AP Biology The Nervous System

Welcome to Educator.com.0000

In this section of animal physiology, we will be focusing on the nervous system.0002

And the nervous system provides a mechanism for the body to receive and send information.0007

And this can be external stimuli that is being received such as sounds and smells or internal information such as temperature,0013

blood pressure that allows the body to coordinate functions and maintain homeostasis.0022

We are going to start out by talking about different types of nervous systems and then, focus in detail on the human nervous system.0031

The simplest nervous system is called a nerve net.0038

And you will recall in the section in diversity of life, I described a nerve net as being found in cnidarians like jellies and Hydra.0042

So, a nerve net is simply a diffused group of interconnected nerve cells. There is no central nervous system.0056

If you look at a simple animal but with a more advanced nervous system than the nerve net, that would be the nervous system of a flatworm.0067

So, the flatworm has longitudinal nerve cords and a pair of ganglia, so nerve cords and ganglia.0079

Recall that ganglia are clusters of nerves, and in the flatworm, these are at the anterior end of the organism.0096

And they allow for processing of sensory input.0109

This gets us to a related point in the evolution and development of the nervous system and that is the concept of cephalization.0113

Recall that in bilaterally symmetrical animals, cephalization developed.0122

And this is the development of a head end in which sensory organisms are clustered or concentrated.0129

Sensory organs are concentrated here and ganglia to process this information, then eventually, in more advanced animals a brain.0141

To give you just examples of an overview of nervous systems and a couple other groups of0155

animals before we go on to talking about vertebrates and particularly humans- arthropods.0160

Arthropods have well-developed sensory organs.0168

They have eyes. They have organs that allow for smell, antennas for touch and even ears in some species.0178

Some have ganglia. Other arthropods actually have brains.0186

Echinoderms such as sea stars have a less well-developed nervous system. They have, recall, a nerve ring with cords that radiate out into their arms.0193

Next, we are going to go ahead and focus in on the vertebrate nervous system.0215

I am going to start with an overview of the organization, discuss the neuron and transmission of nerve impulses and then, revisit the central nervous system0222

in more detail once we have covered the terminology that you need to understand the structure and function of the central nervous system.0233

Here is the nervous system overall, and there are two major divisions.0241

The central nervous system consists of the brain and spinal cord, and the peripheral nervous system is everything else.0245

The PNS can be further divided into two major systems: the autonomic nervous system and the somatic nervous system.0255

The somatic nervous system is responsible for voluntary activities, so voluntary activities such as when you walk or talk or turn your head.0265

Those are all under the control of the somatic nervous system.0279

By contrast, the autonomic nervous system is responsible for involuntary activities.0284

The heart, the GI tract and the endocrine organs are all regulated by the autonomic nervous system.0291

Some systems actually split off into a third division, which is the enteric nervous system, and this is regulation of the GI tract.0297

Instead, they are just putting it under the autonomic system, so you may encounter that.0308

So, nervous system: central versus peripheral.0314

Peripheral is divided into autonomic and somatic, and then, autonomic has two further divisions: the sympathetic nervous system and the parasympathetic.0317

The sympathetic nervous system is responsible for the fight or flight response.0329

Recall that the fight or flight response results in an increase in heart rate, respiratory rate, glucose. Blood is shunted to the skeletal muscles.0335

So, the idea is that if there is a threat like a person is chasing you or something is about to fall on you and you have to run away really quickly,0350

your body triggers this giving you the oxygen, the glucose, the energy, everything you need to either fight or run away.0360

The opposite is parasympathetic. This is often described as rest and digest.0373

With the fight or flight, we see an increase in the heart rate, increase blood pressure, increase in respiration. Blood is shunted to the skeletal muscles.0381

With the rest and digest, things return to calmer. There is a decrease in the heart rate, decrease in the respiratory rate. Blood is shunted to the GI tract.0395

Now we have an overview, we are going to focus on the functional unit of the nervous system,0418

which is the neuron, beginning with the structure of the neuron.0424

And then, we will talk about the way that signals are transmitted along the neuron and from one neuron to another neuron or cell.0427

Beginning with the cell body, the cell body contains the nucleus and organelles for the nerve cell.0437

Extending from the cell body are projections called dendrites.0448

Dendrites receive incoming stimuli, and there are various types of stimuli.0454

It depends on the type of neuron. In the eye, the dendrites are specialized to receive could be light.0467

A stimulus on your skin, there are pain receptors, different touch receptors.0476

The stimulus that is received, it can be specialized.0482

Or the stimulus might just be another neuron that is synapsing on this neuron and stimulating this neuron to transmit the signal.0486

