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Speaker 1: Welcome to BrainStuff, a production of iHeartRadio. Hey BrainStuff, Lauren Vogelbaum here. Living bodies are really weird, in a marvelous way. We are all electrochemically driven meat. You might consider yourself to exist in your thoughts, in the electrical signals in your brain, but Of course, it's impossible to do anything without your muscles, including the muscles that keep your brain fed with blood and oxygen to make thinking possible. Absolutely everything that you conceive of with your brain must be transmitted through your nerves and then expressed as muscular motion. Consider all of the ways you have to express ideas. You have to either use the muscles of your larynx, mouth, and tongue to speak words, or use the muscles of your fingers to write words or form sign language, or use your skeletal muscles to gesticulate or dance or build something, just to name a few methods of expression. The way that your brain communicates with the muscles in your mouth or fingers or heart or groove thing is through electrochemical stimulation. Today, let's talk about that communication and how your nervous system works together with your muscles to get stuff done, from reflexive actions like pulling your hand away from a hot pan, to voluntary actions like singing or dancing, to involuntary actions like digesting food and keeping your heart going. You can think of your body as being any sufficiently complex organization that you want, like a medieval citadel, or a crime syndicate, or Starfleet. There's a headquarters where decisions are made, a central place that both collects information from the periphery and sends out orders to accomplish tasks. That HQ is the central nervous system, consisting of your brain and spinal cord, plus the nerves that connect them, and the cranial nerves that connect your head's sensory organs to your brain. The central nervous system is in constant communication with what's called the peripheral nervous system, which consists of all of the other sensory nerves and the motor nerves that stretch throughout your body. The sensory nerves gather data about the world around you and send it back to HQ, and the motor nerves relay the central nervous system's orders to your muscles. Today, we're talking mostly about those motor nerves. Now, nerve cells, or neurons, are specialized cells that gather and transmit electrochemical signals over long distances, up to several feet or a couple meters. A neuron consists of three parts, the cell body or soma, the variably long transmission bit called the axon, and the nerve endings or dendrites. A motor neuron's cell body contains the same parts as all of your other cells, like a nucleus with DNA and ribosomes to build proteins and mitochondria to make energy. The axon is a long, thin, cable-like projection that can carry nerve impulses. And the dendrites come off the far end of the motor neuron's axon. They're small, branch-like projections that can pass messages to other cells, namely to muscle cells. Okay, when you think of muscles, you might first think about the muscles that you can see, like the biceps and your arms. But there are three unique kinds of muscle in any mammal's body. Skeletal, smooth, and cardiac. Skeletal muscle is the type of muscle that we can see and feel. When you work out to increase muscle mass, skeletal muscle is what's being exercised. These muscles are controlled by a department of your peripheral nervous system called your voluntary or somatic nervous system, meaning that you think about doing some action and your somatic nerves tell those muscles to contract. By contrast, your smooth and cardiac muscles are controlled by another department of your peripheral nervous system called your involuntary or autonomic nervous system, meaning that you don't have to think about contracting them. Smooth muscles are found in the digestive system, blood vessels, bladder, airways, and uterus. Cardiac muscle is specialized muscle found only in your heart. HQ controls these muscles' movements automatically through your autonomic nerves. Side note, there's actually some argument about whether the digestive system should be lumped in with the rest of the autonomic nervous system or whether it's really its own thing, but that's a complication for a different episode. We separate out the voluntary versus autonomic departments of the peripheral nervous system not just because they communicate with different kinds of muscles, but also because they do so through different neural pathways. Voluntary muscle movements start at the top, literally at the top of your head, in part of your brain called the primary motor cortex that runs like a headband across the top of your brain. It sends signals throughout your body in pathways from your brain to your spinal cord and then from your spinal cord out to your muscles. A lot of your autonomic nervous system operates through neural pathways that start lower down in your brainstem in a part of your brain called the medulla oblongata. It sends signals in pathways from your brain to a chain of nerve cell body clusters or ganglia that lie parallel to your spinal cord and then from those ganglia, out to your organs. In both cases, the neurons involved in the first or upper half of the pathway have pretty short axons. But the neurons that actually connect to your skeletal muscles or organ muscles have very long axons, so long that they come covered with a thin layer of myelin, like an insulated electrical wire. Myelin is made of fat, and it helps to speed the transmission of a nerve impulse down a long axon. Peripheral nerves axons get bundled into groups based on where they're coming from and going to. These bundles also contain tiny blood vessels to supply oxygen and remove waste, and are protected by various membranes. Each peripheral motor neuron may have many terminals at its far