Action Potentials

The Action Potential

I like this definition of an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission., “Series of predictable changes.”  An action potential goes through several steps. Once it starts step 1, it proceeds through all of them without going back and without stopping in the middle.  These changes in membrane potential are caused by the opening and closing of sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. and potassium(K⁺): Major ICF cation; essential for muscle and nerve function. channelsProtein passages in the cell membrane that allow specific molecules to pass through..  Voltage-gated channels.  Wiggle your big toe.  An action potential is like a wave. It just left your brain. It traveled down your spinal cordThe central nervous system structure that relays signals between the brain and body.. Then it moved out your sciatic nerveThe largest nerve in the body, arising from the sacral plexus.. Finally, it reached your toe muscle.

 That’s a long distance from your brain to your toe.  In factA statement based on direct observation that is repeatedly confirmed., for certain movements, there is one long neuron that leaves the brain and travels to the muscle.  Have you ever held a rope with another person? It is kind of like setting up for jumping rope.  While you’re standing there, you create a wave in the rope. This wave moves through the rope to the person on the other end.  As the wave travels, the rope goes up then down.  That is pretty much the same for a potential.  It move through a neuron instead of a rope.  It involves ups and downs in voltage rather than physical location in space. 

Terminology

We have seen these terms before. We have also seen this graph when we talked about how a motor neuron excites a skeletal muscle fiber.  Now, we will speak about these terms in application to neuronsThe functional cells of the nervous system that transmit signals..  A Skeletal muscle is like a dead end.  It can receive the action potential but it can’t give that action potential to another cell.  Neurons can.

Let’s review.  A neuron sits or chills out at the resting potential, which we will set as -70mV.  It’s negative because of all those proteinsLarge molecules made of amino acids with various functions in the body. in a cell.  This side of the first hill on this graph represents depolarizationThe loss of electrical charge across a membrane, triggering an action potential..  In this process, the cell is becoming more positive.  This results from the influx of sodium.  The cell gains these cations and they make it more positive. 

This side of the hill is repolarizationThe return of membrane potential to a negative value after depolarization..  We Depolarized the cell, so now we have to Repolarize it back to the resting potential.  In repolarization, potassium cations move out of the cell.  The cell is losing positives and thus becomes more negative again.  Hyperpolarization is also defined as the cell becoming more positive.  The difference is that hyperpolarizationAn increase in membrane potential, making the inside of the neuron more negative. is defined as occurring when the cell is becoming more positives BELOW the resting potential.  Therefore, this is also attributed to the opening of potassium channels.

Any and all channels we are referring to here are channels that are voltage gated.  Those are the channels that open in action potentials.  I know, you’re asking, “Amy, but how does the depolarization start if these are all voltage gated?”  Yes.  Depolarization associated with the local potential opens ligand gated channels.  If we can open enough of them, these voltage gated channels will start to open.

Threshold

-70mV is the resting potential. This is the voltage at which the cell is kind of chilling out and living.  Now, all the time, a neuron is getting poked a prodded by other neurons.  A neuron is always experiencing some degree of depolarization. Repolarization and hyperpolarization occur due to the opening of ligand gated channels, not voltage gated.  These little changes in voltage are called graded or local potentials. 

Now if these little local potentials become large enough, they hit a specific voltage value. This causes them to open up voltage gated channels. These channels will become an action potential moving over the axon hillockCone-shaped region of the neuron where the axon originates and where action potentials begin.. It travels down the axon.  The value at which this happens is called thresholdThe minimum voltage needed to trigger an action potential. and we are going to use -50mV as our threshold.  So, this is not the resting potential, in fact, quite opposite.  This is the voltage at which and action potential happens and there is no going back. 

All-Or-Nothing Principle

When a neuron hits threshold, there is no going back, an action potential will happen.  When I lived in a rental in Trumansburg, we had a washing machine that was older than me.  It had a dial that only turned in one direction.  Let’s say you had a wet towel that you just wanted to spin dry.  No.  That washing machine would only let you rinse and spin, not only spin.  You had to use the dial to advance through the pre wash cycle. Then, continue through the wash cycles. Finally, turn the knob to rinse. 

That was the only way it worked.  Action potentials are kind of like this.  You gets all the steps of an action potential, all the time.  There is no going back. There is no stopping at step 2, or 3, or any step that is not the last one.  All the steps of an action potential occur in the same sequence every time.  This is called the all-or-nothing principleAn action potential either fires completely or not at all, depending on whether the threshold is met.  When threshold is hit, the all-or-nothing principle takes over. It forces the neuron to go through all the steps of an action potential.

Steps of an
Action Potential

There are specific steps to an action potential.  Different books, websites, or teachers might provide a different number of steps. However, the sequence is generally the same.  Ket’s just make some hard and fast rules here so that we can then explore them in more detail elsewhere.

Step 1 – the neuron is just chilling out at its resting potential. Suddenly, some neurotransmittersChemicals released by neurons to transmit signals across a synapse. connect to its ligand-gated channels in a dendrite.  These neurotransmitters open ligand gated channels, allowing sodium to influx into the cell.  This depolarizes the cell, making it more positive.  But, this time, A LOT of ligand gated channels open.  In fact, enough to get to and over threshold.  Once threshold is hit, we will increase to about +30 mV. This happens by opening all the voltage gated sodium channels in the area.  So, these first two steps are about depolarization, but the first step involves ligand-gated channels and we are below threshold.  The second step involves voltage gated channels and we are above threshold.

OK, so now the cell has opened up all the sodium channels that it can possibly open.  This means that we are here at the top of this peak.  If another stimulus came, this neuron would be like, “Yeah?  Well. I can’t have another action potential, but all my sodium channels are open.  I couldn’t make the cell more positive even if I wanted to. 

We’ve depolarized the neuron, now we have repolarize it.  Potassium channels start opening, sodium channels start to close.  Potassium cations leave the cell. The extracellular environment becomes more negative to restore the resting potential.  As soon as those sodium channels start to close, the neuron COULD have another action potential. 

Unfortunately, the neuron overshoots resting potential.  At this point, all the sodium channels are closed, but some of those potassium channels are still open.  So, resting potential is missed.  That’s OK.  The neuron is just going to use the sodium/potassium pump to get back to resting potential.  Remember that the pump will return 2 potassium cations to the cell while removing 3 sodium cations.  This imbalance means that the pump can have the overall effect of making the cell more negative.