So, it could be an outside stimuli or another cell synapsing on this one.0496

This section right here, leading away from the cell body, is called an axon hillock.0501

And this is the region where the action potential that we are going to talk about or impulse that0509

transmits the signal from the dendrites here along down this axon, so this is the axon.0515

The action impulse is initiated here, action potential initiated here.0525

This is the axon. We will come back to this in a second.0540

And then, here at the end of the axon are the synaptic terminals.0542

We are going to look at a close up view of this later, but the neurotransmitter is released from the synaptic terminals.0546

Some nerve cells have myelinated axons, so what they have is what is called a myelin sheath that encases much of the axon.0562

Myelin in the peripheral nervous system is produced by Schwann cells.0579

Now, there are segments that are unmyelinated even on a myelinated neuron, and these are called nodes of Ranvier.0589

When I talk about action potentials, you will learn how the action potential really appears to skip from one node to the other in a myelinated neuron.0600

So, right now, I am just focusing on structure, and then, we will talk about function.0609

Here, you can see several nerve cells together, and these two nerve cells are synapsing on this nerve cell giving it input.0619

As I said, the neurotransmitters are released from the synaptic terminal here. They can, then, diffuse to the postsynaptic cell, give it input.0631

So, what I want you to understand here, is that these cells are called presynaptic cells.0644

Here, we have the synapse, this connection between one cell and another, and then, here, I have a postsynaptic cell.0655

Now, the nerve cell does not always synapse on another nerve cell. In fact, the nerve cell might synapse on a muscle cell.0671

So, what happens, then, is that the nerve cell is giving the muscle cell a signal to contract or inhibiting contraction.0681

Or a nerve cell can also synapse on an endocrine gland. It could, then, cause the endocrine gland to release a particular hormone.0693

There is communication not just obviously between one nerve cell and another but between the nerve cell and other systems of the body.0701

There are three groups of neurons.0712

The first group are the sensory neurons, and the sensory neurons receive information from the environment.0714

So, they receive input in the form of...it could be sound. It could be light, touch, smell.0729

And then, they transmit the input to the central nervous system to the brain,0742

to the spinal cord and to the brain for processing so that sensory input is received.0751

For example, when you look, your optic nerve is receiving that input of light, but it is your brain that interprets the image.0762

So, processing occurs in the central nervous system.0772

Information within the body is also collected: changes in blood pressure, changes in their stretch receptors in the GI tract.0775

So, the internal environment is monitored as well through the sensory neurons.0785

The second group of neurons are the motor neurons, and these transmit impulses to the muscle. They can, then, stimulate the muscle to contract.0792

Some motor neurons, as I mentioned, synapse on endocrine glands, so they might transmit to an endocrine gland stimulating a secretion of a hormone.0811

The cells that motor neurons act on...motor neurons act effector cells, so an effector cell could be a muscle for example.0824

You may have heard of Lou Gehrig's disease, also known as ALS, and this is a motor neuron disease.0840

It is degeneration of the motor neurons resulting in muscle atrophy and weakness throughout the body.0858

Finally, interneurons: interneurons connect sensory and motor neurons, so the input is being received by a sensory neuron.0866

Then, that signal may be transmitted to an interneuron which will, then, convey the information to a motor neuron.0886

These are also located in the central nervous system, the brain and the spinal cord.0895

And this is just giving you a very simple example, but the interactions between all of the different neurons are very complex.0901

But, in general, there are these three types, and this is how they interact.0909

Next, we are going to talk about how signals are transmitted by the nervous system.0915

And to understand that, you need to understand a concept called resting potential.0920

Just starting out talking about membrane potentials, a membrane potential is the difference in the electrical charge across the cell membrane.0926

So, if I have a cell and I am going to draw the cell here as a tube like if I am looking at the axon, the long axon membrane as a cylinder here,0936

there is a difference in electrical charge between the inside of the cell and the outside of the cell.0952

It is a result in a difference in concentration of ions between the inside and the outside of the cell.0959

The resting potential is the membrane potential of a neuron that is not conducting a signal, so that is why it is the resting potential. The cell is at rest.0967

So, the resting potential - this is the resting potential - is around -70 mV in the neuron.0978

And it is the result between the difference in sodium and potassium ion concentration inside and outside of the cell.0989

The concentration of sodium ions outside of the cell is much higher than inside of the cell, so lots of sodium outside, not so much inside.1000

The situation with potassium is the opposite. Lots of potassium inside the cell relative to the outside of the cell.1015

This gradient is maintained a couple of ways.1028

First of all, there is the sodium-potassium pump that I have mentioned elsewhere in the course.1031

What this pump does is it hydrolyzes ATP and uses the ATP for active transport of sodium out of the cell and potassium into the cell.1038

