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  • Say it's late at night, you're home alone drifting off to sleep, just, entering that

  • dream about Fritos, and then suddenly there's a banging at the door! Suddenly you're wide

  • awake and it feels like your heart's gonna explode. You jump up ready to run out the

  • back door, possibly grab a Phillips head screwdriver and stab it into the darkness until it sticks

  • into something. Now whether it's a Weeping Angel or your neighbor looking to borrow a

  • can of beans, it doesn't really matter because when you heard that sudden noise, your startled

  • brain released an icy typhoon of chemicals. And everything that's now going through your

  • mind, like your urge to flee, your urge to defend yourself, that internal debate about

  • whether Weeping Angels are even real and "Woah! Where's the cat?" All that? Is just a result

  • of those chemicals.

  • Our brains and our nervous systems and the substances they produce and are always bathed

  • in are amazingly complex nuanced systems. And even though we're always talking about

  • our mental activities being somehow separate from all the biological stuff going on in

  • our bodies, in reality, the moods, ideas, impulses, that flash through our minds are

  • spurred by our biological condition. As psychologists like to say, "Everything psychological is

  • biological." So one way to understand how your mind works is to look at how the chemistry

  • of your body influences how you think, sense, and feel about the world around you. To do

  • that, we begin at the simplest level, the system with the smallest parts, it's all about

  • the neuron, baby.

  • [Intro]

  • Neurons, or nerve cells, are the building blocks that comprise our nervous systems.

  • Neurons share the same basic makeup as our other cells, but they have electrochemical

  • mojo that lets them transmit messages to each other. Your brain alone is made up of billions

  • of neurons, and to understand why we think or dream or do anything, you gotta first understand

  • how these little transmitters work. You actually have several different types of neurons in

  • your body, from ones that are less than a millimeter long in your brain to ones that

  • run the whole length of your leg! Yes, you have cells as long as your legs, which is

  • nothing compared to the hundred and fifty feet the nerve cells of some dinosaurs had

  • to be, I'm getting off topic, sorry.

  • No matter how big a nerve is, they all have the same three basic parts: the soma, dendrites,

  • and axon. The soma, or cell body, is basically the neuron's life support; it contains all

  • that necessary cell action like the nucleus, DNA, mitochondria, ribosomes, and such. So,

  • if the soma dies, the whole neuron goes with it. The dendrites, as bushy and branch-like

  • as the trees they're named after, receive messages and gossip from other cells. They're

  • the listeners, whispering what they hear back to the soma. The axon is the talker. This

  • long, cable-like extension transmits electrical impulses from the cell body out to other neurons

  • or glands or muscles. Whereas the dendrites are short and bushy, the axon fiber is long,

  • and, depending on what type of neuron it is, is sometimes encased in a protective layer

  • of fatty tissue, called the myelin sheath. It's almost like an insulated electrical wire,

  • the myelin sheath speeds up the transmission of messages, and if it degrades, as it does

  • with those affected with multiple sclerosis, those signals are degraded as well, eventually

  • leading to lack of muscle control.

  • Neurons transmit signals either when stimulated by sensory input or triggered by neighboring

  • neurons. The dendrites pick up the signal and activate the neuron's action potential,

  • or firing impulse, that shoots an electrical charge down the axon to its terminals and

  • towards the neighboring neurons. The contact points between neurons are called synapses.

  • All those bushy little dendrites are decorated with synapses that almost but don't quite

  • touch the neighboring axon in the tiniest game of "I'm not touching you!" of all time.

  • They're less than a millionth of an inch apart. And that microscopic cleft is called the synaptic

  • gap. So, when an action potential runs down to the end of an axon, it activates the chemical

  • messengers that jump that tiny synaptic gap, flying like that little air kiss and landing

  • on the receptor sites of the receiving neuron. Those messengers are neurotransmitters.

  • Although neurotransmitters slide right into their intended receptors like a key into a

  • lock, they don't stay bonded to the receiving neuron. They just sort of pop out, having

  • excited or inhibited the receiving neuron's trigger, then the extras immediately get reabsorbed

  • by the neuron that released them in the first place in a process called reuptake. Kinda

  • like, "Here you go, oh, psych!" So neurons communicate with neurotransmitters which in

  • turn cause motion and emotion; they help us move around, make jazz hands, learn, feel,

  • remember, stay alert, get sleepy, and pretty much do everything we do.

  • Some of them just make you feel good, like the endorphins we get flooded with after running

  • ten miles or falling in love or eating a really good piece of pie. We've got over 100 different

  • kinds of these brilliant neurotransmitters -- some are excitatory and others are inhibitory,

  • and all are good reminders that everything psychological is also biological. Excitatory

  • neurotransmitters rev up the neuron, increasing the chances it will fire off an action potential.

  • Norepinephrine is one you're probably familiar with, it helps control alertness and arousal.

  • Glutamate is another, involved in memory, but an over-supply of it can wig out the brain

  • and cause seizures and migraines which is why some people are sensitive to all that

  • MSG, or monosodium glutamate, in their Ramen. Inhibitory neurotransmitters on the other

  • hand, chill neurons out, decreasing the likelihood that the neuron will jump into action. GABA

  • gamma-aminobutyric acidis a major inhibitory neurotransmitter, and you've probably heard

  • of serotonin which affects your mood and hunger and sleep. Low amounts of serotonin are linked

  • to depression, and a certain class of antidepressants help raise serotonin levels in the brain.

