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It is now time to meet the system that helps your crazy brain stay in touch with the outside world. We follow up last week's tour of the central nervous system with a look at your peripheral nervous system, its afferent and efferent divisions, how it processes information, the reflex arc, and what your brain has to say about pain.

Table of Contents
Peripheral Nervous System 0:38
Afferent and Efferent Divisions 5:42
Information and Responses to Pain 3:12
Five Steps of the Reflex Arc 4:35
Different Kinds of Reflexes 6:44
What the Brain Says About Pain 8:09


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When it comes to the nervous system, or just your body in general, let’s face it, your brain gets all the props. And it deserves those props! It’s a complicated, and crucial, and sometimes crazy boss of an organ. But your brain would be pretty useless without a support team that kept it connected to the outside world. Because frankly, like any leader, the more isolated your brain gets, the weirder it gets. Put a person in a watery, pitch-black sensory deprivation tank, and you’ll see the brain do some really weird stuff. Without a constant flood of external information, the brain starts to confuse its own thoughts for actual experiences, leading you to hallucinate the taste of cheeseburgers, or the sound of a choir singing, or the sight of pink stampeding elephants. It’s your peripheral nervous system that keeps things real, by putting your brain in touch with the physical environment around you, and allowing it to respond. This network snakes through just about every part of your body, providing the central nervous system with information ranging from the temperature, to the touch of a hand on your shoulder, to a twisted ankle. The peripheral nervous system’s sensory nerve receptors spy on the world for the central nervous system, and each type responds to different kinds of stimuli. Thermoreceptors respond to changes in temperature. photoreceptors react to light, chemoreceptors pay attention to chemicals, and mechanoreceptors respond to pressure, touch, and vibration. And then we’ve got specialized nerve receptors called nociceptors that, unlike those other receptors, fire only to indicate pain, which is the main thing I want to talk about today. Because, as unpleasant as a stick in the eye or tack in the foot may be, pain is actually a great example of where everything we’ve talked about over the last few weeks all comes together, as we trace a pain signal through your nervous system, from the first cuss to the Hello Kitty band aid. By the end of this episode of Crash Course Anatomy & Physiology you’ll never think of a stubbed toe, pounding headache, or burned tongue the same way again. Most people go to great lengths to avoid pain, but really, it’s an incredibly useful sensation, because it helps protect us from ourselves, and from the outside world. If you’re feeling physical pain, it probably means that your body is under stress, damaged, or in danger, and your nervous system is sending a cease and desist signal to stop twisting your arm like that, or to back away from that bonfire, or please seek medical attention, like, RIGHT NOW. So in that way, pain is actually good for you -- that’s why it exists. I’m not saying it’s pleasant, but if you’ve ever wished for an X-Men-like power to be impervious to pain, I’ve gotta say, that is one foolish monkey’s paw of a wish. Just ask Ashlyn Blocker. She’s got a genetic mutation that’s given her a total insensitivity to any kind of pain. And as a result, she’s absent-mindedly dunked her hands in pots of boiling water, run around for days without noticing broken bones, and nearly chewed off her own tongue. Luckily, such congenital conditions are very rare. The rest of us have a whole nervous system dedicated to making sure our bodies react with a predictable chain of events at the first sign of damage. Like say you just wake up and you’re extraordinarily hungry for some reason, so you run downstairs to grab some clam chowder, but you didn’t put any shoes on and suddenly you’re like, “YOWW!” There’s a tack, fell out of the wall, and you stepped right on it -- of course. Your foot immediately lifts off the ground, and then you’re assuring your dog that you’re not yelling at her, you’re just yelling, and then you limp over to the couch, and sit down, and you pull up your foot, and remove that spiny devil from your flesh. You want to talk physiology? So what exactly just happened in your body? Well, the first step was a change in your environment -- that is, a stimulus that activated some of your sensory receptors. In this case, it was a change from the probably completely ignored feeling of bare skin on a smooth floor to a distinct feeling of discomfort -- the sharp metal tack piercing your skin. Your peripheral nervous system’s mechano- and nociceptors provided that base sensation, or awareness that something had changed. Then it went to your central nervous system -- first to the spinal cord that caused the immediate reflexive action of pulling up your foot, and then your brain eventually interpreted that awareness into the perception of pain, and decided to pull the tack out and probably say an expletive or two. Pain itself is a pretty subjective feeling, but the fact is, we all have the same pain threshold. That is, the point where a stimulus is intense enough to trigger action potentials in those nociceptors is the same for everybody. But, you and I might have different tolerances for discomfort. In general, most doctors think of pain as the perception of pain -- whatever any given brain says pain is. So, you’ve got the stimulating event -- foot meets tack -- and then the reception of that signal, as the nociceptors in your foot sense that stimulus, and then the transmission of that signal through your nerves to your spinal cord and eventually up to the brain. Now remember back how every neuron in your body has a membrane that keeps positive and negative charges separated across its boundaries, like a battery sitting around waiting for something to happen? Well that tack in your flesh is that something. And it snaps