Three sodium are transported into the cell per two potassium.1049

Excuse me, correction. Three sodium are transported out of the cell per every two potassium transported into the cell.1061

So, when one ATP is hydrolyzed, the result will be per ATP hydrolyzed.1075

One ATP is hydrolyzed. That transports three sodiums out and two potassiums in.1082

So, here is this pump right here, and it is sending three sodiums out for every two potassiums in.1089

And this is active transport because these are being transported against their concentration gradients.1098

So, concentration of sodium outside the cell is much, much greater than sodium inside.1108

What the sodium wants to do is diffuse into the cell or not diffuse actually because it is an ion, but it wants to enter the cell. It wants to enter the cell.1119

It cannot though unless it goes through a channel.1128

Therefore, to get it out, you need to transport it against its gradient.1132

The concentration of potassium outside the cell is much, much lower than potassium inside the cell.1139

What potassium wants to do is it wants to leave the cell. To get it to enter the cell requires this active transport.1150

So, this maintains this concentration gradient.1158

Now, what happens is since these are charged, there also ends up being an electrical gradient.1161

So, this is a chemical gradient where we have lots of high sodium concentration out and high potassium concentration inside- chemical gradient.1170

But, this creates an electrical gradient, and let me talk about how this is created and maintained.1181

Well, for every three sodium out, only two potassium are pumped in. This is a net loss of +1 charge from the cell.1195

So, for every turn of the pump, one unit of positive charge is lost overall, a net loss of positive charge.1209

As a result, the inside of the cell ends up with a negative charge relative to the outside of the cell.1218

And this difference in charge ends up giving a membrane potential of -70 at rest.1232

The other issue here is that sodium channels...there are sodium channels.1245

And the sodium wants to go down its concentration gradient at the end of the cell.1252

The problem is these are mostly closed, so sodium channels are mostly closed. Meanwhile, many potassium channels are open.1258

So, what happens is some potassium can leave the cell, and that is a loss of more positive charge to make the cell negative.1275

Now, to balance this, chloride wants to follow so that the positive sodium leaving the negative chloride is going to go out with it.1284

And it will balance out in terms of charge. However, chloride channels are closed.1295

So, the positive charge sodium cannot really enter the cell.1301

The negatively charged chloride cannot leave the cell, but potassium, which is positively charged can leave the cell.1306

The result is we have more positive charge leaving.1315

We already have this in balance to the sodium potassium pump where we have more positive charge leaving the cell than coming in.1319

Now, we have more potassium leaving, and that causes this negative charge inside the cell relative to the outside.1324

What will happen is that there will be a net loss of potassium until the pull of the chemical gradient1334

of potassium is exactly counterbalanced by the negative charge pulling the potassium back in.1343

So, there are two forces acting on potassium.1352

There is a chemical gradient, which is drawing potassium out of the cell because potassium wants1354

to go down its concentration gradient to outside the cell where potassium is lower concentration.1362

So, the chemical gradient pulls - you can think of it as pulls - potassium out.1371

However, the electrical gradient, because potassium is negatively charged, draws potassium in, and these two are working in opposition.1379

And so, potassium will leave the cell until there is an equilibrium reach where the concentration gradient is no longer sufficient of potassium1391

- the chemical gradient - to oppose the attractive force, the negative charge drawing the positively charged potassium in.1402

That point of equilibrium occurs at about -70 mV, and that is the resting potential.1409

So, resting potential is a result of the selective permeability of the cell membrane for ions,1417

the fact that potassium channels are open but sodium and chloride are not.1425

And the resting potential is also maintain by the sodium-potassium pump.1431

What we say is that this cell is polarized. When we say that it has a resting potential, we say that it is polarized.1439

And I am going to talk about depolarization and hyperpolarization and repolarization.1450

And we are starting out with the cell that is polarized because of this difference in electrical charge compared with the inside versus the outside of the cell.1454

Now, I am talking about sodium channels and chloride and potassium channels, and these are a different type of channel that I am about to talk about.1466

I am going to talk again about sodium and potassium channels but a different type than I just discussed.1474

And these other types of channels are called gated ion channels, and these are key to understanding the action potential.1481

So in addition to the sodium and potassium channels I discussed, there are these channels that are gated.1489

Gated, meaning a stimulus triggers the opening or closing of an ion channel, so, gated means stimulus triggers the opening or closing of a channel.1498

And some of the channels that I talked about could involve some type of stimulus opening or closing.1520

But, I really want to focus on voltage-gated ion channels for the action potentials.1525

Voltage-gated channels would open or close due to a change in membrane potential.1532

So, these are regulated, the opening and closing of them is regulated by a change in membrane potential.1540