end, allowing it to connect to many muscle fibers. Big movements, like, say, moving your leg to walk, might involve one neuron connecting to a few thousand muscle fibers. But the more precise a movement needs to be, like moving your thumb to pick up a pencil, the fewer muscle fibers each neuron involved will connect to. But okay, by connect, I don't mean that your neurons actually physically connect to your muscle cells. It is not like how electricity can flow from a power plant to your home and then from your home to a lamp and make it light up. Or rather, your neurons communicate with your muscles chemically, across tiny gaps called neuromuscular junctions, or synapses. We talked a bit about this recently in our episode about myasthenia gravis. It's really interesting because it allows our nervous system to have very fine control over our muscles. Of course, our muscles can be triggered to contract by direct electrical stimulation— for example, internally through a pacemaker or externally through a TENS machine. But your body's electrochemical method is more directable. Let's say you want to wave your hand at a friend you see across the way. You think about waving, and your brain sends a series of electrical signals from the motor cortex to the spinal cord and then out to the muscles in your arm and hand via motor neurons. The dendrites at the end of the motor neurons are also basically looking at a friend across the way, a muscle fiber. So they communicate across the gap via chemicals called neurotransmitters. In vertebrates, like humans, the main neurotransmitter involved in muscle stimulation is called acetylcholine. So the motor neuron releases molecules of acetylcholine into the junction. they'll be caught by receptors on the surface of the muscle fibers, which can then recognize that a request for movement has been made and can trigger muscle contraction. The exact electrochemistry that's happening when a muscle contracts is more than we need to get into today. Anyway, after a molecule of acetylcholine helps trigger a muscle contraction, it doesn't stick around for long. It binds only briefly with a receptor and then releases. After that, it's quickly broken down by the enzyme acetylcholine esterase, which is plentiful in the fluid around neuromuscular junctions. From there, the component molecules will be recycled by your body, and some will be used to make new acetylcholine later. In the meanwhile, if everything in your nervous system and muscles has gone right, based on exactly which muscle fibers received the request and how strongly and for how long, your muscles will contract and relax in the correct pattern, and you'll wave. You'll also form the movement based on sensory feedback from other neurons that your central nervous system receives and processes about how your hand is moving. You know, like, are you doing a friendly wave or more like a weird jazz hand that you should pull back on? And has your friend seen you so you can stop or should you keep going? The same basic process happens in order to make both voluntary and involuntary movements. But there are also separate and simpler neural pathways for reflexive movements. Take, for example, the knee-jerk reflex. your brain doesn't have to get involved there at all. A tap at the right spot on your knee will send a signal through a sensory neuron to your spinal cord where it's routed to the motor neuron that will then signal the appropriate leg muscle to contract. The response is a quick jerk that you don't even have to think about. But okay, whether the result of neuromuscular communication is a hand wave or a jerk of the knee or your heart beating or that's all assuming that everything has gone right. Because this is a complicated process, and there are lots of things that can go wrong. For example, some types of injuries can crush, pinch, or even sever neurons in your limbs or spine. This can result in pain, numbness, complete loss of feeling, or loss of movement, depending on the severity and location of the damage. These sorts of injuries can include broken bones, but also, say, sciatica, in which strained or damaged tissue compresses the large nerve that runs through the hips into the leg. Or a carpal tunnel syndrome, in which repeated motions of the wrist cause swelling where the nerves pass through the wrist and into the fingers. And those are fairly straightforward issues. The condition multiple sclerosis can also cause numbness or weakness because the In it, your immune system attacks the myelin coating on your axons, thus degrading the conduction of nerve signals to and from your central nervous system. The aforementioned myasthenia gravis is another immune condition, in which your immune system attacks the acetylcholine receptors on your muscle fibers. Some neurotoxins that can cause our muscles to stop working, like botulism toxin and black widow venom, do that by preventing your neurons from releasing acetylcholine. You'll probably know if you get botulism or a black widow bite, and I probably don't have to tell you to seek medical care if you do. Same thing goes for broken bones. But more broadly, if you notice either numbness or pain that you can't explain, or if you find yourself or a loved one experiencing a loss of motor control, talk to a health care provider about it. Our nervous system is complicated, so it might take some doing to figure out what's going on. But we're learning about our marvelously weird bodies all the time, which means that there are more and better treatment options all the time, too. Today's episode was written by me, with additional material from the articles How Muscles Work and How Nerves Work, both by Craig Freudenreich on HowStuffWorks.com. BrainStuff is a production of iHeart Podcasts in partnership with HowStuffWorks.com and is produced by Tyler Klang. For more shows from iHeart Podcasts, visit the iHeart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.
