Neurologist and Assistant Professor Dr. Natalie Cheng joins WIRED to answer your questions about neurology from Twitter. Why do our limbs sometimes "fall asleep?" Why do we experience headaches? Or itchiness? Is the sympathetic versus parasympathetic nerve system concept real?
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00:00 I'm Dr. Natalie Chang, neurologist and assistant professor of clinical neurology.
00:04 I'm here today to answer your questions from the internet.
00:06 This is Nerve Support.
00:12 @emilyflower asks, "If your brain doesn't have any sensory nerves, that's why you can
00:16 be awake during a brain surgery, then what the heck are headaches?"
00:20 It's true that the brain does not have any sensory nerves.
00:22 It has no pain receptors.
00:24 Many other structures in the head and neck do.
00:26 So the meninges, which is the covering of the brain, as well as the blood vessels in
00:31 the head and the muscles in the head and neck all have pain fibers.
00:35 These can all be affected when you have a headache.
00:38 @kellychakour asks, "Why do they call it a funny bone?
00:41 Ain't nothing funny about hitting that."
00:43 The funny bone sensation is when you actually hit the ulnar nerve, which is pretty superficial
00:49 right here as it comes around the medial epicondyle of your upper arm bone.
00:54 Most people think that it's called the funny bone because it feels funny when you hit it,
00:57 but it's actually a really bad dad joke from the 19th century.
01:01 This bone is called the humerus, and some people thought back then that instead of calling
01:06 it the humerus, they would call it the funny bone.
01:08 @coconoir asks, "Why do our limbs fall asleep?"
01:11 The sensation of our limbs falling asleep is called paresthesias, that pins and needles
01:16 sensation when we've compressed a peripheral nerve for too long.
01:20 And what happens isn't really that the blood flow has stopped to that limb.
01:23 It's more that that nerve can't send those electrical signals back to the brain to tell
01:28 them that there's feeling in that limb.
01:31 When you stop that compression of that nerve, like when you wake up and you feel that pins
01:35 and needles, it's really the electrical signals kind of going crazy because they're like,
01:40 "Oh, finally, we're no longer being compressed."
01:42 @erlinhenriquez asks, "So how does general anesthesia work?"
01:47 General anesthesia is actually one of the great modern marvels of medicine and really
01:51 laid the foundation for surgery as we know it today.
01:54 Before general anesthesia, if you had to have, let's say, a limb amputated, you were awake,
01:59 you could see everything, and there was really not much pain control.
02:03 Now you would undergo general anesthesia, which is made up of several components.
02:07 So number one is being unconscious.
02:10 Number two is not remembering anything.
02:12 Three is pain control.
02:14 Four is maintaining your body's functions while you're out.
02:17 And then lastly, we want to make sure that you have muscle relaxation and paralysis.
02:22 You can imagine all these things are important because if a surgeon is operating on you,
02:27 you don't want a moving patient.
02:29 You also don't want a muscle to be really rigid, and you really don't want to smell
02:33 your burning flesh.
02:34 @kellyhogaboom asks, "Is the sympathetic versus parasympathetic nerve system concept real?
02:40 And also, if so, what is the difference?
02:42 In terms I can understand.
02:43 For reference, I am like a wily but well-meaning caveman."
02:46 Well, well-meaning caveman.
02:48 Imagine a saber-toothed tiger is running after you, and you need to respond.
02:52 The sympathetic nervous system is part of the autonomic nervous system, and it helps
02:58 mediate your automatic functions like breathing, heart rate, sweating, and digestion.
03:03 If the saber-toothed tiger is chasing you, you want your heart rate to go up so that
03:07 blood can get to all the organs in your body.
03:09 You want the arteries that supply your muscles to dilate so that you can run away from the
03:14 saber-toothed tiger.
03:16 And you really don't want to send blood to your gastrointestinal system because now is
03:19 not the time to digest that food that you just ate.