There are also what is called ligand-gated channels.1552

And in lagan-gated ion channels, opening or closing is triggered by the binding of a molecule to the channel.1556

So, ligand-gated, the stimulus would be binding of a ligand opens or closes the channel.1562

Extremely important in action potentials is voltage-gated ion channels.1578

These are sodium channels and potassium channels whose opening is controlled by changes in potential.1589

So, there are two types you need to be aware of.1597

The ones in orange are the sodium- voltage-gated sodium channels. The ones in purple are voltage-gated potassium channels.1600

This is a cell membrane, and this is the inside of the cell. Here is the outside of the cell.1610

Recall that outside the cell, very high relative to the inside of the cell concentration of sodium.1617

Inside the cell, the situation...and this is going to curve around the cell membrane1628

Inside the cell of the opposite, I have relatively high potassium, relatively low sodium concentration.1636

Opening these voltage-gated sodium channels causes a massive influx of sodium into the cell.1658

So, sodium is going to open up. Tons of these will open up once during an action potential, and the result is going to be sodium enters the cell.1665

When that positive charge enters the cell, it is going to make the cell less negative.1677

The cell was at -70 mV. If you put tons of sodium into the cell, it will go up to -50 and -40 and then, +10 and on up.1684

So, sodium enters the cell when these are open, and the result is the cell becomes depolarized; and I am going to put these all in context in just a minute.1694

Now, what is happening here actually these arrows should be going the other way. Let me go ahead and correct that, so correction there.1712

Now, what is happening is that since the concentration of potassium is very high in the cell relative to outside,1723

the potassium wants to go down its concentration gradient and leave the cell, and what that is going to do is cause positive charge to leave the cell.1731

When potassium leaves the cell the result will be the membrane potential will become more negative.1743

When the membrane potential becomes more negative, if it drops below the resting potential baseline, then, we say it is hyperpolarized.1765

Sometimes what is happening is that the membrane potential became plus negative due to depolarization.1774

And all the potassium is doing by coming back into the cell is resetting it to baseline, is repolarizing.1781

The thing you should understand is that sodium enters the cell. That causes the cell to become more positive inside.1788

That is depolarization.1794

When potassium leaves the cell, the cell becomes more negative inside.1797

That is positive charge leaving, and the cell becomes repolarized or even hyperpolarized.1802

An action potential transmits the signal along the axon of the neurons.1811

So, an action potential is also known as an impulse, and it is a large change in the membrane potential.1815

As you can see, this change down here is a quick increase where the membrane potential becomes more positive.1823

And then, it drops rapidly and becomes more negative and returns to its baseline.1832

So, here, at -70 is the resting potential, and I will give you an overview; and then, we will talk about what happens at each step.1837

As this is increasing, what we have here is depolarization. The cell is becoming depolarized until it reaches the peak.1849

Here, the cell is repolarizing, so it is repolarization.1866

It actually undershoots and goes past baseline, so the repolarization really tends to be a little more precise. It would be right around here.1875

And then, this segment here, when it passes rest, so repolarization is occurring as it is dropping, dropping, dropping and then, hyperpolarization.1888

The cell is depolarizing, becoming more positive, repolarizing, but then it shoots past the resting potential and then, hyperpolarizes briefly.1912

And then, it goes back to the baseline.1925

Let's look at what happens during each section here.1928

First, the cell is at rest. There is no stimulus.1931

Then, a stimulus excites the neuron and causes entry of sodium into the cell, and that can occur from a stimulus. It could be another neuron.1937

It could be light. It could be sound.1954

Some kind of stimulus excites the neuron, and it is going to cause sodium to enter the cell here.1956

Remember that when sodium enters the cell, sodium is positively charged.1962

So, these positive charges entering the cell are going to cause the membrane to become less negative.1969

It is going up, kind of, creeping up towards what is called threshold.1974

So, the stimulus causes the entry of sodium into the cell and causes some depolarization.1983

If the stimulus is strong enough, enough sodium will enter the cell that the cell reaches threshold.1993

If the membrane potential, the cell membrane, reaches threshold potential...let me put this right here, threshold.2004

If the cell membrane reaches threshold potential, then, the voltage-gated sodium channels open.2021

When the voltage-gated sodium channels open, that is going to be a massive influx of positive charge into the cell.2042

Here, you get some entry. Hopefully, it is enough to get you up to threshold, get things going.2050

But at this point, this is what we call all or none response that either you are going to get to threshold, and these are all going to open;2056

or you are not going to get to threshold, and none of these voltage-gated sodium channels will open.2069

If they do open though, there is going to be a very fast change in the membrane potential.2073