Speaker 1: Welcome to BrainStuff, a production of iHeartRadio. Hey BrainStuff, Lauren Vogelbaum here. Living bodies are really weird, in a marvelous way. We are all electrochemically driven meat. You might consider yourself to exist in your thoughts, in the electrical signals in your brain, but Of course, it's impossible to do anything without your muscles, including the muscles that keep your brain fed with blood and oxygen to make thinking possible. Absolutely everything that you conceive of with your brain must be transmitted through your nerves and then expressed as muscular motion. Consider all of the ways you have to express ideas. You have to either use the muscles of your larynx, mouth, and tongue to speak words, or use the muscles of your fingers to write words or form sign language, or use your skeletal muscles to gesticulate or dance or build something, just to name a few methods of expression. The way that your brain communicates with the muscles in your mouth or fingers or heart or groove thing is through electrochemical stimulation. Today, let's talk about that communication and how your nervous system works together with your muscles to get stuff done, from reflexive actions like pulling your hand away from a hot pan, to voluntary actions like singing or dancing, to involuntary actions like digesting food and keeping your heart going. You can think of your body as being any sufficiently complex organization that you want, like a medieval citadel, or a crime syndicate, or Starfleet. There's a headquarters where decisions are made, a central place that both collects information from the periphery and sends out orders to accomplish tasks. That HQ is the central nervous system, consisting of your brain and spinal cord, plus the nerves that connect them, and the cranial nerves that connect your head's sensory organs to your brain. The central nervous system is in constant communication with what's called the peripheral nervous system, which consists of all of the other sensory nerves and the motor nerves that stretch throughout your body. The sensory nerves gather data about the world around you and send it back to HQ, and the motor nerves relay the central nervous system's orders to your muscles. Today, we're talking mostly about those motor nerves. Now, nerve cells, or neurons, are specialized cells that gather and transmit electrochemical signals over long distances, up to several feet or a couple meters. A neuron consists of three parts, the cell body or soma, the variably long transmission bit called the axon, and the nerve endings or dendrites. A motor neuron's cell body contains the same parts as all of your other cells, like a nucleus with DNA and ribosomes to build proteins and mitochondria to make energy. The axon is a long, thin, cable-like projection that can carry nerve impulses. And the dendrites come off the far end of the motor neuron's axon. They're small, branch-like projections that can pass messages to other cells, namely to muscle cells. Okay, when you think of muscles, you might first think about the muscles that you can see, like the biceps and your arms. But there are three unique kinds of muscle in any mammal's body. Skeletal, smooth, and cardiac. Skeletal muscle is the type of muscle that we can see and feel. When you work out to increase muscle mass, skeletal muscle is what's being exercised. These muscles are controlled by a department of your peripheral nervous system called your voluntary or somatic nervous system, meaning that you think about doing some action and your somatic nerves tell those muscles to contract. By contrast, your smooth and cardiac muscles are controlled by another department of your peripheral nervous system called your involuntary or autonomic nervous system, meaning that you don't have to think about contracting them. Smooth muscles are found in the digestive system, blood vessels, bladder, airways, and uterus. Cardiac muscle is specialized muscle found only in your heart. HQ controls these muscles' movements automatically through your autonomic nerves. Side note, there's actually some argument about whether the digestive system should be lumped in with the rest of the autonomic nervous system or whether it's really its own thing, but that's a complication for a different episode. We separate out the voluntary versus autonomic departments of the peripheral nervous system not just because they communicate with different kinds of muscles, but also because they do so through different neural pathways. Voluntary muscle movements start at the top, literally at the top of your head, in part of your brain called the primary motor cortex that runs like a headband across the top of your brain. It sends signals throughout your body in pathways from your brain to your spinal cord and then from your spinal cord out to your muscles. A lot of your autonomic nervous system operates through neural pathways that start lower down in your brainstem in a part of your brain called the medulla oblongata. It sends signals in pathways from your brain to a chain of nerve cell body clusters or ganglia that lie parallel to your spinal cord and then from those ganglia, out to your organs. In both cases, the neurons involved in the first or upper half of the pathway have pretty short axons. But the neurons that actually connect to your skeletal muscles or organ muscles have very long axons, so long that they come covered with a thin layer of myelin, like an insulated electrical wire. Myelin is made of fat, and it helps to speed the transmission of a nerve impulse down a long axon. Peripheral nerves axons get bundled into groups based on where they're coming from and going to. These bundles also contain tiny blood vessels to supply oxygen and remove waste, and are protected by various membranes. Each peripheral motor neuron may have many terminals at its far end, allowing it to connect