03:22 In contrast, the parasympathetic nervous system is more our rest and digest or feed and breed
03:27 functioning.
03:28 And this is where digestion becomes a priority as well as reproduction.
03:33 @dunkelugless asks, "Is the nervous system what causes anxiety?
03:37 Because my system is freaking nervous."
03:40 Technically, yes.
03:41 The nervous system does promote the actual feeling of anxiety.
03:45 So fast heart rate, trouble breathing, kind of queasiness in our gastrointestinal system.
03:50 This is the fight or flight response being activated.
03:53 Now when people have anxiety, this response is on all the time, which can be pretty taxing
03:58 for our minds, our bodies, as well as our emotions.
04:02 @mohosaofficial asks, "About how fast do nerve impulses travel?
04:07 Impulsive potentials can travel as quickly as 100 meters per second, which is about 224
04:12 miles per hour for us non-metric system users.
04:15 They can travel more quickly if the nerve is covered in myelin.
04:18 Myelin acts like insulation, just like on an electrical wire that helps the impulse
04:23 travel more quickly.
04:24 Instead of having the electrical signal travel continuously down, it can jump in between
04:29 these fatty segments."
04:30 @filmcarstairs asks, "I want to know how I can move my body.
04:34 Like how can my brain send signals to my toes and tell them to move?
04:38 I don't get it."
04:39 The way your body moves is through a system called the corticospinal tract, and this is
04:44 a big part of your motor systems.
04:46 Let's say you want to move your toes.
04:48 Our brain actually has a map of the body in the motor cortex, and there is a toe center
04:54 actually here in the middle that will then send a signal to the spinal cord.
05:00 It sends fibers to your spinal cord on the opposite side of the body.
05:04 It crosses in the medulla, which is at the base of your brain stem.
05:08 @saphrodite asks, "The nervous system is so weird.
05:11 Like what is itching?"
05:13 Itching is pretty annoying, but it's a pretty smart defensive mechanism.
05:17 So let's say you have a mosquito who's crawling on your arm.
05:20 You definitely want to know about that so that you can hopefully flick it away and it
05:24 doesn't bite you.
05:25 This is sort of like a pain response, but it kind of tickles, but that's what itching's
05:31 purpose is.
05:32 @jenna_t asks, "Why can we be tickled?
05:35 Like what response is that?"
05:37 Tickling is a type of itch almost, and we found that when people have done functional
05:42 neuroimaging studies of the brain while being tickled, that two areas light up.
05:47 So the somatosensory cortex lights up when you're being tickled, so that's our touch
05:51 processing center, but also the anterior cingulate cortex, which is right here, and this is our
05:58 emotional center.
05:59 So there definitely is a touch, but also emotional component to being tickled.
06:05 In terms of why we laugh when we're tickled, some think it's a form of social bonding,
06:10 like between a baby and its parent.
06:12 Others think it's more of a sign of submission to your tickler that you're done being tickled
06:17 and they've won.
06:19 @melodyb123 asks, "The difference between central nervous system and peripheral nervous
06:24 system is confusing me, and I think it's meant to be straightforward.
06:28 The central nervous system includes your brain and spinal cord.
06:31 The peripheral nervous system is basically anything that is outside of the skull or outside
06:36 of the spinal column."
06:38 So in this diagram, the peripheral nervous system starts basically where these branches
06:44 come off the spinal cord, and you can see more peripheral nerves in the limbs here and
06:50 here.
06:51 @kaganellen asks, "Does acupuncture work for chronic pain?"
06:55 Yes.
06:56 So far, 39 clinical trials found that doing acupuncture significantly improved the control
07:02 of chronic pain.
07:03 @jessdoza asks, "Why does pain have to be so painful, you know?"
07:08 So pain is a really smart defense mechanism by our bodies to avoid things that might potentially
07:13 injure the body permanently.
07:14 Like if you step on a nail, you don't want to keep walking on that foot because every
07:18 time you step down, you're injuring your foot further.
