Step 2: The Resting Potential
What gives a neuron its unique functionality?
The ability to use electrochemical gradients to communicate over long and short distances very rapidly.
What enables neurons to create electrochemical signals?
The creation of a concentration gradient coupled with an electrical gradient due to the differential segregation of certain ions across the neuron’s membrane.
Ions are charged particles, in neurobiology the important ions are Na+, K+, Cl- and Ca+2. They cannot pass through the cell membrane unless a channel or a pump provides a passage-way. The channels that we will examine here are all highly specific as to the type of ion that they let through, i.e. there are channels for Na+, and there are separate channels for K+. There are also different types of channels. The 4 of immediate concern are:
One neuron can possess thousands of these channels, as well as energy-requiring pumps that move ions against their electrochemical gradient across the membrane. Through combinations of pumps and channels, two important gradients: electrical and chemical, are established and maintained. Watch the steps involved in the creation of an electrochemical gradient across a neuron's membrane in the following animation.
You'll need the Shockwave plug-in, but first see if you can watch the animation below to find out if the computer you are using already has this plug-in. If not, click on the Shockwave plug-in link and choose "download" from the choices on the left. Download the plug-in (following the instructions), then install the plug-in (you'll probably also have to restart your browser).
Note the symbol key below before you start the animation:
Notice you can pause and play the movie at any point along the way.
Questions
- Did you see how the pump concentrated K+ on the inside of the cell and Na+ on the outside?
- Since there is now so much more K+ inside the cell than outside, there is a driving force acting on K+ pushing it out of the cell so that its concentration will be equal on both sides of the membrane. Na+ is in the same situation, except the driving force acting on it works to push it into the cell.
- Since there is now so much more K+ inside the cell than outside, there is a driving force acting on K+ pushing it out of the cell so that its concentration will be equal on both sides of the membrane. Na+ is in the same situation, except the driving force acting on it works to push it into the cell.
Neurons have many more K+ leak channels than Na+ leak channels, so more K+ can leak out following its concentration gradient.
- Did you see how when this happened it left a net negative charge on the inside of the cell?
- K+, a + ion leaving the cell and not enough + ions replacing it cause there to be more - charged particles inside the cell relative to the outside of the cell. K+ will leak out of the cell until this negative charge or potential becomes so great inside that now K+ (a positive ion) is electrically attracted to the negativity inside the cell. This equilibrium occurs when the driving force pushing K+ out, due to its concentration gradient, is balanced with the force pulling K+ in, due to the electrical gradient.
- K+, a + ion leaving the cell and not enough + ions replacing it cause there to be more - charged particles inside the cell relative to the outside of the cell. K+ will leak out of the cell until this negative charge or potential becomes so great inside that now K+ (a positive ion) is electrically attracted to the negativity inside the cell. This equilibrium occurs when the driving force pushing K+ out, due to its concentration gradient, is balanced with the force pulling K+ in, due to the electrical gradient.
- Did you see how if you insert a recording microelectrode into a neuron in this resting state (not currently signaling) you record a stable membrane potential of -65 millivolts (mV) which is called the neuron's resting potential.
Watch the animation again and examine the creation of a neuron’s resting membrane potential.
Neurons can be modeled with electrical circuits. Modeling can help in the understanding of how the neuron creates and propagates signals.
Visit this web site and see how much energy the membrane potential provides the neuron.
