How Do Skeletal Muscles Contract??

Want to know a fun fact? You've been lied to your entire life.

That's right. Everything you know is built upon a facade.

What is it this time? Aliens? Talking dolphins? Did Stella really not get her groove back?

Nope. Not even close...

Ready? Prepare yourself...

Muscles. Don't. Flex.

Okay, so maybe it's not the biggest lie ever told, but you gotta admit that you've probably thought it was true your entire life.

I guess the only question now is, what exactly is it that muscles do?

The answer is very simple — they contract. But what exactly is a contraction?

To figure this out, we need to learn some of the basic anatomy of muscle tissue.

3 Types of Muscle Tissue

If you recall from a previous post about muscle tissue, there are 3 different types of muscle in the body:

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Smooth and cardiac muscle are both involuntary, meaning that you can’t decide when they contract. Smooth muscle is found all throughout the body, including inside of blood vessels and your digestive tract.

Cardiac muscle is found exclusively in the heart, and is some of the most unique tissue in the body.

Skeletal muscle however is voluntary, meaning that you can decide when to contract it. This is the type of muscle most of us think of when we hear the word “muscle".

To make things easier, we will only be speaking about skeletal muscle from here on out.

At a later date, I will delve much deeper into muscle tissue in general, but specifically smooth and cardiac. For now, simply understand that they use similar, yet different processes in their contraction.

Either way, they contract.

Skeletal Muscle

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Skeletal muscle is what we call multi-nucleated, meaning that one individual cell has many nuclei that assist in its cellular functioning. Skeletal muscle cells can also be very long, bordering on multiple feet in length at times. A good example of this would be the Sartorius muscle, which depending on your height, can obviously reach differing lengths.

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Let's Look Inside

If we zoom into an individual skeletal muscle cell, we find even smaller structures called myofibrils. Myofibrils are made up of the functional units of a muscle cell, called sarcomeric units, or sarcomeres, which are stacked end-to-end throughout the length of the myofibril.

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Sarcomeres are defined as the region between the Z-Lines, Z-Membranes, or Z-Discs. Each name corresponds to the same structure, so whichever one you prefer to use is acceptable. The Z stands for the German word Zwischen, which literally means between.

As with every other biological system, if work is going to be done, proteins are required. The proteins of muscle tissue are myosin and actin.

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Myosin consists of a head and a tail/rod. When several myosin proteins are stacked onto one another, the structure is now called the thick filament.

Actin molecules are tiny little bead like proteins, that when stacked together form a long and slender structure called the thin filament.

Myosin and actin are so extremely attracted to one another, a protein complex is required to block the receptor/attachment sites on the actin molecules, otherwise the muscle would be in a state of permanent contraction.

This protein complex consists of two proteins:

Troponin
Tropomyosin

Tropomyosin is another slender protein that acts to cover the receptor/attachment sites. To do this, troponin binds tropomyosin down to the thin filament.

Time to Contract

In order for a muscle contraction to occur, calcium ions are diffused into the skeletal muscle cell. For as much as troponin loves tropomyosin, it enjoys the presence of calcium infinitely more so.

Calcium will then bind to troponin, causing a change in conformation, and thus relaxing its grip on the tropomyosin. As the tropomyosin starts to relax, the binding sites on actin become available, and myosin takes advantage.

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The myosin heads will now bind to actin, and a change in conformation will occur. This results in what’s called the power stroke.

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It’s these power strokes that bring the Z-lines closer together, or what we can now call a contraction. When you add up the hundreds of thousands of power strokes, the end result can be quite dramatic.

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All Good Things Must Come to an End

In order for a muscle contraction to stop, a few things need to occur. First, ATP (adenosine triphosphate) needs to bind to the myosin head and hydrolyze, becoming ADP (adenosine diphosphate) and a spare phosphate. When this occurs, the myosin head is “cocked” back into a position of high energy, ready for another power stroke.

Next, the calcium ions need to be removed from the troponin. This is accomplished by ATP binding to the calcium, and hydrolyzing into ADP, effectively "pushing" the calcium out of the cell. When this occurs, troponin once again binds to tropomyosin, blocking the receptor sites on the actin molecules, and the muscle is now in a relaxed state.

There's Plenty More to This Story

As they say, this is only the tip of the iceberg. In upcoming posts, I'll be diving much deeper into all 3 muscle tissue types, the different types of contractions, muscle cramps, and even how the brain sends the signal to contract in the first place.

So then, what is a contraction?

Contractions are a muscle generating tension. When calcium ions are introduced into the skeletal muscle cell, they bind with the protein troponin, which is "holding" the protein tropomyosin down against the thin filament, blocking its receptor sites. When the receptor sites become available, myosin will bind to the actin, and a power stroke will occur.

The fact is, flexion is what we call a joint action. Muscles can cause flexion to occur, but they don't flex themselves.

For example, when you are doing the typical biceps curl, your elbow joint is flexing. Now, the contraction of biceps brachii caused the elbow to flex, but the joint is the only thing flexing. Not the muscle.