07:20 It's a signal to you to fix whatever is hurting you so that you prevent further damage.
07:26 @profcooler asks, "How do nerves regenerate?"
07:30 Nerves in the peripheral nervous system can regenerate, and what happens is, let's say
07:35 there's some injury to a nerve, the immune system is activated, and it's kind of like
07:39 New Year's Day in Times Square.
07:41 The cleaning crew comes up and just gets rid of all that debris.
07:44 Until that debris is cleaned up, the cell body in the main part of the neuron can't
07:49 really give out directions to produce new axon endings.
07:54 @thymaster asks, "During a physical, why does the doctor tap your knee with a hammer?"
07:59 When we're testing your reflexes in the exam room, we're checking to see if your motor
08:04 and sensory neuron reflex is intact.
08:07 When we tap your, let's say your patellar tendon, that causes a stretch in a muscle
08:13 spindle.
08:14 This sends a signal back to the sensory neuron, which activates a motor neuron in the spinal
08:20 cord to cause your knee to kick up.
08:23 If this reflex is absent, it could indicate that you have a lower motor neuron problem.
08:28 If it's overactive, meaning it's too jumpy, it can signal a problem with an upper motor
08:32 neuron, something in the spinal cord above the level of the knee jerk reflex, which is
08:37 L3, L4, or something in the brain.
08:40 @emartin11 asks, "Why do people have such different pain tolerances?"
08:45 Pain is actually both a physical and an emotional phenomenon.
08:48 It's been studied that various factors like history of prior trauma, lack of sleep, female
08:54 sex can give folks lower pain tolerances.
08:57 Higher pain tolerance is associated with older age, exercise activity, as well as more social
09:02 support.
09:03 @eater asks, "Seriously, what is carpal tunnel?
09:06 What are you guys doing with your wrists?
09:08 I have never suffered this issue, but I'm intrigued."
09:11 So, carpal tunnel is when there's compression of the median nerve here in your wrist, which
09:17 is where the carpal tunnel is.
09:18 It's a pretty small tunnel where the tendons of a lot of the forearm muscles run through
09:22 as well.
09:23 If you get any type of repetitive stress injury, like the common one is typing, other people
09:29 notice it with cooking or other types of frequent manual labor, they can cause compression of
09:35 this nerve.
09:36 @amnon96 asks, "How does local anesthesia work?"
09:40 Medications like lidocaine or xylocaine, which you typically get at the dentist or for some
09:44 minor procedures, basically block the opening of voltage-gated sodium ions, which means
09:51 that you can't really have those action potentials spread.
09:54 So it really stops electrical impulses.
09:56 @anastasiabeave asks, "Would you fill me in on what brain zaps are?
10:01 That sounds terrifying."
10:02 So brain zaps are actually something that's more recently described in the early 2000s,
10:06 where people would notice that they would feel these electrical shocks in their head
10:10 or it feels like buzzing.
10:12 We found that they're associated with stopping antidepressant medications.
10:16 It's also been described after people use ecstasy or stop taking other medications like
10:21 benzodiazepines.
10:22 It might be some change to neurotransmitter levels as you've stopped taking these medications,
10:27 but we're still trying to figure out why they happen.
10:30 @ragdaybooyon31 asks, "Why do nerves in the teeth have to exist?
10:34 Freaking toothaches, man."
10:35 Your teeth are kind of precious in terms of, you know, once you have your permanent teeth,
10:40 you don't get new ones.
10:41 If you think about it, it's also in the mouth, which is where we put a lot of things that
10:44 could harm us, right?
10:45 You could eat bad food, you could eat something sharp that could cause a perforation of your
10:50 stomach.
10:51 So if there's a problem with your teeth, you want to know quickly rather than letting it
10:54 slowly decay and potentially cause an infection.
10:57 Those are all the questions.
10:58 Hopefully you learned something today.
11:00 See you next time.
11:01 (upbeat music)
11:03 (bells chiming)