  • Some neurotransmitters like acetylcholine and dopamine play both sides and can both

  • excite or inhibit neurons depending on what type of receptors they encounter. Acetylcholine

  • enables muscle action and influences learning and memory; Alzheimer's patients experience

  • a deterioration of their acetylcholine producing neurons. Dopamine, meanwhile, is associated

  • with learning, movement, and pleasurable emotions, and excessive amounts of it are linked to

  • schizophrenia as well as addictive and impulsive behavior.

  • So neurotransmitters are basically your nervous system's couriers. But they aren't the only

  • chemical messengers delivering the news; they've got some competition brewing in the endocrine

  • system. And if you've been through puberty, you know what I'm talking about: hormones.

  • Like neurotransmitters, hormones act on the brain, and indeed some of them are chemically

  • identical to certain neurotransmitters. Hormones affect our moods, arousal, and circadian rhythm,

  • they regulate our metabolism, monitor our immune system, signal growth, and help with

  • sexual reproduction. You could say that most of them boil down to the basics: attraction,

  • appetite, and aggression.

  • Whereas neurons and synapses flick on and off, sending messages with amazing speed,

  • the endocrine system likes to take its time, delivering the body's slow chemical communications

  • through a set of glands that secrete hormones into the bloodstream where they're ferried

  • to other tissues, especially the brain.

  • So while the nervous and endocrine systems are similar, in that they both produce chemicals

  • destined to hit up certain receptors, they operate at very different speeds. It's like,

  • if the nervous system wants to get in touch with you, it sends you a text. But if the

  • endocrine system has a message, it will like lick the stamp, and put it on, and write your

  • address, and then a note and a pen on paper, and then fold it up and put and mail it to

  • you with the Post Office. But fast isn't always better, and your body will remember that letter

  • longer than the text. Hormones, they linger. Which helps explain why it takes some time

  • to simmer down after a moment of severe fright or anger.

  • And our endocrine systems have a few important hormone brewing glands. We've got a pair of

  • adrenal glands snuggled up against our kidneys that secrete adrenaline, that famous fight

  • or flight hormone that jacks up your heart rate, blood pressure and blood sugar, giving

  • you that tidal wave of energy preparing you to run like heck or punch that charging baboon

  • in the throat; the pancreas sits right next to the adrenal gland and oozes insulin and

  • glucagon hormones that monitor how you absorb sugar, your bodies main source of fuel. Your

  • thyroid and parathyroid glands at the base of your throat secrete hormones that regulate

  • your metabolism and monitor your body's calcium levels; if you have testicles, they're secreting

  • your sex hormones like estrogen and testosterone, and if you've got ovaries, they're doing that

  • job.

  • And all those glands are super important, but there is one gland that rules them all,

  • and in the darkness binds them: the pituitary gland. Although it's just a little pea-sized

  • nugget hidden deep in the bunker of the brain, it is the most influential gland in this system.

  • It releases a vital growth hormone that spurs physical development and that love hormone,

  • oxytocin, that promotes warm, fuzzy feelings of trust and social bonding. What really makes

  • the pituitary the master gland is that its secretions boss around the other endocrine

  • glands, but even the pituitary has a master in the hypothalamus region of the brain, which

  • we will talk more about next episode.

  • So, AHHHHHHHHH! if I managed to scare you, sorry, but I'm illustrating a point. You have

  • no control over being scared, but maybe now you do understand a little more clearly how

  • your nervous and endocrine systems worked together to call the shots.

  • First, the sensory input from your eyes and ears went to your brain, the simplest bits

  • of your hypothalamus without even letting you analyze it and were like ahhhh, and then,

  • that ran down the chain of command from your pituitary to your adrenal glands, to the hormone

  • adrenaline, to the rest of your body and then back to your brain, which then realized that

  • I was just messing with you and told everybody to just calm down for once!

  • The whole deal is a feedback loop: your nervous system directs your endocrine system which

  • directs your nervous system, brain, gland, hormone, brain. And of course each of these

  • systems is fantastically complex. Way more than we can get into here.

  • So, in our next lesson, we're gonna get all up in your brain, and delve deeper into the

  • different components of your nervous system, find out what your old brain is, and learn

  • about how much of your brain you actually use.

  • In the meantime, thank you for watching this lesson in Crash Course Psychology which was

  • brought to you by Zane Ice, who wants to say hi to his friend Harrison. Thank you, Zane.

  • If you'd like to sponsor an episode and give your own shout-out, you can learn about that

  • and other perks available to our subbable subscribers, just go to subbable.com/crashcourse.

  • This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant

  • is Dr. Ranjit Bhagwat. Our director and editor is Nicholas Jenkins, the script supervisor

  • was Michael Aranda, who was also our sound designer, and the graphics team is Thought

  • Cafe.

Say it's late at night, you're home alone drifting off to sleep, just, entering that

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