 and then out to the muscles in your arm and 0:08:11.930 --> 0:08:15.890 hand via motor neurons. The dendrites at the end of 0:08:15.910 --> 0:08:19.410 the motor neurons are also basically looking at a friend 0:08:19.430 --> 0:08:23.830 across the way, a muscle fiber. So they communicate across 0:08:23.870 --> 0:08:29.430 the gap via chemicals called neurotransmitters. In vertebrates, like humans, 0:08:29.730 --> 0:08:34.089 the main neurotransmitter involved in muscle stimulation is called acetylcholine. 0:08:35.230 --> 0:08:39.689 So the motor neuron releases molecules of acetylcholine into the junction. 0:08:40.590 --> 0:08:43.429 they'll be caught by receptors on the surface of the 0:08:43.470 --> 0:08:47.170 muscle fibers, which can then recognize that a request for 0:08:47.230 --> 0:08:51.490 movement has been made and can trigger muscle contraction. The 0:08:51.550 --> 0:08:54.990 exact electrochemistry that's happening when a muscle contracts is more 0:08:55.030 --> 0:08:58.589 than we need to get into today. Anyway, after a 0:08:58.650 --> 0:09:02.970 molecule of acetylcholine helps trigger a muscle contraction, it doesn't 0:09:02.990 --> 0:09:06.219 stick around for long. It binds only briefly with a 0:09:06.250 --> 0:09:10.589 receptor and then releases. After that, it's quickly broken down 0:09:10.670 --> 0:09:14.360 by the enzyme acetylcholine esterase, which is plentiful in the 0:09:14.390 --> 0:09:19.319 fluid around neuromuscular junctions. From there, the component molecules will 0:09:19.360 --> 0:09:22.120 be recycled by your body, and some will be used 0:09:22.179 --> 0:09:27.020 to make new acetylcholine later. In the meanwhile, if everything 0:09:27.160 --> 0:09:30.500 in your nervous system and muscles has gone right, based 0:09:30.640 --> 0:09:34.430 on exactly which muscle fibers received the request and how 0:09:34.530 --> 0:09:38.050 strongly and for how long, your muscles will contract and 0:09:38.090 --> 0:09:43.089 relax in the correct pattern, and you'll wave. You'll also 0:09:43.200 --> 0:09:46.640 form the movement based on sensory feedback from other neurons 0:09:46.740 --> 0:09:50.939 that your central nervous system receives and processes about how 0:09:51.000 --> 0:09:53.660 your hand is moving. You know, like, are you doing 0:09:53.710 --> 0:09:56.470 a friendly wave or more like a weird jazz hand 0:09:56.510 --> 0:09:59.270 that you should pull back on? And has your friend 0:09:59.309 --> 0:10:02.000 seen you so you can stop or should you keep going? 0:10:03.179 --> 0:10:06.160 The same basic process happens in order to make both 0:10:06.220 --> 0:10:12.100 voluntary and involuntary movements. But there are also separate and 0:10:12.200 --> 0:10:17.420 simpler neural pathways for reflexive movements. Take, for example, the 0:10:17.520 --> 0:10:20.819 knee-jerk reflex. your brain doesn't have to get involved there 0:10:20.840 --> 0:10:23.660 at all. A tap at the right spot on your 0:10:23.700 --> 0:10:26.300 knee will send a signal through a sensory neuron to 0:10:26.340 --> 0:10:29.210 your spinal cord where it's routed to the motor neuron 0:10:29.290 --> 0:10:32.350 that will then signal the appropriate leg muscle to contract. 0:10:33.390 --> 0:10:35.990 The response is a quick jerk that you don't even 0:10:36.010 --> 0:10:40.230 have to think about. But okay, whether the result of 0:10:40.250 --> 0:10:44.170 neuromuscular communication is a hand wave or a jerk of 0:10:44.220 --> 0:10:47.910 the knee or your heart beating or that's all assuming 0:10:48.030 --> 0:10:52.980 that everything has gone right. Because this is a complicated process, 0:10:53.280 --> 0:10:55.679 and there are lots of things that can go wrong. 0:10:56.740 --> 0:11:00.580 For example, some types of injuries can crush, pinch, or 0:11:00.780 --> 0:11:04.520 even sever neurons in your limbs or spine. This can 0:11:04.570 --> 0:11:08.390 result in pain, numbness, complete loss of feeling, or loss 0:11:08.429 --> 0:11:12.090 of movement, depending on the severity and location of the damage. 0:11:13.170 --> 0:11:17.849 These sorts of injuries can include broken bones, but also, say, sciatica, 0:11:18.210 --> 0:11:21.800 in which strained or damaged tissue compresses the large nerve 0:11:21.820 --> 0:11:25.400 that runs through the hips into the leg. Or a 0:11:25.620 --> 0:11:28.979 carpal tunnel syndrome, in which repeated motions of the wrist 0:11:29.160 --> 0:11:31.880 cause swelling where the nerves pass through the wrist and 0:11:31.940 --> 0:11:37.800 into the fingers. And those are fairly straightforward issues. The 0:11:37.850 --> 0:11:41.929 condition multiple sclerosis can also cause numbness or weakness because 0:11:41.950 --> 0:11:45.570 the In it, your immune system attacks the myelin coating 0:11:45.670 --> 0:11:49.160 on your axons, thus degrading the conduction of nerve signals 0:11:49.320 --> 0:11:53.860 to and from your central nervous system. The aforementioned myasthenia 0:11:53.900 --> 0:11:57.340 gravis is another immune condition, in which your immune system 0:11:57.400 --> 0:12:03.160 attacks the acetylcholine receptors on your muscle fibers. Some neurotoxins 0:12:03.220 --> 0:12:06.209 that can cause our muscles to stop working, like botulism 0:12:06.230 --> 0:12:09.730 toxin and black widow venom, do that by preventing your 0:12:09.770 --> 0:12:15.250 neurons from releasing acetylcholine. You'll probably know if you get 0:12:15.340 --> 0:12:18.540 botulism or a black widow bite, and I probably don't 0:12:18.559 --> 0:12:20.860 have to tell you to seek medical care if you do. 0:12:21.580 --> 0:12:25.560 Same thing goes for broken bones. But more broadly, if 0:12:25.620 --> 0:12:29.240 you notice either numbness or pain that you can't explain, 0:12:29.940 --> 0:12:32.640 or if you find yourself or a loved one experiencing 0:12:32.720 --> 0:12:35.319 a loss of motor control, talk to a health care 0:12:35.340 --> 0:12:39.420 provider about it. Our nervous system is complicated, so it 0:12:39.500 --> 0:12:41.770 might take some doing to figure out what's going on. 0:12:42.550 --> 0:12:46.590 But we're learning about our marvelously weird bodies all the time, 0:12:47.190 --> 0:12:50.449 which means that there are more and better treatment options 0:12:50.630 --> 0:12:58.340 all the time, too. Today's episode was written by me, 0:12:58.530 --> 0:13:01.520 with additional material from the articles How Muscles Work and 0:13:01.580 --> 0:13:06.260 How Nerves Work, both by Craig Freudenreich on HowStuffWorks.com. BrainStuff 0:13:06.300 --> 0:13:09.620 is a production of iHeart Podcasts in partnership with HowStuffWorks.com 0:13:09.820 --> 0:13:12.860 and is produced by Tyler Klang. For more shows from 0:13:12.920 --> 0:13:16.710 iHeart Podcasts, visit the iHeart Radio app, Apple Podcasts, or 0:13:16.870 --> 0:13:18.580 wherever you listen to your favorite shows.