Autonomic nervous system | Organ Systems | MCAT | Khan Academy
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Autonomic nervous system | Organ Systems | MCAT | Khan Academy

Voiceover: In this video,
I want to introduce the autonomic nervous system,
autonomic nervous system, which is part of the
overall nervous system, and this is a functional
division of the nervous system, not a structural division,
like the central nervous system and the peripheral nervous system. The autonomic nervous system consists of efferent neurons in the
peripheral nervous system that do specific jobs. So, these are efferent
neurons, and these neurons control three different types of cells. The first are smooth muscle cells, which are in all sorts of
structures all over our body, like around our blood vessels, and they control cardiac muscle, the muscle that makes up our heart tissue, so, cardiac muscle, and
these muscle tissue types are different than the skeletal muscle, the muscle that’s all over,
attached to our skeleton that moves us around,
because those are controlled by different efferent neurons of the peripheral nervous system. Those are controlled
by lower motor neurons, not autonomic neurons. The last thing that autonomic neurons control are gland cells. Some gland cells are controlled by the autonomic nervous system. Now, the autonomic nervous
system is called this because it tends to
control all these things without conscious involvement. It doesn’t require the involvement of consciousness to control these things. So, it’s kind of autonomous. It kind of does this stuff on its own, without our conscious selves
having to be involved, for the most part, and we divide the autonomic nervous system
into two big subsystems. So, let me write two big arrows here. And this part we call the sympathetic nervous system, the sympathetic nervous system, which is the first big part of the autonomic nervous system. So, I’ll just write SNS for short, for sympathetic nervous
system, and this other big part we call the parasympathetic
nervous system. Parasympathetic, so I’ll
just write PNS for short, for parasympathetic nervous system, and there are a number of big differences between these two parts of the autonomic nervous system
that we can talk about in this kind of introductory talk. The first big difference
is kind of where they start in the central nervous system. The sympathetic nervous system starts in the middle of the spinal cord, and at the middle part of the spinal cord, let me draw a bunch of somas here, and I’ll just take one of these here, and I’ll draw a little short
axon on the first neuron that’s coming out of the
central nervous system, and then it’s going to
synapse with the second neuron in a ganglia close to
where the first neuron is, and then the second
neuron is going to send a longer axon to reach its target cell. So, let me just draw a big T, to represent some kind of target cell that
it’s going to synapse on, and this target cell will
be a smooth muscle cell, a cardiac muscle cell, or a gland cell. And here’s an illustration
of kind of the entire autonomic nervous system,
and here they’re showing kind of the middle part of the spinal cord that all these first neurons in the sympathetic nervous system are starting, and then there’s a short
axon until they synapse in a ganglia that’s
pretty close to the spine. Here’s a set of ganglia, and
here are a few other ganglia, but they all tend to be
pretty close to the spine. This set of ganglia are
actually often linked together in kind of a chain, which we actually call the sympathetic chain, and here’s just a different illustration of the same thing. So, here it’s showing in the
middle part of the spinal cord that first axon’s coming
out, synapsing at a ganglia close to the spine, with
a lot of these ganglia linked together in a chain,
and then the second neuron sending a longer axon out to
synapse on the target cell in whatever tissue you’re
talking about that contains smooth muscle cells, cardiac
muscle cells, or gland cells. Now, the parasympathetic nervous system has its first neurons
start in a different place in the central nervous system. They start either up here in
the brain stem, or they start way down here at the
bottom of the spinal cord, and then their first neuron tends to send a long axon out to synapse
with the second neuron in a ganglion at a distance
from the first neuron, and then that second
neuron usually sends out a short axon to synapse
on its target cell. I’ll just write a big
T here for target cell. And here this illustration
is showing this as well, where it’s showing the
first neurons of the parasympathetic nervous
system either up here in the brain stem or
down here at the bottom of the spinal cord, and then it’s showing these long axons on the first neuron, until it reaches a ganglia at a distance from the first neuron’s
soma and then a shorter axon on that second neuron, until
it reaches its target cell. And here’s another illustration, just showing the same thing. So, here’s these first axons coming out of either the brain stem up high or the bottom part of
the spinal cord down low, and then these first
long axons go all the way until they meet a ganglion at a distance from the first neuron soma,
and then the second neuron sends a shorter axon to the target cells. So, the similarities in the structure of the different parts of the autonomic nervous system
are that they both usually consist of a chain of two neurons connecting the central nervous
system to the target cell, but the differences are
where those first neurons start and whether there’s
a short first axon and a long second axon
or a long first axon and a short second axon. But, more importantly than
these structural differences between the different parts of the autonomic nervous system, are
the functional differences, and these neurons do so
many different things in so many tissues of the body, that it’s a little hard to
talk about them in general, but there are these great
phrases that can kind of help think through lots of