to many muscle fibers. Big movements, like, say, moving your leg to walk, might involve one neuron connecting to a few thousand muscle fibers. But the more precise a movement needs to be, like moving your thumb to pick up a pencil, the fewer muscle fibers each neuron involved will connect to. But okay, by connect, I don't mean that your neurons actually physically connect to your muscle cells. It is not like how electricity can flow from a power plant to your home and then from your home to a lamp and make it light up. Or rather, your neurons communicate with your muscles chemically, across tiny gaps called neuromuscular junctions, or synapses. We talked a bit about this recently in our episode about myasthenia gravis. It's really interesting because it allows our nervous system to have very fine control over our muscles. Of course, our muscles can be triggered to contract by direct electrical stimulation— for example, internally through a pacemaker or externally through a TENS machine. But your body's electrochemical method is more directable. Let's say you want to wave your hand at a friend you see across the way. You think about waving, and your brain sends a series of electrical signals from the motor cortex to the spinal cord and then out to the muscles in your arm and hand via motor neurons. The dendrites at the end of the motor neurons are also basically looking at a friend across the way, a muscle fiber. So they communicate across the gap via chemicals called neurotransmitters. In vertebrates, like humans, the main neurotransmitter involved in muscle stimulation is called acetylcholine. So the motor neuron releases molecules of acetylcholine into the junction. they'll be caught by receptors on the surface of the muscle fibers, which can then recognize that a request for movement has been made and can trigger muscle contraction. The exact electrochemistry that's happening when a muscle contracts is more than we need to get into today. Anyway, after a molecule of acetylcholine helps trigger a muscle contraction, it doesn't stick around for long. It binds only briefly with a receptor and then releases. After that, it's quickly broken down by the enzyme acetylcholine esterase, which is plentiful in the fluid around neuromuscular junctions. From there, the component molecules will be recycled by your body, and some will be used to make new acetylcholine later. In the meanwhile, if everything in your nervous system and muscles has gone right, based on exactly which muscle fibers received the request and how strongly and for how long, your muscles will contract and relax in the correct pattern, and you'll wave. You'll also form the movement based on sensory feedback from other neurons that your central nervous system receives and processes about how your hand is moving. You know, like, are you doing a friendly wave or more like a weird jazz hand that you should pull back on? And has your friend seen you so you can stop or should you keep going? The same basic process happens in order to make both voluntary and involuntary movements. But there are also separate and simpler neural pathways for reflexive movements. Take, for example, the knee-jerk reflex. your brain doesn't have to get involved there at all. A tap at the right spot on your knee will send a signal through a sensory neuron to your spinal cord where it's routed to the motor neuron that will then signal the appropriate leg muscle to contract. The response is a quick jerk that you don't even have to think about. But okay, whether the result of neuromuscular communication is a hand wave or a jerk of the knee or your heart beating or that's all assuming that everything has gone right. Because this is a complicated process, and there are lots of things that can go wrong. For example, some types of injuries can crush, pinch, or even sever neurons in your limbs or spine. This can result in pain, numbness, complete loss of feeling, or loss of movement, depending on the severity and location of the damage. These sorts of injuries can include broken bones, but also, say, sciatica, in which strained or damaged tissue compresses the large nerve that runs through the hips into the leg. Or a carpal tunnel syndrome, in which repeated motions of the wrist cause swelling where the nerves pass through the wrist and into the fingers. And those are fairly straightforward issues. The condition multiple sclerosis can also cause numbness or weakness because the In it, your immune system attacks the myelin coating on your axons, thus degrading the conduction of nerve signals to and from your central nervous system. The aforementioned myasthenia gravis is another immune condition, in which your immune system attacks the acetylcholine receptors on your muscle fibers. Some neurotoxins that can cause our muscles to stop working, like botulism toxin and black widow venom, do that by preventing your neurons from releasing acetylcholine. You'll probably know if you get botulism or a black widow bite, and I probably don't have to tell you to seek medical care if you do. Same thing goes for broken bones. But more broadly, if you notice either numbness or pain that you can't explain, or if you find yourself or a loved one experiencing a loss of motor control, talk to a health care provider about it. Our nervous system is complicated, so it might take some doing to figure out what's going on. But we're learning about our marvelously weird bodies all the time, which means that there are more and better treatment options all the time, too. Today's episode was written by me, with additional material from the articles How Muscles Work and How Nerves Work, both by Craig Freudenreich on HowStuffWorks.com. BrainStuff is a production of iHeart Podcasts in partnership with HowStuffWorks.com and is produced by Tyler Klang. For more shows from iHeart Podcasts, visit the iHeart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.
