Have you ever tried to catch a falling ruler? Specifically, as part of a reaction time lab? It’s a pretty popular lab; you’ll find all kinds of different versions of this lab online. The idea of it is basically that you drop this ruler and you try to catch it and you can use it to get a rough calculation of your response time based on how long it fell before you caught it. It goes into reaction time. How long it takes you to react to some stimulus. I remember as a kid being fascinated by this. And wondering –how can we respond to things so quickly? I see the ruler drop and then the muscles in my hand try to grab it and --- how do my brain cells and muscle cells respond so quickly? And a huge part of that answer? Action potential. Action potentials are something you find in what you call excitable cells. An excitable cell is a cell that, from a stimulus, can generate an electrical signal. Excitable cells include neurons and skeletal muscle cells, which would both be directly involved in the reaction time with the ruler. The action potential is an electrical signal that excitable cells can generate. It can also be sent to a neighboring cell, if the excitable cell does that. So, how to explain how the action potential works; how is a cell able to generate and potentially send an action potential? We’re going to need to fill you in on a little background information first. Ok, so you know all cells have membranes – membranes are critical in controlling what goes in and out of the cell. Including controlling what ions go in or out. Ions are charged particles and they generally don’t get through the cell membrane by themselves. They typically need proteins of some type where the ions can passively flow through or, if the ions are being transported against their gradient, they typically need a protein pump to help. We have a video on the sodium potassium pump – highly recommend watching that video before this one – that sodium potassium pump is critical in getting cells to what is called their resting membrane potential, which we’ll explain in just a bit. The sodium potassium pump, with the help of ATP, moves ions against their concentration gradients, and as the name of the pump would imply, those ions it moves are sodium ions (Na+) and potassium ions (K+). Both of those ions are positively charged ions and the pump moves more sodium ions out than potassium ions in – 3 sodium ions out for every 2 potassium ions in. If only looking at the sodium and potassium, that would make more positive ions outside the cell than inside. But, also, there are a lot of negatively charged ions and negatively charged proteins inside the cell. All of this contributes to the cell being more negative inside compared to the outside, when at resting membrane potential anyway. But at this resting membrane potential state, I don’t want to give a misconception that the ions just sit there once they’re pumped – because they don’t - there are some membrane ion channels that are a bit leaky. Some sodium ion channels leak – and because it’s higher sodium concentration outside the cell – the sodium ions passively want to have a net movement into the cell to follow their high -> low concentration gradient movement. And some potassium ion channels leak – and because it’s a higher potassium concentration inside the cell – the potassium ions want to have that passive net movement out of the cell to follow their high->low concentration gradient. Overall, the combo of the slow leaking and the sodium potassium pump maintain the cell’s resting membrane potential and overall, this is steady for the cell. Now we need to talk about membrane potential; what is it? It’s the difference of the electric potential between the inside and the outside of the cell. How would you know what that is? So, you could potentially use a recording microelectrode inside the cell to compare the charge in the cell with the charge outside of the cell using a reference electrode. If the reference electrode – the electrode outside – is set at a reference point at zero then this recording microelectrode can show how the electrical potential changes inside the cell as a comparison. Let’s consider an excitable cell in the brain, a neuron. So generally, at rest, its membrane potential is -70 mV. In this context, what that means is that this neuron is 70 millivolts more negative than its surroundings, the outside. This is its resting membrane potential. We’re going to have a little graph here that has membrane potential (mV) on the Y axis and time on the X axis and this is roughly going to show how the membrane potential changes in an action potential. In the beginning, you’ll see that the cell is polarized because the cell inside is negatively charged compared to its surroundings. Then on this graph, you can see the cell will become depolarized, but how does that happen and what does that even mean? Let’s walk through an action potential using this graph. An action potential is going to require gated sodium channels to open. These are not leak channels, but gated ion channels that will allow sodium to rush through. We’ll talk more about what might have triggered these to open and the different gated channel types later. So where are the sodium ions going? That’s right, they’re following their gradient and rush into the cell. Sodium ions have a positive charge so their entry into the cell is going to make the cell more positive. The effect is that the cell is depolarized because the difference in that electrical potential between the inside and outside of the cell is lessening. The cell is LESS polarized compared to it's surroundings. If and only if the membrane potential