the changes that these different parts of the
autonomic nervous system do, and for the sympathetic nervous system, the phrase is fight or
flight, fight or flight, that the sympathetic nervous system, when it’s activated, will cause
lots of changes in the body that’ll prepare to
either fight or run away, which can kind of help you deal with threatening or dangerous situations. So, I’ll write that in red here for the sympathetic nervous system, whereas the parasympathetic nervous system I’ll write in a nice cool green
here, because its phrase is rest and digest, rest and digest. So, when it’s active, it
often causes lots of changes in the body that are more
important for homeostasis and just maintenance of the body in nonthreatening situations. So, let’s take a few
examples of a few tissues where these different responses happen, to get a feel for what this means. So, first let’s look down here at the gastrointestinal system,
the intestines or the gut, and both the sympathetic
and the parasympathetic nervous system play a role
in a lot of activities of the gastrointestinal system, but one is blood flow to the intestines,
because the amount of blood flowing through the
intestines plays a big role in how much digestion
the intestines can do. Blood flow to intestines,
and it also plays a big role in how much blood is available
for other parts of the body. So, when the sympathetic
nervous system is activated in some kind of fight or flight situation, blood flow to the intestines decreases, and that blood is actually diverted away from the intestines,
often to skeletal muscle. So, all our muscles all over our body that can help us move to deal with dangerous situations,
the blood is going to leave the intestines and go to that, because during a dangerous situation is not the time to be digesting food. It’s the time to be moving, so the blood’s flow
decreases to the intestines and is diverted to skeletal muscle, whereas most of the time, when you’re in a nonthreatening situation and
it’s time to rest and digest, the peripheral nervous
system is activated, and that increases blood
flow to the intestines. That’ll divert blood away
from skeletal muscle, because now you’re not in a
fight or flight situation, and you want to rest and digest. So, it’s going to bring
the blood flow back to the intestines, to increase
your ability to digest food. If we look at the heart, both the sympathetic and parasympathetic nervous systems innervate the heart, and we look at the heart
output, kind of how much blood the heart is pumping out over
any particular unit of time. Heart output of blood. When the sympathetic
nervous system is activated, the heart output increases. The heart pumps harder and pumps faster and pushes more blood out, so that things like skeletal muscle
can get more blood flow. In addition to diverting
blood flow from the intestines to skeletal muscle, the heart’s
just going to push more out, so there’s more available
for the skeletal muscle. When the parasympathetic
nervous system is activated, the heart output goes down. The heart is pumping less hard, and it’s beating less often. It’s just working less,
because you don’t need as much blood flow to
the muscles for movement, so you go to kind of a baseline
level that’s sufficient for activities that involve
resting and digesting. So, these examples of blood
flow involve the activity of smooth muscle, because smooth muscle is around our blood vessels and determines where the blood is going to flow to, and cardiac muscle,
because the cardiac muscle makes up the heart, and then
if we think about gland cells, there’s a bunch of different glands that the autonomic
nervous system controls, and they tend to be activated kind of differently at different times. So, one type of gland that’s activated during fight or flight situations, when the sympathetic
nervous system is active, are sweat glands out here in the skin, and the sweat glands are
activated to secrete sweat, which helps cool us down,
which increases our ability to move faster and farther,
if we’re able to stay cool, whereas some glands that
are activated by the parasympathetic nervous
system include things like the salivary glands that
produce saliva in our mouth, because saliva is very
useful for digestion, and it’s part of a number of activities that happen that help us digest food. So, I find these phrases
helpful when I’m thinking about what effects the autonomic nervous system will have on different tissues of the body during different situations, because like in all of these examples, most of the things that the
sympathetic nervous system does when it’s activated
increase the body’s ability to turn stored energy into movement, to deal with dangerous situations, like moving blood from the
intestines to skeletal muscle and increasing the amount of blood being pumped around from the heart and increasing sweat
production from sweat glands to keep us cool while we’re moving to deal with a dangerous situation, whereas all of these things
that the parasympathetic nervous system is doing, make sense in nonthreatening situations,
where we’re actually trying to conserve and store energy, like diverting blood flow
away from skeletal muscle to the intestines, to increase digestion, decreasing heart/cardiac
output to conserve energy, and increasing saliva production from the salivary glands to
help with digestion as well. But the autonomic nervous system affects many more structures and
has many more functions than I can cover in this
little introductory video. For instance, autonomic
neurons play a role in changing the size of
your pupils in your eyes, in sexual responses, and in secretion from a whole bunch of other glands, and because it does so
many different things, I find it best to
actually not cover it all in one sitting, but instead
to cover these things as you’re studying each
individual organ system. Because almost any organ system you’re going to cover is going to have autonomic neurons coming in and affecting how that
organ system functions.

