© 2010
  University of Idaho
  All rights reserved.
  Psychology Dept.
  University of Idaho
  Design - P&D  CTI

As we can see in slide two, the neuron at rest contains a variety of different ions at different concentrations.  As you can see on the outside of the membrane, there are large amounts of sodium ions balanced by chloride ions, while on the inside you have large amounts of potassium ions balanced by structures that are within the membrane of the neuron  These are collectively called anions.

Well, how do we measure this, and how do we know what’s going on within the inside of the system?  Well, the best way that we have developed is to take the axon from the giant squid Aplasia.  What we do is we place this neuron in a sea water recording chamber.  We then take a microelectrode and stick it inside of the axon.   We take another wire and stick it into the chamber of the solution.  What we can do is determine the difference between the inside of the axon and the outside.  As we can see in the following figure, this is what is called the resting membrane potential.  Basically it’s the concentration or the difference in voltage between the inside and the outside of an axon membrane.  So, let’s go back to that picture again and look at it for a real quick second.  What we normally see, if we look at the voltmeter, is a voltage that usually ranges about minus 70 millivolts.  This can vary, though, depending upon the width of the axon and where it’s located.  Usually, it’s a little more positive than that.  So a resting membrane potential is about a minus 65 or minus 60 millivolts.  However, again it varies, depending upon where it is and the width of the axon in and of itself.

Now there are several major concepts that we need to discuss when we talk about this axon and how it’s going to work.  These are shown in slide five.  The first concept is what we call influx.   Influx is generally defined as material moving into some membrane, whatever that may be.  In this case within the axons it may be ions.  Efflux, on the other hand, is basically where material moves from the inside of a structure to the outside of the structure.  In this case a membrane.  Finally, we can have equilibrium where material basically is the same on both sides of the membrane or both sides of a structure.

Well, why do ions move in these neurons, in these axons, etc?  They move for two different reasons and this is shown in slide six.  The first reason that ions move is because of concentration differences.  In essence compounds of any type move from higher concentrations to lower concentrations.  So if you stick some food coloring into a clear water glass, initially there’s high concentrations and it’s real dark at one particular spot where the drop of food coloring has gone into the water.  But over time, it diffuses out and basically becomes neutral and basically all then over a period of time looks the same.  So that’s a concentration difference.  The other reason that ions move is because of what is called electrostatic pressure.  This works similar to a magnet where “like” charges repel each other and “opposite charges” attract each other.

So now that we have these concepts clarified a little bit, let’s talk about why there is a resting state.  Why do you get this difference between the inside and the outside of an axon and the voltage potential being about a minus 65mv?  Well the first thing we need to know, as we see in slide seven, is that the membrane of a neuron is selectively permeable to different types of ions. It’s permeable to sodium, to potassium, to chloride and to calcium.

At rest, potassium ions can leave the inside of the axon and go out.  Basically potassium ions can cross at will.  Generally they cross because of the concentration gradients and a little bit of electrostatic pressure.  However, while potassium ions can leave really easily, few sodium ions can come to the inside.  So, what you have are higher concentrations of potassium ions on the inside and fewer on the outside; or you have very few sodium ions on the inside and a lot on the outside.  As a consequence of this, the outside is going to be more positive than the inside.  Ultimately the exterior of the nerve cell is more positive than the inside of the axon, and as a result of that, the inside is much more negative. 

So as we can see in slide nine here, what we have are high concentrations of sodium and chloride on the outside of an axon and high concentrations of potassium and structures (anions) on the inside.  So you have this differential of electrical voltage (minus 70 millivolts ).

Slide 10 shows this in a pictorial diagram where you have high concentrations of sodium balanced out by chloride and low potassium on the outside, and high concentrations of potassium and ions and low amounts of sodium on the inside.  Again slide 11 is a picture showing that process.

Well in addition to that, we also have channels that are in this axon membrane.  For us, there are two types of channels or pores for the different types of ions.  These are called passive channels or voltage-gated channels.   So let’s talk about each of these for a minute in a little bit more detail.

Passive channels, as we can see in slide 12 and moving on into slide 13, are basically open all the time.  Consequently they allow ions to go through the membrane of the cell from the outside to the inside.  However, let’s examine one other concept while we’re here.  These channels are also ion particular.  For example, a sodium ion cannot go through a potassium ion channel and a potassium ion cannot go through a sodium ion channel as well.

So, what we see here (slide 14) is that you have lots of passive channels that are open in an axon and you also have some passive sodium channels.  But since they’re selective, you see different rates of ions going through the channels at different rates.  What you usually (about 2/3s of the way down) here, is 12 potassium ions going back and forth across the membrane to every one sodium ion.  So, we’ve having some ions that are moving out there, but not a lot, ok.  If you’ve get lots and lots of movement you go to equilibrium very quickly.

The other type of channel that we talk about is what we call voltage gated channels.  Voltage gated channels are extremely, extremely important and they are needed for the action potential to occur.  Again, these channels are ion specific, and they open at some level of depolarization.  That is, some level of voltage change.  As a result, you get changes when they open and when they close. 

Now there are two different types of gates that are occurring in voltage gated channels.  These are shown in slide 16 with the example of a sodium voltage gated channel.  As you can see, there are two gates; an active gate and an inactive gate.  At rest, the active gates are usually closed and the inactive gate is usually open.  However, when you get depolarization or you get a charge or a change of 15 millivolts (mv) the active gate will open and sodium will enter to the inside of the cell.

The steps of an action potential and the way this works is shown in slide 17.  Initially you’re going to get some kind of depolarization and if this depolarization reduces 15mv, that is it goes from a minus 60mv to a minus 45mv, the active gate will open, the sodium will enter, and you get the action potential.  However, the most important thing here is that if it doesn’t depolarize 15mv nothing happens; the gate remains closed, and the charge basically diffuses out and goes away.  We’ll talk about this in more detail a little bit later.  So, what you get is an all or nothing type of phenomenon.  The thing that’s most important about this is that this occurs at the axon hillock.  So, this is where you start seeing it first, and this process will cause you to develop the action potential that will travel down the axon.  However, if the gate doesn’t open, if the depolarization or becoming more positive does not occur, then nothing happens.

Well, what about a potassium voltage gated channel, how does that work.  Well, it works the same way (as you can see in slide 18).  Again, it has an active and inactive gate but in this case, the active gate is going to open to the outside, rather to the inside.  So, again, the active gates are usually closed, and again with depolarization (that is the inside is going to become more positive) the active gates open

So in slide 19, what we see is that the potassium voltage gated channel is very much similar to the sodium voltage gated channel and they basically work the same way.  The inactive gates are going to close based on a couple of things:  One, how much depolarization is occurring (which we’ll talk about in a little bit later) and Two, they seemed to close automatically as well ( but they don’t close instantaneously; it takes a little while for them to close).  So in you’re going to have is a change in the polarity, and a change in the amount of the potentials that are going on inside the different axons that are sending action potentials.

Well in the next section we’re going to continue on discussing these concepts.  We are going to talk about passive potentials, the action potential, and how they all tie together.  So until then, keep studying hard and have a great day.