reaches a threshold level, which is about -55 mV in this case, an action potential is triggered. This is a really important point because the action potential is considered “all or nothing” - meaning - you hit this target and it starts. It’s on. If you don’t reach it, it’s off. There’s no partial action potential. You don’t have little action potentials or big action potentials; if triggered by reaching threshold level, you get the same action potential. So, action potential starts when that threshold level is reached. We’re now in the rising phase as depolarization will result in most sodium channels opening and thus lots of sodium entering. The membrane potential will reach zero, but it doesn’t stop there, as the sodium keeps entering the cell – thanks largely to the negative proteins in the cell that attract those positive sodium ions. When the membrane potential gets to around +30 mV – meaning it’s 30 millivolts more positive than its surroundings – those gated sodium channels start to get inactivated - so sodium can no longer enter through them- and gated potassium channels open. Potassium follows its concentration gradient to leave the cell. This all results in repolarization which starts to bring it back down to the resting potential of -70 millivolts EXCEPT it actually goes beyond that, and that is called hyperpolarization. A little undershoot if you will, until those gated potassium channels close. Those channels will close and with the help of the sodium potassium pump, which is continually working all this time, we can get back to resting membrane potential. So that may be the overall actions happening in an action potential with this graph but we have 3 major concepts we want to mention before we go. 1. We said “gated ion channels” - what are some different types of those and how do they work? 2. How are these channels involved in actually initiating the action potential? And 3. How are action potentials spread - or if you want to be fancy, how are they propagated? Ok so let’s answer #1 and #2 first. A reminder that an action potential won’t start until enough ions enter the cell to depolarize it to the threshold level. So some initial depolarization must occur before reaching threshold for the action potential to actually begin. Ligand-gated ion channels or mechanically-gated ion channels can respond to a stimulus and open to start this initial depolarization. Ligand-gated ion channels let ions go through only when some kind of ligand - a ligand being a signal molecule of some type - binds to the channel protein. Consider a ligand that is a neurotransmitter and it is released from a neuron to a muscle cell. Ligands (which would be neurotransmitters in this example) would bind to ligand-gated ion channels on the muscle cell’s membrane. That can result in the channels opening and ions rushing in. Then there are mechanically-gated ion channels. These channels can respond to some mechanical action, some type of physical stimulus. For example - touching the skin - which could signal mechanically-gated ion channels on a sensory neuron to open and ions to rush in. Voltage-gated ion channels are big players in the action potential events, and they respond to specific voltage changes. Overall these different types of gated channels are critical in depolarization being able to start and then the events in the action potential being able to happen. To emphasize how important these channels are, there are medical conditions that can be caused when these gated ion channels are not functioning as they typically do. For example, problems with voltage-gated ion channels can lead to epilepsy - which can cause seizures - and this can make voltage-gated ion channels a potential target of medications for this condition. #3: How do action potentials spread or, fancier, propagate? So when we were talking about an action potential, we were talking about it in one spot of the neuron. But in excitable cells like neurons, the neurons have long axons and the action potential needs to spread along this axon. So if we take a neuron - here is the axon. The depolarization in this first segment of the axon can reach threshold which will trigger an action potential. The action potential in this first segment causes a change in voltage that triggers voltage-gated sodium channels in the neighboring region to open. This will result in that neighboring part depolarizing. Meanwhile, the previous region of the axon where the action potential had occurred, will start to repolarize and enter a refractory period, which means for a little while, it cannot be restimulated. By little while - it could be like 2 milliseconds or even less. But it’s important to understand the refractory period happens largely due to the sodium channels being inactivated during the events in the action potential and restimulation cannot happen during that period and that’s important because it sets a max rate of how often a neuron can fire and it keeps the action potential moving forward in one direction. So the action potential travels along this neuron, with the old segments repolarizing and eventually reaching resting membrane potential. This is how the action potential will be propagated along the axon. We do need to point out that many neurons are insulated with myelin and the spread of an action potential in myelinated neurons is a little different - see our further reading in our description. So many things we do every day - moving, thinking, catching falling rulers - depend on excitable cells. And those excitable cells? Well, they shall always rely on the action potential. Well, that’s it for the Amoeba Sisters, and we remind you to stay curious.