It is going to be more and more positive until it reaches a certain peak, and action potential is a stereotypic size and shape.2079

Even if you put a super strong stimulus, this is still going to peak at, say, 35.2086

It is not going to peak at 50, or it is not going to make the graph wider somehow or end up more hyperpolarized.2093

The size and shape are always going to be the same for a cell.2098

What is going to change though, is if there is a stronger stimulus, the action potentials will become more frequent, not larger.2101

They do not become larger.2113

Alright, now, the voltage-gated sodium channels have opened, big influx of sodium into the cell.2115

However, they quickly close. They quickly close.2123

So, then, the next thing that happens is sodium channels close, and here, voltage-gated potassium channels open.2127

Because right now, the cell has become depolarized.2149

The inside of the cell is now relatively positive and to end this action potential, we need to get back to the resting state.2154

Here, we have sodium rushing into the cell, then, the sodium channels close. The potassium channels open.2162

And potassium is going to go down its concentration gradient and leave. Potassium leaves the cell, so potassium rushes out of the cell.2172

During depolarization, we get sodium in.2184

During repolarization, potassium rushes out taking that negative charge with it, bringing the membrane potential back down, down to the negative range.2188

However, potassium continues to leave the cell not just until it hits this resting potential, but it does what is called an undershoot.2200

And at that point, there is hyperpolarization.2212

The cell has not just repolarized, it is actually gone past its hyperpolarized, and this section of the curve is called the refractory period.2217

And you cannot initiate another action potential during that time or during the latter part of it.2230

It is harder to initiate a potential, although, you maybe able to initiate it.2237

The reason that you cannot initiate an action potential during the refractory period is that the sodium channels remain closed.2241

And then, what happens is the sodium potassium pumps get to work and return things to normal, to baseline.2255

They cause return to the resting potential.2266

Initially, we had a stimulus triggering some entry of sodium into the cell.2273

So, the membrane potential becomes less negative, and then, if it hits threshold, the membrane potential's threshold is right around -50,2278

the voltage-gated sodium channels will open, and then, there will be a massive influx of sodium into the cell.2290

It will peak. The sodium channels will close, and then, the potassium channels will open.2299

There will be a massive influx or outflow of potassium from the cell. The membrane potential drops.2305

It undershoots. The cell is hyperpolarized.2313

That is the refractory period, a time during which an action potential cannot be initiated at all, or at certain points, it can be, but it is more difficult.2316

And then, as the sodium-potassium pump gets all this sodium back out and potassium back in, puts things back to where they were,2327

the cell will return to its resting potential state, and then, another action potential can be initiated.2336

The next thing is to talk about how action potentials are transmitted along the axon, and I am going to just draw a schematic.2346

Let this represent the axon.2357

A stimulus initiates the action potential at first, but then, that action potential initiates an action potential in a nearby segment of the membrane,2364

which initiates an action potential in the next segment and then, in the next segment.2377

This is often compared to having a row of dominos, and you push the first domino. That is the initial stimulus.2381

And then, that domino causes the next one to fall and the next one and the next one, and that is how an action potential is propagated along the axon.2387

So, remember at rest, what we are going to have is a relatively negative charge inside the axon and a relatively positive charge outside the axon.2396

The action potential comes along and switches that, so now, we end up with a positive charge inside and a negative charge outside.2409

The depolarization of the cell that causes this action potential is, then, going to cause depolarization in this nearby segment,2421

which will initiate the action potential there, which will open all those sodium channels and cause major depolarization, and that action potential gets going.2433

Meanwhile, this one is recovering from the previous. It has become repolarized, and it is returning to its resting state.2447

And now, it is going to be back at resting potential.2455

So, one action potential initiates another and so on along the cell, and in that way, the signal is propagated along the axon.2459

Now, the speed of conduction of an action potential is dependent on a couple of factors, so speed of conduction.2480

Larger diameter axon conducts more quickly. Myelinated axon conducts more quickly.2493

So, the larger the diameter, the less resistance there is. That action potential would be conducted more quickly along.2531

The other thing that can help is insulation.2539

Recall that Schwann cells produce myelin and the myelin sheet around the axon.2542

We have this long axon, and if it is myelinated, that serves us an insulator.2549

Let's say this is myelinated, and in between the myelinated segments are the nodes of Ranvier.2565

Conduction works like this on myelinated axons.2577

What happens is the electrical current moves quickly through these myelinated sections and then, initiates an action potential in the node of Ranvier.2582

The only place that there are the voltage-gated ion channels in a myelinated cell are in the nodes of Ranvier.2591

They are not exposed here, or they are not found in myelinated segments.2600

The current is going to connect quickly along these myelinated segments.2607

And then, when it gets to a node of Ranvier, it is going to initiate an action potential.2610

