The Wires in my Head

Today marks the beginning of our review in my neural systems class on the most basic principle of the neuron- the action potential.  It's a surprisingly complex phenomena which most people only understand as "a spike in electrical current the sends the signal down the axon to the terminal end."  You could see a series of spikes drawn in a textbook and anyone with a small background in biology could name it as an action potential.

This is a standard action potential drawn the
world over in physiological and biological
and neurological classes the world over.

But do they understand what it actually is caused by?  In my biopsychology class I was told the neuron receives chemical signals which when there are enough different signals from other neurons (spacial summation) or when it got enough signals within a certain time constraint (temporal summation), the signal would be passed along the axon of the cell in the form of an action potential to be used to spread the signal to the other cells.

So what's actually happening when an action potential occurs?  Well I'm by no means a teacher or a
professional writer but I'll try to explain to the best of my knowledge.  Bear in mind, this subject will be covered in multiple lectures in class so I suspect it will take me several blog posts as well.

Let's just start with the basics shall we?  The first question to really ask is what is an action potential?  Sure we all see the spikey line on a graph but what does it mean?  What you are seeing is actually the membrane voltage of the neuron change in response to the influx and efflux of ions.  Which ions? you might ask.

Well there are actually many ions involved with the regulation of cellular membrane in a neuron.  But for the most part, the work of the action potential is being done by two main ions: potassium and sodium.

Let's start with sodium (Na) since it's what initiates the action potential.  As the cell is receiving signals from the network, it starts to open (primarily) ligand-gated Na channels.  When enough of these are open, sodium -which is normally pumped out of the cell- begins to flood the cell.

Here's where it gets a little tricky.  The important things to remember are:

1. Sodium is being constantly pumped out of the cell (when there's no action potential) which sets up a gradient.  This "makes" sodium want to flow into the cell to put the ion at equilibrium but it must have specific gates open in the membrane to do so.

2. Potassium is being constantly pumped into the cell (except when, once again, the neuron is experiencing an action potential) which also causes an unequal distribution of the ion.  This gradient wants to move in the opposite direction of sodium.  That is, potassium wants to flow out of the cell if it can find the opening to do so.

3. Sodium is more positive than potassium.  Hence, because these two major ions have differing charges a voltage is created across the membrane with the inside more negative than the outside.

This image depicts the traditional sodium-potassium pump
which maintains the cell's membrane potential by keeping most
of the sodium on the outside and the potassium inside.

Now back to our story.  When enough of these sodium channels open, they set off a special kind of sodium channel which is voltage-gated.  That is, when the cell becomes positive enough, tons of these sodium channels open.  This is the upward part of the spike on an action potential.

The cause of the fall of the spike is for the exact opposite reason.  The sodium channels have a time-dependent element which makes them inactivate shortly after opening.  This stops the "depolarization" of the cell which is that upward part.  And as the sodium channels inactivate, potassium channels open up and potassium leaks out.  That changes the cell's membrane back to being more negative than the surroundings.  However, these additional potassium channels usually overshoot (though not in all neural cells) and hence you see the action potential dip below the line it started at.  This is called hyperpolarizing and it's during this period of time that the sodium channels are re-setting themselves for the next spike.  It's often called the refractory period because during this time the cell cannot fire because it's still resetting its various gates.

I know this is a lot of information to try to understand clearly but those are the bare bones of an action potential.  In actual life, there are an unbelievable number of variations on this general theme.  Some cells use chloride ions and some have different types of gates.  The numbers of the different gates vary across cell types and the overall population of these channels is only just being investigated.

I hope I've provided a little clarity on reading and understanding the picture of an action potential for those new to the topic.  I hope my further posts are more interesting and maybe a little more detail-oriented.

If you're more into circuits like some of my electrical engineer student friends,
this might be a little easier to understand. But honestly to me it looks super confusing.