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  • 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,