Current will travel through here, initiate an action potential and so on.2614

And so, it seems like what is happening is the signal is just jumping from one node to another to another.2617

And that is called saltatory conduction where what we have is saltatory conduction.2623

It is the action potential skips from one node to the next rather than being transmitted right from2630

one section to the next, to the next, to the next, as it would if the myelinated sheath were not there.2650

Alright, we talked about conduction of the action potential along the axon.2656

The dendrites receive a signal. The action potential is initiated at the axon hillock, and then, it is propagated along the axon.2662

Here, we have a presynaptic cell and postsynaptic cell, and this communication between the two that is the synapse.2672

So, the action potential is traveling along here. It gets to the synaptic terminal, so this end-region is the synaptic terminal.2687

Finally, we have this space right here. This is called the synaptic cleft.2702

Alright, the action potential travels along the axon. It gets to the synaptic terminal, and it causes the release of neurotransmitters.2711

In these vesicles are neurotransmitters, and they are already packaged in vesicles. They are ready to go.2721

And when the action potential comes along and depolarizes the synaptic terminal,2729

these vesicles will fuse with the cell membrane and release neurotransmitters into the synaptic cleft.2735

Action potential travels to the synaptic cleft - excuse me - synaptic terminal. This causes exocytosis of these little packets of neurotransmitter.2747

The neurotransmitters diffuse across the synaptic cleft to receptors on the post synaptic cell where they bind to the receptor and trigger a response.2774

So, what you should note is that when we were talking about action potentials,2809

we were talking about the communication or a transmission of an electrical system.2813

Here, the electrical signal has been converted to a chemical signal.2819

So, electrical signal of the action potential is now, converted to the chemical signal via the neurotransmitter.2826

To add a little bit more detail, calcium plays a role in mediating the release of neurotransmitters.2835

What happens is that depolarization of the presynaptic cell at the synaptic terminal causes voltage-gated calcium channels to open.2842

So, depolarization results in the opening of voltage-gated calcium channels, then, there are many of these channels in the synaptic terminal.2862

And then, here, we are going to have calcium enter the cell through these channels.2878

And it is the calcium, the increase in calcium level that triggers the release of the neurotransmitters.2885

So, increased calcium in the synaptic terminal triggers the release of the neurotransmitters.2895

The neurotransmitter, then, can do one of several things.2923

It can either just diffuse a way, diffuse out of the synaptic cleft because the neurotransmitter, if it just stayed here,2927

it would continue to stimulate the postsynaptic cell and generate a response from it.2934

And instead of it just sitting here and continuing to do that, it is going to either diffuse away, so removal of neurotransmitter from the synaptic cleft.2940

It can diffuse away. It can be broken down by an enzyme, or it can be taken up by the presynaptic cell.2951

So, in some cells, the neurotransmitter is taken back up, repackaged and then, reused by the presynaptic cell, so it is recycled.2968

In others, it just diffuses away, and then, there are esterases- enzymes that breakdown neurotransmitter.2978

And the response that this neurotransmitter binding to the cell will cause depends on the particular cells involved and the neurotransmitter.2986

The result could be to stimulate the cell or inhibit the cell, and that differs from situation to situation.2996

Some examples of neurotransmitters: one is acetylcholine; serotonin is another; dopamine.3006

There are many kinds of neurotransmitters.3026

Acetylcholine acts at the neuromuscular junction, so it is released by a neuron and then, stimulates the contraction of a muscle cell.3029

There is an enzyme - just to give you an example - called acetylcholinesterase.3039

So, you just take acetylcholine and add esterase, acetycholinase - excuse me - acetylcholinase, and that will breakdown the acetylcholine in here.3043

So, again, just to give you a review before we move on, the action potential is propagated along the axon.3060

It causes the depolarization of the synaptic terminal resulting in the opening of voltage-gated calcium channels, and that causes the influx of calcium.3071

The increase in calcium concentration triggers the fusion of vesicles containing neurotransmitters with the cell membrane.3088

Those neurotransmitters are released into the synaptic cleft.3096

They diffuse over to the post synaptic cell where they can bind receptors and illicit a response.3101

So, that is how the electrical signal is converted into a chemical signal at the synapse.3109

Now, as I said, we are going to revisit the topic of the organization and structure of the nervous system now that you3117

have the terminology that you need to understand focusing on the central nervous system, the brain and the spinal cord.3123

The brain, just giving you an overview, it has many twists and convolutions, and those increase the surface area of the brain.3130

The brain and spinal cord are covered with connective tissue called meninges.3140

So, meninges is a connective tissue that covers the brain and spinal cord, and you may have heard meningitis; so that is inflammation of the meninges.3148

