Now that we've discussed anatomy of a neuron, we're going to talk about a neuron's main function, and that is to send electrical signals. Remember that neurons are the cells that are electrically excitable. They can send and receive electrical signals, while the glial cells are the support components of the nervous tissue. Now, there are actually two different types of electrical signals that neurons can send, graded potentials and action potentials.
Up until now, We probably haven't mentioned graded potentials unless you've moved ahead and read about them. Now, for any type of potential to occur, though, regardless if it's graded or action, this is going to depend on certain things in the cell. It's going to depend on a resting membrane potential and on ion channels. So, when we talk about a resting membrane potential, Basically what we're saying is that there's a difference in charge across the membrane.
So we can see that here the inside of the membrane, which is where the cytosol is, is going to have a more negative charge, and here the outer side of the membrane, where the extracellular fluid is, is going to have a more positive charge. Now our whole goal with this resting membrane potential is that we need to maintain a more negative inside than the outside. But when we're ready to send signals, we're going to manipulate those charges.
But there's one slight problem, the membrane. We have to remember that the tails of the membrane are hydrophobic, are also known as nonpolar. Basically, they don't like anything with charges. So they're going to reject anything that carries a charge.
So what we have to talk about is how these charges are going to be able to move through the membrane. and as they move through the membrane, how are they going to affect the resting membrane potential, the inside charge. So before we actually talk about how these charges get through the membrane, let's talk about what ions are present. Now, your body is full of tons of different ions, but there are certain ions that are really going to contribute to movement.
So the first one we're going to talk about are going to be potassium. Potassium I want you to think about as kings. So potassium is represented by the letter K.
Notice it has a positive charge. And since it's a letter K, we're going to call it a king. And where do kings live?
Do they live inside a castle or outside a castle? If you said inside, you're correct. So we're going to find potassium in really high amounts on the inside of the cells.
Now naturally, when anything is found somewhere in high amounts, it's going to want to move somewhere where it's in less abundance. So potassium is going to want to move. to the outside.
Now the next one that we have is sodium. Sodium stands by Na plus, so it's a positive charge. And I want you to think of the plus as a T, so it spells the word nat. Now where do we find nat, those little bugs?
If you said outside, you're correct. So we're going to find sodium in high amounts on the outside. Now anytime something is found in high amounts on the outside, it's naturally going to want to move to where it's not as abundant. So sodium is naturally going to want to move to the inside. Now, there are other ions.
There's going to be calcium and chloride. Calcium and chloride are also going to be found on the outside in high amounts. An easy way to remember this is that kings have to have lots of protection, so a lot of people cannot be around them. So calcium and chloride are basically going to be rejected from the inside, so they're going to hang with sodium on the outside.
But since they're in high amounts on the inside, they're also going to naturally want to move to the... Since they're in high amounts on the outside, they're going to want to move to the inside. Now remember, all of these ions contain charges, either positive or negative.
So these charges are going to contribute to that resting membrane potential, the more negative inside than the outside. Now the problem is these charges cannot move through the membrane on their own. So they are going to depend on ion channels. So if you remember, we said that the production of the signal depends on two things, resting membrane potential and ion channels.
So how do we know what the inside is? Well, we just basically measure it with an electrode. So you can stick an electrode through a membrane and it's going to tell us that the inside is more negative than the outside. And if you notice, the inside of a membrane when resting is negative 70. So we're going to talk about a number line and what happens with movements. Resting membrane potential can be lots of different numbers.
Every neuron in the body is going to have a different number, but if we look at all the restings in the body and take the average, the average is most likely going to be negative 70. So we're going to use negative 70 in this class as our resting. Now resting doesn't mean that nothing's going on. What resting means is that the inside is more negative than the outside and there's no signals being sent.
Now our goal though is to send a signal. We have to manipulate these ions where they're located by moving them into or out of the cell. And remember, ions carry charges, so when we move these charges in and out of the cell, it's going to affect the number. So our goal, though, is to eventually get to negative 55. Negative 55 is special because it's called threshold.
And threshold is where a new action potential will begin. Now, once we hit threshold, that means we're going to send a new action potential, and this is actually going to be a more positive movement. If you look at negative 70 to negative 55, negative 55 is getting closer to zero. So does that mean we're getting more positive or more negative?
If you said more positive, you're right. This event of becoming more positive is called depolarization. So our goal to send a new signal is to depolarize a neuron. But a new signal will not be sent unless we hit threshold, which is negative 55. But sometimes, depending on the movement, negative 70 will actually become more negative and go towards negative 80, negative 90. If we get more negative, we're going to call this hyperpolarization.
So if the inside of the membrane becomes more positive, moves closer to negative 55 to 0 to 30, that's depolarization. If the inside of the membrane becomes more negative towards negative 80, negative 90, that's hyperpolarization. So let's discuss on what can cause something to become depolarized or hyperpolarized. So we're back to this picture, and remember that the tails are hydrophobic, so they don't let these charges in.
So we will need ion channels so these charged ions can move through. But once we have these channels... depending on where the charged ions move, is going to change what happens to the membrane's potential. So the first ion that we talked about is potassium. Now remember that potassium carries a positive charge, and it's naturally going to want to move to the outside of the cell.
