I went to visit my sister at University recently, and found pinned to her notice-board a picture that I drew and posted to her when she first moved in. The picture is a comic strip describing an action potential. An action potential is the series of events that occur in your nerves to transmit messages along them. Clearly I know EXACTLY what the cool kids want on their walls at university.
I've spoken before about how much I love action potentials, and how one of the proteins involved was my first molecular love affair. That protein, the voltage-gated sodium channel, will be appearing the in Really Awesome Protein series soon, because it's a phenomenally impressive molecular machine. For today though, I thought I'd share the comic strip I produced for my sister. And use it to explain how your nerves transmit messages (I should note that I drew the picture, however Emma went out and bought pens so she could colour it in, so all credit for the colour goes to her).
Now I meant it when I said I love action potentials. Learning about them was one of the first things in science that really made me sit up and take notice. The complexity of it, and the speed. When you get your head around it, the speed just Blows. Your. Mind. Truly, stick with me, it will.
To understand how nerves transmit messages, one of the first things you need to know is that your nerves exist in a state of relative negativity. The inside of the nerve cells is negative compared to the outside. The difference between the two, known as the resting potential, is -70 millivolts (mV). This resting potential is maintained by a sodium potassium pump (called, er.. Joe), which actively pumps two positively charged sodium ions (Na+) out of the nerve, each time it lets a potassium ion (K+) in. It does this all the time.
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| "Once there were three proteins living in a membrane |
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| "Joe used to spend all his time trying to keep the resting potential at -70 mV" |
The voltage-gated sodium channel (or Bob, as he is known to his molecular buddies) can sense this negative charge. In general, when the charge is -70 mV, it doesn't do much. Chats to neighbouring proteins, about Glee apparently. However, if it senses that the voltage is changing slightly, it starts to get excited. If it senses the voltage change to the threshold level, which is -55 mV, it gets REALLY excited. What it does at this point is open its gate, and through this gate rushes sodium. Sodium is a positively charged ion (Na+) and so the moment it has access to a negatively charged region, it's heading STRAIGHT THERE. Sodium (and other positive ions) can't bear to see negativity, they need to rush in and get things positive.
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| "But sometimes Bob and Patrick get a bit... excited" |
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| "Bob sense some action in the neighbourhood and lets sodium rush through him" |
The sodium channel has a second gate, one that isn't affected by voltage. Instead, it is operated by time. Nanoseconds after the voltage gate opens, the second gate slams shut, so that the positivity can't get TOO carried away. In the short time it had though, the sodium managed to generate an impressive amount of positivity, rocketing the voltage from -55 mV up to +40 mV. This change in voltage is known as the depolarisation of the membrane.
Following depolarisation, another nearby protein, Patrick the potassium channel, opens. Potassium then rushes out of the cell, forcing the inside of the cell to become negative again.
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| "Patrick wants to join in so he opens himself to potassium" |
In fact, Patrick is a bit over-zealous and lets the membrane potential get too low. This undershoot is known as the refractory period, and during this phase, whilst Joe is restoring normality, no other action potentials can start in this region of the membrane.
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| "Joe tries desperately to restore normality while their neighbours Bert and Paul joined in..." |
However, the sudden sharp depolarisation at Bob, Joe and Patrick's house causes the voltage in a neighbouring region of membrane to increase to the threshold level. This increase causes another sodium channel (Bert, now) to open, and the whole cycle begins again. And again. And again. A wave of depolarisation passes along the nerve. It only ever moves in one direction because the refractory period of each cycle prevents the depolarisation from going backwards. Bob can't get excited by the activity at Bert's place, because his region of the membrane is currently just TOO negative.
This mexican wave of positivity parties along a nerve membrane is the transmission of a nervous impulse. Now, of course my comic strip is an over-simplification (not to mention a demonstration of a stunning lack of artistic talent). There are MANY more sodium channels than just dear old Bob. In fact each depolarisation is the result of multiple channels opening. The basic principle, however, is the same. And the main thing you need to take away from this isn't necessarily the details of the molecular behaviour but how fast they do what they do. To demonstrate, just do me a favour, right now... wave at me. To make that action you had to send hundreds of thousands of nervous impulses. That's millions of tiny depolarisations occurring to shoot the message along each nerve. And how long did it take you to wave? It felt almost instantaneous. If the thought of that intricate and beautiful molecular orchestration of depolarisation happening at such insane speed doesn't blow your mind, then I think there's probably something wrong with you. It is absolutely awe-inspiring. And that is why ALL the cool kids want a picture of it on their walls at Uni.
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