This is a schematic showing you the major parts of the brain.3171

The cerebrum is the forebrain, and it is divided into left and right hemispheres; so there are left and right hemispheres.3178

So, we are looking at a side view, so we are just seeing one hemisphere, and there is a thick band of tissue called the corpus callosum.3193

And the corpus callosum connects the two hemispheres. It is very important because it allows for communication between the two hemispheres.3202

The surface of the brain, of just the overall structure is consisting of grey matter.3219

So, the surface of the cerebrum is grey matter in the brain, and deeper in is the white matter.3228

Grey matter consists of cell bodies, dendrites and some unmyelinated axons.3239

Deeper in below the surface is the white matter, which consists of bundles of myelinated axons.3256

Alright, in the cerebrum are voluntary activities and thought or cognition, so voluntary activities. This is what allows you to consciously think.3267

Sensory input is processed here.3283

Speech, emotions, personality, motivation, memory, your ability to think, reason, plan and make decisions is all located in the cerebrum.3286

And for example, if somebody gets an injury or a tumor to the frontal lobe, an area of the cerebrum called the frontal lobe,3303

then, what can happen is there might be a personality change and because this is an area that controls personality.3311

So, somebody who is very laid back and, kind of, calm and collected before might become more volatile or agitated, act differently than before.3321

The cerebrum is divided into four lobes, so cerebral lobes.3332

It is divided into left and right hemispheres and the lobes, which are the frontal lobe, the parietal lobe, the occipital lobe and the temporal lobe.3343

The next section of the brain, structure of the brain we are going to talk about is the cerebellum.3360

The cerebellum is very important in coordinating movement.3367

And if a person has an injury to the cerebellum, they might walk in a certain, kind of, staggering off-balanced way, and it is called ataxia.3373

Cerebellum is very important for balance and in the coordination of movements.3385

The brain stem controls some of the basic functions of life, and the brain stem consists of three parts: the medulla, the pons and the midbrain.3390

Recall that - let's over here write this out - the medulla in the respiratory section,3411

I talked about how the medulla is one of the structures responsible for regulating breathing, so regulates breathing.3417

It also regulates the heart rate and the amount of constriction that blood vessels have, vasoconstriction versus vasodilation.3426

I will just put vasoconstriction, so constriction or non-constriction of blood vessels.3438

The next structure in the brain stem is the pons.3444

And the pons is responsible for relaying information from the cerebrum to the cerebellum- relays info from the cerebrum to the cerebellum.3447

It also helps in the regulation of breathing.3465

The midbrain plays a role on various functions, for example vision, auditory function, so various different roles and some of the sensory functions.3475

Oops, thinking about test, this should be midbrain not midterm.3497

OK, next, structure that you should be familiar with is the thalamus.3502

The thalamus, which lies outside the cerebellum and brainstem, relays sensory information to the cerebrum.3510

So, it relays sensory information to the cerebrum. The information passes through the thalamus, goes up into the cerebrum.3520

We talked in detail about some of the functions of the hypothalamus when we talked about the endocrine system.3537

And we are going to talk about a couple more right now.3544

So, remember that the hypothalamus, located right around here, regulates the anterior pituitary. It also has a very important role in homeostasis.3548

It helps with the regulation of temperature, hunger, thirst, so a very important function of maintaining homeostasis, regulates the anterior pituitary.3561

And recall that the hypothalamus also produces oxytocin and ADH hormones stored in the posterior pituitary.3575

There are cavities in the brain called ventricles, and these are cavities that contain cerebrospinal fluid.3591

Arterial blood is filtered to form CSF or cerebrospinal fluid.3610

And what the cerebrospinal fluid does is it supplies the brain with substances such as nutrients, and it also helps to cushion the brain, OK?3616

So, these are some of the major structures found in the brain.3625

The spinal cord serves as the connection between the peripheral nervous system and the brain.3630

Here, it is the opposite set up in terms of grey and white matter as it was in the brain. Here, the white matter in the spinal cord is outside.3639

It is on the surface, and grey matter is deeper. It is on the inside.3648

I will just say outside and inside or surface and deeper.3655

So, there is sensory input. There are sensory stimuli, and that is sent to the spinal cord.3663

And then, it goes up to the brain, and then, there is a response that goes out via the motor neurons.3672

Remember, there are different types of neurons, and the signals travel into the3688

spinal cord via sensory neurons and then, out of the spinal cord via the motor neurons.3694

Now, many most actions are initiated in the brain.3702

However, there are reflexes for when we need to do a really fast response, and the injury could happen if we had to wait for the brain to process it.3706

So, there are reflex arcs, and the best way to understand this is through example.3716