Now if the inside is negative 70 and potassium starts to exit, we're going to be losing positive charges. So what's that going to make the inside of the membrane do? Is it going to go more negative or more positive? If you said more negative, you're correct. So if potassium exits the membrane to the outside of the cell, we're going to go more negative, and then this is going to be called hyperpolarization.
What happens if sodium moves into the membrane? Look what sodium carries. Sodium carries a positive. If sodium moves into the cell, what's going to happen?
We're adding positives to the negative 70. So if we add positives to the negative 70, it should become... more positive or more negative? If you said more positive, you're correct. This is called depolarization.
Okay, but remember we also have calcium and chloride. Okay, calcium carries positive charges just like sodium. So if calcium moves into the membrane, the same thing is going to happen.
We're adding positive charges. So that's also going to be depolarization. But let's talk about chloride. If you look at chloride, chloride carries a negative charge.
And when a negative charge gets moved into the membrane, we're adding it to negative 70. So negative 70 is going to get more negatives. It's going to become more negative. And so that's also hyperpolarization.
So to recap, depolarization can be caused by the influx of sodium and calcium. Hyperpolarization can be caused by the influx of chloride or the efflux, the exiting of potassium. Knowing where the charges are and knowing which way they'll move helps us decide if something's going to become more positive through depolarization or more negative through hyperpolarization. Remember that these ions cannot move through the membrane.
What they have to do is they have to move through different types of proteins. There's actually going to be four different proteins that we're going to talk about. The first type are going to be called channels because they are going to basically open and close randomly.
And when they open, ions can move through. When they close, ions cannot move through. Leaked channels are going to be found over the entire neuron. So we're going to find leakage channels in the neuron's dendrites, the neuron's soma, and the neuron's axon. Now remember these channels are embedded in the membrane, so we'll find them throughout the entire neuron.
Now leakage channels, there's going to be two major types. We have sodium leakage and we're going to have potassium leakage. There are going to be more potassium leakage channels than sodium, and this is going to be important in just a little bit. So leakage channels, we have more potassium than sodium. They're found over the entire neuron, dendrite, soma, and axon.
And one of their main functions is to help contribute to a resting membrane potential. They help contribute to the RMP. Now the next one is going to be ligon gates. We've talked about ligon gates previously when we talked about muscle contractions.
Now ligon gates are going to be gates that open in response to a chemical. The chemical can be a hormone or it can be a neurotransmitter. So there are two different types of ligon gates.
The ligon is the chemical. The first type is going to be ionotropic. If we look at this picture, what we see is we see that the receptor for the ligand is on the same protein as the channel for the ligand is. So here is the receptor. Here is the channel.
They are located on the same protein. So once the... Ligon, which is the chemical, binds to its receptor. The channel now opens and ions can move in or out. Ionotropic, basically there's only one protein channel.
Now look at this picture. These are metabotropic. If you notice, there's two different proteins in this picture.
This first protein is the one that has the receptor for the ligon. The second protein is the channel. Once the ligand binds to the protein, it's going to send a signal, so it's going to send a message to the channel, and notice that the channel opens so material can move in or out. Ionotropic has one channel that contains both the receptor and the channel. Metabotropic has two channels, or two proteins.
One protein contains the receptor, the other protein contains the channel. Now where do we find ligon gates? We're going to find ligon gates only on somas and dendrites.
The only place we find these gates that open to chemicals are somas and dendrites. Now remember we're talking about neurons so we only find them on somas and dendrites and neurons. Can anyone remember where we found them on muscle cells?
On muscle cells, we find ligand gates on the sarcolemma, but on neurons, we find ligand gates on the somas and dendrites. Our next gate are mechanical gates. Mechanical gates are going to be located on sensory neurons. They're only going to be located on somas and dendrites, and these gates open to some type of stimulus, such as touch, pressure.
They're going to open to Audio they're going to open to light. So some type of physical stimulus will open these gates. Now since everything I named helps us detect different sensations, that's why we say they're only found in sensory neurons and they're only going to be found on the neurons, somas, and dendrites. The last gates are voltage gates, which we've also talked about before with the muscle cells when we talked about NMJs.
Voltage gates are gates that open to some type of change in charge. So if we look in this picture, what we see is that the cell is at resting negative 70, and we can see that the gate's closed. All of a sudden, there's a change in the charge of this cell to negative 50, and now the gate is open. So voltage gates are going to open to a change in charge. We only find voltage gates on the axon.
And there's going to be different types. The great thing about voltage gates is that they're named for what they let through. So on the axon part that's green, we're going to find both sodium and potassium voltage gates. But on the synaptic end bulbs, we're going to find calcium voltage gates.
there's still voltage gates. They're still found on the axon. It's just on the thread portion in axon terminals, we find sodium and potassium voltage gates.
And then on the very end, the little swellings, the synaptic end bulbs, we find calcium voltage gates. So right now, the cell is at resting. For us to manipulate the cell, we're going to have to move these ions. either into or out of the cell.
The only way to do that is to use channels and gates. So in review, the leakage channels are going to be found over the entire cell. There's more potassium leakage channels than sodium, and they're going to help contribute to resting membrane potential.
On the somas and dendrites, we're going to find ligand gates and mechanical gates. And then on the axon, we're going to find voltage gates.