How do Muscles Work?

And I'm back again, folks, working to conquer a textbook over the summer. We have a long way to go. Today I present to you:

Chapter 1, Part 1: Structure and Function of the Muscular System (Part 2/2)

Sliding Filament Theory of Muscular Contraction

We left off last time discussing muscles and their makeup in depth. Serious depth, actually. Before we continue on today with part 2 of the muscular system, I want to remind you of our friends actin and myosin, two myofilaments within a myofibril that are in charge of contracting the muscle.  I’m also going to bring this picture back from the other day because it’s awesome.

First on today’s agenda we have the sliding-filament theory, which simply states that
the thin filaments (actin) slide over the thick filaments (myosin) to pull the z lines closer to one another, causing the H-zone and the I-band to shrink, thus contracting the muscle. That’s it. So nonchalant. But that’s why our muscles contract. As you can imagine, one little set of actin sliding over its myosin neighbor doesn’t move your muscle a whole lot, so this has to happen countless times throughout the muscle to make some real movement occur. Luckily our muscles are all interconnected via our nerves and T-tubules, which, as I mentioned the other day, are a big deal. And now you can see why. This interconnection allows the whole muscle to work as one unit.

Let’s rewind again and refresh on the sarcoplasmic reticulum. Remember what that does? I actually do and didn’t have to read back, so that’s good news. I’m learning, friends. The sarcoplasmic reticulum stores and releases calcium within the myofibril. At a resting state, very little calcium is present in the myofibrils, so very few of the myosin cross-bridges (the hook-of-the-hanger looking things) are bound to the actin, meaning there’s no tension in the muscle. Whenever your brain tells your muscles to contract, the sarcoplasmic reticulum releases calcium ions into the myofibril that bond with troponin, a calcium-loving protein within the actin filament. It is this reaction that allows myosin cross-bridges to connect to the actin filament. This is key because the amount of force you can produce at any given moment is directly dependent on the number of myosin cross-bridge heads that are connected to the actin filaments. Better connection = more force production. 

Where does all this energy come from? Sure, we understand what’s happening (or do we?), but what’s powering it? The hydrolysis of ATP (adenosine triphosphate) to ADP (adenosine diphosphate) and phosphate, which is catalyzed by ATPase (adenosine triphosphatase). So many big words—microsoft word is having an underlining field day. Let’s say that again without the big words: the breakdown of ATP to ADP and phosphate, spurred on by ATPase, powers the cross-bridge flexion. Roger roger. Another molecule of ATP must replace the ADP on the myosin cross-bridge head in order for the head to detach from the active actin site and recock. That sentence was directly from the book. I’m still decoding it. Calcium must be present for this contraction process to continue. No calcium, no muscle contraction.

In review, for a muscle to contract, the following must happen: You decide you want to move > your brain sends these signals out to your muscles> calcium is released from the sarcoplasmic reticulum > the calcium binds to troponin > the myosin cross-bridge couples with actin > actin and myosin dissociate > the myosin cross-bridge head recocks > more calcium binds > etc etc etc. This process will continue repeatedly throughout the muscle as long as: 

• Calcium is available
• ATP is available to assist in the uncoupling
• ATPase is available for catalyzing the breakdown of ATP

Wow. I literally wrote “wow” right here in my book. Insanity. So many things happening within a tiny myofibril (which is about 1/100^th of the size of a strand of hair), which is within 1 single muscle fiber (about the diameter of a single stand of hair), which is within 1 fasciculus, which is within 1 muscle. And there are hundreds of myofibrils within a muscle fiber. And there are up to 150 muscle fibers in a fasciculus. And there are I-don’t-know-how-many fasciculi in a muscle. And there are (usually) numerous muscles involved in a movement. The complexity of our system is just mind blowing to me. Glory to God. Anyway, I digress.

As I was saying, this process continues repeatedly so long as all the aforementioned ingredients are present. When one of these isn’t present, however, actin and myosin filaments return to their unbound state, and the muscle relaxes. Calcium is called home by its mother, the sarcoplasmic reticulum, the sun sets, the street lights come on, and everything is calm. Until you decide to move again, then BAM! (read as fast as you can) Calcium is released and binds with troponin, myosin couples with actin, and your muscles move again (good work). In the blink of an eye, at that. Less than the blink of an eye, I suppose, seeing as these things have to happen in order for your eyes to blink. Madness, I tell you. 

This concludes the muscular system portion of chapter 1 within my text. Good news: there's only 3 more systems in chapter 1. Goodness. I'm already behind and I'll be knocking those out this week, bing-bang-bam, so buckle up. 

As I said before, much of this is out of the text, "Essentials of Strength Training and Conditioning" (Third Edition), sometimes even directly. I don't have quotes because I'm trying to make this easier to read. The random ridiculous comments are my own. Pretty much everything else is from them. That is my disclaimer.