Reflex arcs are actions that are initiated by the spinal cord without even having to wait for the brain to figure out a response.3722

Let's say that you touched a hot iron. The information from the receptors in your hand are sent to the spinal cord via the sensory neurons.3731

So, the sensory neurons bring the information to the spinal cord. This sensory neuron will, then, synapse on the motor neuron.3748

Motor neuron causes a response. The sensory neuron will bring the information to the spinal cord.3770

The motor neuron will stimulates the response, and that response would be is to stimulate certain muscles to contract so that you draw your hand away.3786

Interneurons maybe involved, as well. Interneurons recall, they draw the connection between the sensory and the motor neurons,3796

So, it might, then, go sensory to an interneuron and then, motor neuron, and part of what the interneurons can help do is also sent inhibitory responses.3810

So, to grab your hand away certain muscles need to contract.3825

Those need to be stimulated, and others need to relax so that they do not oppose that movement.3828

And the interneuron will help to send out that inhibitory response.3833

The knee jerk response is another reflex arc, so when the doctor taps just below your kneecap, that causes a stretch in the tendon.3837

And then, what ends up happening is that it gets sent to your spinal cord, and it creates a contraction that will cause you to kick out your leg.3847

And that is just a reflex. It is not controlled in your brain.3858

Now, at the same time this is happening, the interneurons may also help to send the information up to your brain about you have just touched3862

something hot, and more responses might follow initiated by your brain; but that initial very fast response is part of a reflex arc.3868

Now, we are going to go on and review the nervous system by doing some practice questions.3877

Example one: what are the two divisions of the autonomic nervous system and describe the function of each.3882

The two divisions of the autonomic nervous system are one the sympathetic nervous system, and the other is the parasympathetic.3890

The sympathetic is responsible for the fight or flight response.3900

This is a response that is going to result in an increase in heart rate, increase in respiration, increase in blood sugar, blood sent to the skeletal muscles,3910

the pupils will dilate, so pretty much a response that allows a person to run away or fight, somehow respond to a threat.3929

Parasympathetic is often called, known as the rest and digest response. It returns things back down to calm.3938

Blood is shunted to the GI tract, so you can digest your food, which you would not want to waste energy digesting your food if you run away from something.3946

So, The heart rate will decrease, respiratory rate will decrease, so the opposite response from the sympathetics.3957

Which structure in the brain is responsible for regulating breathing and heart rate?3970

That is the medulla. Although, the pons does also help to regulate breathing.3976

Example two: describe the events at the synaptic terminal that lead to the release of neurotransmitters.3984

Remember that we have this synaptic terminal, and then, we have our postsynaptic cell with receptors.3991

And what happens is action potential travels along the axon, and it depolarizes the cell membrane in the synaptic terminal.4000

That triggers the opening of these voltage-gated calcium channels- voltage-gated calcium channels open.4021

As a result, calcium enters the cell, so calcium enters the cell.4038

The increase in calcium, increase in the calcium concentration causes exocytosis of a neurotransmitter4045

of these neurotransmitters already packaged in vesicles waiting to fuse with the cell membrane.4066

They fuse with the cell membrane and are released and then, can bind to receptors on the postsynaptic cell.4071

Which voltage-gated ion channels are open during section one of the action potential below? So section one, I am going to mark as this section.4082

Which voltage-gated ion channels are open section two? Section two, I am going to mark as this section right here.4097

We see a typical action potential, and during this section one, what is happening is the depolarization is occurring.4108

And depolarization is the result of the opening of sodium channels, voltage-gated sodium channels.4116

Positively charged sodium ions enter the cell membrane potential, more and more positive.4129

It peaks, and then, here, during section two which is repolarization, voltage-gated potassium channels open, and the sodium channels closed.4136

The result is that the potassium is going to leave the cell, bringing negative charge with it4148

and cause the cell to repolarize and even hyperpolarized as the positive charge leaves the cell.4157

Example four: label the following structures on the figure below starting with axon.4167

Axon carries the action potential away from the cell body, so the action potential goes along. It goes down the axon.4173

Cell body: the cell body is where the nucleus and organelles of the neuron are located.4184

Axon hillock is where the action potential is initiated.4191

If the stimulus is strong enough, the incoming stimulus then, an action potential will be initiated here and propagated away from the cell body along the axon.4196

Dendrite: dendrites receive an incoming stimulus. They project out from the cell body.4207

So, we did axon. We did dendrite, cell body, axon hillock and finally, synaptic terminals here at the end of the axon.4218

And these are the site of the release of neurotransmitters.4227

So, that concludes this lesson on the nervous system here at Educator.com.4232

Thank you for visiting.4237