About James Carlton

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57 thoughts on “Autonomic nervous system | Organ Systems | MCAT | Khan Academy

  1. You don't even have to take notes. He took the liberty of writting them for you! Thanks guy! That's One flashcard down…

  2. My mind was boggled when I first started learning this, and that was as an EMT-B, the first level of entering into EMS.
    I had all kinds of trouble remembering that afferent means 'to' and efferent means 'away from' until I could recall they were talking about the brain as opposed to the ANS when efferent means it's coming TO the nerves and afferent means it's going away from it.

  3. I'm kind of curious why PNS has longer pre-synapse neuron. What is the reason behind that? Is it about how it functions? And about the difference between their neurotransmitter (acetylcholine and norepinephrine). Why are they different? If anyone can give me answers, that would be awesome. Thanks!

  4. Good overview, but it would have been nice to have at least touched on the different neurotransmitters specifically associated with the steps of transmission along each division of the system (e.g. metabotropically active muscarinic AChRs receiving ACh released by efferent postganglionic depolarization events in the PNS).

  5. Is computationalism reserved strictly for (personal level) cognitive activity or does computation happen in the autonomic systems?

  6. you state that PNS activation of the heart causes the heart is "pumping less hard and less often". My understanding is that the PNS innervates the nodes but not the ventricle. So the cardiac output goes down because heart rate goes down, but not inotropic effect.

  7. good video but the guy's vocal fry gets pretty annoying.

    not very hard to get rid of, and its not something you'd expect from a producer as established as khan.

  8. When you combine some of the stuff around kundalini theories with the parasympathetic nervous system there's some correlation going on.

  9. I think 'Matthew' should watch this video.
    Matthew 6:25: “Therefore I tell you, do not be anxious about your life, what you will eat or what you will drink, nor about your body, what you will put on. Is not life more than food, and the body more than clothing?" Who is Matthew? The Bible editor?
    1) To think about what we will eat or what we will drink is not 'anxiety', but it is useful for healthy life.
    2) 'life' and 'food' are not things to be compared. To take food is a healthy thing to the life.
    3) 'body' and 'cloth' are not things to be compared. To put on good cloth is a healthy thing for the body.
    4) If Matthew will not agree with 1) to 3), he will be anxious about his life and his body.

  10. I had to search intestines on merriam bc he said it too many times and got it stuck in my head lol I don’t think I’ll forget this 😂 Great video!! Thanks!!

  11. Great video, but unfortunately largely insufficient even at the paramedic level. Would have been nice to talk about neurotransmitters in more detail.

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