Negativity is the natural resting state of your cells. It’s related to a slight imbalance between the charged atoms located inside and outside the cells.
Those atoms are known as ions — and the imbalance we just mentioned sets the stage for your electrical capacity.
Now a lot of the ions in question (not all of them, but ... a lot) are either sodium or potassium atoms. Pay attention to these two elements because they’re about to become very, very important to our discussion.
Both potassium and sodium ions carry a positive charge. And when your cell isn’t transmitting electrical signals, there’ll be a higher concentration of sodium ions outside the cell than inside the cell. On the flip side, you’ll also have more potassium ions inside the cell than outside it.
Overall, the space surrounding the cell is going to have a charge that’s relatively more positive than the space within the cell. So the charge inside this cell will be negative by comparison.
It’s a state of being that scientists call the cell’s resting membrane potential, or RMP.
Meanwhile, the charge difference on each side of the cell’s membrane will establish an electrochemical gradient between what’s inside the cell and the area immediately outside it.
OK, so when a cell is in the RMP stage, sodium and potassium ions are both present on either side of the membrane. Cool beans.
But — how do they cross the barrier? How does an ion enter or exit a cell? Well, that’s where ion channels come in. As the name implies, these are channels located in the membrane that grant passage to specific kinds of ions. (Note: In most cells, the potassium channels outnumber the sodium ones.)
Let’s take a minute to explain how they function. To quote Harvard Extension School’s official YouTube channel, the “difference in total charge inside and outside of the cell is called the membrane potential.” (The term “resting membrane potential” derives from this concept. Go figure.)
Once a cell’s membrane potential changes — once the interior total charge fluctuates in relation to the exterior total charge — that can activate some of the relevant ion channels which are embedded in the membrane.
Many channels only open up and allow the transfer of ions when the cell’s membrane potential has shifted by just the right amount. The formal name for those pathways is voltage-gated ion channels.
Each voltage-gated ion channel will only let a particular kind of ion enter or exit the cell.
Your neurons, which are specialized cells in your nervous system responsible for transmitting information across the body, contain both sodium voltage-gated ion channels and potassium voltage-gated ion channels in their membranes. Capiche?
By letting certain ions enter a neuron from the outside, these channels can alter the cell’s membrane potential. And if enough ions pass through, the cell will no longer be at its RMP.
OK, the last few paragraphs have laid a lot of groundwork. But there’s another term we really need to unpack before we go any further: action potential.
“An action potential is a rapid sequence of changes in the voltage across a membrane,” explains the U.S. National Library of Medicine. Basically, an action potential is a shockwave that comes in two phases: depolarization followed by repolarization.
Let’s say you accidentally touch a hot stove. It happens to the best of us.
Action potentials within your neurons help those vital cells communicate with each other. In this case, the neurons in your hand need to send an important message all the way to your brain about that hot, hot stovetop.
Now remember, at RMP, there will be more sodium ions outside these cells than there are inside them. But the stimuli in our “hot stove” example will provoke an opening of sodium voltage-gated ion channels along the cellular membrane of the closest neuron. Suddenly, loads of sodium ions will come pouring into the cell.
Friends, this marks the “depolarization” state. And it’s a total gamechanger. The rapid increase of sodium ions is going to make the inside of the cell more positively charged than the space surrounding it, which is the exact opposite of the situation we had at RMP.
The sodium influx causes the internal voltage to rise. But that’s only phase one. Next, the neuron enters its “repolarization” phase. With the help of sodium-potassium pumps that eject sodium ions and pull in potassium ones, the cellular membrane reinstates RMP by — once again — making the inside of our neuron more negatively charged than the outside.
Depolarization and repolarization are the one-two punch behind action potentials. Those electrical shockwaves can set off a chain reaction among the neurons, giving your brain a signal to interpret and act upon.
A Well-tuned Network
So there you have it. The secret to those electrical signals which tell your heart muscles to contract and tell your brain — by way of your eyes — that what they just saw is the word “brain.” You know, important stuff like that.
Naturally, any breakdown in your body's electrical system is a real problem.
When you get an electric shock, it interrupts the normal operation of the system, sort of like a power surge. A shock at the lightning level can cause your body to stop. The electrical process doesn't work anymore — it's fried. But that’s a story for another time.
Originally Published: Sep 22, 2008