those nociceptors to attention. Some neurons have mechanically-gated receptors that respond to a stretch in their membranes -- in this case, that happens when the tack punches through them. Meanwhile, other neurons have ligand-gated receptors that open when the damaged skin tissue releases chemicals like histamine or potassium ions. These channels allow sodium ions to flood into the neuron, causing a graded potential, if that hits the right threshold, it activates the electrical event that sends the signal all the way up the axon and gets one neuron talking to another -- the action potential. When that action potential races down the length of its axon to the terminal, the message hits the synapse that then flings it over that synaptic gap to another neuron that’s in your spinal cord. Remember, signals travel between neurons either by electrical or chemical synapses. The electrical ones send an electrical impulse, while the chemical ones -- the ones I’m talking about now -- first convert that signal from electrical to chemical, by activating neurotransmitters to bridge the synaptic gap, before the receiving neuron converts that chemical signal back into an electrical one. In this case, news of the tack-attack is carried by specific neurotransmitters whose sole job is to pass along pain messages. Now, so far, your body’s response to the stimulus has been handled by the sensory, or afferent, division of your peripheral nervous system. This is the part that’s involved expressly in collecting data and sending it to the central nervous system. But at this point, the responsibility changes hands. The torch is passed. Because the pain signal has just triggered an action potential in a neuron in the spinal cord, which is part of the central nervous system, and there it reaches an integration center. From here, the response is taken over by the motor, or efferent division. Once the integration center interprets the signal, it transmits the message to motor neurons, which send an action potential back down your leg, where it reaches an effector. And an effector is just any structure that receives and reacts to a motor neuron’s signal, like a muscle contracting or a gland secreting a hormone. From here, the motor neurons complete the whole foot-lifting response until the rest of your nervous system gets engaged in the complicated tasks of figuring out what the problem is, and fixing it. Those are the five steps that your highly specific neural pathways go through to produce what’s known as a reflex arc. A lot of your body’s control systems boil down to reflexes just like this -- immediate reactions that can either be innate or learned, but don’t need much conscious processing in the brain. Lifting your foot when you step on a tack is an innate, or intrinsic, reflex action -- a super fast motor response to a startling stimulus. These reflexes are so invested in your self-preservation that you actually can’t think about them before you respond. All this processing happens in the spinal cord, so that the control of muscles can be initiated before the pain is actually perceived by the brain. Learned, or acquired reflexes on the other hand, come from experience. Like how you learn to dodge obstacles while riding a bike or driving a car. That process is also largely automatic, but you learn those reflexes by spending time behind the wheel, or behind the handlebars. And reflex arcs stimulate some muscles, while inhibiting others. For example, the tack in your right foot ended up activating the motor neurons in your right hip flexors and hamstring, causing that knee to bend and your foot to lift up. But it also told the quad muscles in your left leg to extend and stand tall, allowing you to shift your body’s weight off the tack. Of course not all reflexes come from pain, as you’ve probably experienced when a doctor tapped your knee and your foot kicked. Your muscles and tendons are very sensitive to being stretched too far, or too fast, because that kind of movement can cause injury. So for this we have receptors called muscle and tendon spindles that specifically sense stretching. If triggered by an over-stretch, they generate a reflex arc that contracts the muscle to keep it from stretching further. So, when does the brain actually get involved in all this? Well, when your spinal cord sent impulses down the motor neurons, it also sent signals up your spinal cord toward the brain. News of the tack arrived first at your thalamus, the information switchboard that then split the message and sent it to the somatosensory cortex -- which identifies and localizes the pain, like: “sharp, and foot”; as well as the limbic system, which registers emotional suffering -- like, “why tack? Why me?!” And it also went to the frontal cortex, which made sense of it all, assigning meaning to the pain -- like, “oh, I see this tack fell from the Crash Course poster on the wall here.” So basically, although your body has been reacting all along, it’s not until those pain signals hit the brain that you have the conscious thoughts of both “dang, that hurt,” and “oh, that hurt because I stepped on a specific pointy thing.“ And this is where I want to point out that we here at Crash Course cannot be held responsible for any injuries sustained in the process of owning a Crash Course poster. Enjoy them at your own risk. Today you got your first look at the peripheral nervous system, by learning how the afferent and efferent divisions provide information about, and responses to, pain. You learned about the five steps of the reflex arc, the different kinds of reflexes you have, and what your brain has to say about all that pain, once the news is finally broken to it. Crash Course is now on Patreon! Big thanks to all of our supporters on Patreon who make Crash Course possible for themselves and for the whole rest of the world through their monthly contributions. If you like Crash Course and you want to help us keep making great new videos like this one, you can check out This episode was written by Kathleen Yale. The script was edited by Blake de Pastino, and our consultant, is Dr. Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, and our graphics team is Thought Café.