There is no way not to be awestruck at the speed and capabilities of the human (or any) body. Examples of it are everywhere, and every one of them baffled me in school. I still don’t fully comprehend them. So let’s start with a simple demonstration, shall we?
Just so you can have this in mind as we go through the post, I want you to look down at your hand. Either hand. No, your dominant hand, let’s go with that one. Now extend all your fingers out, just stretch your hand out wide.
Good. Now as fast as you can, clench them into a fist.
Good! Now fingers out, now fist. Now go back and forth as fast as you can. Faster! Good. Look at how fast that is, how many inches each fingertip is moving in the span of a few fractions of a second.
Okay, got it? Like I said, just keep this in the back of your mind.
Today I want to talk about muscles.
My high school had an interesting approach to dissections and animal anatomy. We learned about muscle connectivity using Tyson chicken wings (yes, the big bag of frozen ones you get at Costco). If you hold up a chicken wing at its base (you know, where it used to meet the chicken) and then grab onto its little bicep and pull toward the absent chicken body, the whole wing will flap toward you. Because the muscle is pulling the bone, right? And that’s what muscles do, they contract and haul whatever they’re attached to along with them. The muscles in your fingers are pulling them in toward your palm and out again.
But did you ever wonder how they do it?
I mean on a molecular level, what is pulling on what, exactly – what’s the machinery that makes it all contract like that?
It all comes down to two elements of a muscle cell: actin and myosin.
(Before I get going about these two, we need a quick background. Your body is made up of cells. All kinds of cells. All these cells contain thousands of types of proteins, which get to do all the action. I always think of the proteins as the ants all running around doing things, while things like carbohydrates and fats make up the walls and food supply and stuff. In fact – well, hopefully we’ve all heard of DNA here. You know what DNA really is? It’s a code for how to make proteins. That’s it. DNA tells your cell the exact recipe for every protein, and supplies information about where it goes and when to use it. So proteins are the actors, the workers, the important molecules in a cell. Okay, end background.)
So back to actin and myosin, which are both proteins. Actin is a protein that, when put together with a bunch of other actin molecules, assembles into long filaments that help support the basic structure of a cell. In a muscle fiber these filaments all run in parallel down the length of the muscle.
Myosin, on the other hand, is literally a little guy that walks along the actin filament. (Okay, first off, how cool is that? A protein – just a simple string of amino acids – can fold itself into a structure that looks like a pair of legs with big clubby Mickey Mouse feet! That alone is mind-numbingly awesome.)
The myosin family of proteins. Figure borrowed from here.
All right, I lied, there’s one other critical chemical in this story: adenosine triphosphate, or ATP. ATP is an essential chemical in the body, it acts as an energy source. Often when a protein needs energy, it just grabs onto an ATP molecule and rips off one of its three phosphates. The breaking of this bond converts it from a triphosphate to a diphosphate (ADP) and produces the energy required to move.
So this is already a lot to take in, right? Take a breather for a second and look down at your hand again. Think about all the microscopic actin filaments and myosins and ATPs hanging out in there. That’s right, this stuff is starting to get heavy. Shall we continue?
So we have to get a little smaller than the muscle fiber level to see where actin and myosin come in. Each muscle fiber is composed of a bundle of myofibrils, each of which consist of a string of sarcomeres. It looks like this:
Figure borrowed from here.
The sarcomeres are units of overlapping thick and thin filaments – the thin filaments are made of actin attached to each end of the sarcomeres, and the thick filaments are made up of a bunch of myosins all strung together, sitting in the middle of the sarcomere.
When the muscle contracts, here’s what happens.
1. We’ll start with the myosin floating above the actin filament, with an ATP molecule bound to it. The ATP separates into ADP and a phosphate, which shoves the myosin’s little foot (scientists call it a head – go figure) forward.
2. When the myosin head gets close to the actin, the phosphate is released (leaving ADP behind) and the myosin head binds tightly onto the actin filament and yanks it backward.
3. The myosin releases the ADP and another ATP comes in to take its place. When ATP binds in there, the myosin releases its hold on the actin filament.
4. The ATP converts to ADP and a phosphate, the myosin head cocks forward, and the process starts all over again.
If I have just befuddled you beyond comprehension, go here and look at this fantastical video.
Also, keep in mind that all of these steps are nothing more than the probabilistic functions of a bunch of molecules lounging around in a cellular soup – they’re just events that are likely to happen. It’s not like these are the directed, intentional acts of a bunch of conscious little players, in other words. No. This is just… chemistry.
But here’s the real point. Do you see how long it took me to explain this? Do you see that the video to which I directed you took a full thirty seconds just to show you a detailed depiction of a single pull of a myosin head on an actin filament?
Now go back and look at your hand. Flex it a couple of times again, fast. Every single time you twitch a muscle anywhere in your body, thousands of ATP molecules are being used up to allow thousands of little myosins to pull many times over on thousands of actin filaments. And somehow they can do it that fast and still be ready for another pull an instant later!
How can anyone reconcile this? How on Earth can it work this way? It’s crazy.
Next post: the magnificent neuronal synapse.
