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

Let me start this blog post off with the following disclaimer: this blog post (as well as all those like it to follow) is more for my own benefit than yours. I’m sorry to be selfish in this way, however as I previously mentioned, one of my goals is to read through my strength and conditioning textbook over the summer, and I’ll be using this blog to hold me accountable. So lucky you, you’ll be getting my own typed-up summary of this book. Maybe by the end, you can just go take the CSCS exam and be certified along with me.

Anyway, if you're interested in this kind of thing, I think I made it somewhat readable. If you're not interested, I apologize. 

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

(wait a second…chapter 1, part 1, part 1 of 2? HOW MANY PARTS ARE THERE?!) 

All exercise involves forces. These forces
:  

• Are developed through the muscles
• Use the skeleton as a lever system
• Are control by the cerebral cortex
• By means of motor neurons
• Which activate the skeletal muscle cells and fibers
• Which is continuously supported by oxygen delivery and carbon dioxide removal
• Via the respiratory and cardiovascular system.

At the most basic level, strength and conditioning coaches aim to maximize physical performance via programs that maximize physical performance by increasing flexibility and muscular strength and endurance, all while understanding how the cardiovascular and respiratory systems interact with the neuromuscular system to provide an optimal environment for sustaining muscular work. In other words, it's important to understand not just what to do to make someone a better athlete, but why. This was on the forefront of the minds of the two individuals that created the Sports Physiology program at A&M not too long ago. Their mission was to take the exercise physiologist and the strength and conditioning coach and combine them into one. Anyway, I digress. Onward!

The Muscular System 

Macrostructure and Microstructure

Every bone is covered in bone periosteum, which is a specialized connective tissue. Every muscle connects to the bone by means of its tendon latching onto to the bone periosteum. The tendon is simply a tough, fibrous band located on the end of the muscle. So we have a muscle with tendons on the ends connecting to the bone periosteum on the bone. Got it.

All limb muscles have two attachments to the bone, proximal and distal. The proximal attachment is the one closer to the trunk, whereas the distal end is further from the trunk. But what about the actual trunk muscles? Their attachments have their own names: superior (closer to the head) and inferior (closer to the feet). The origin of each muscle is the attachment closest to the center of the body (ie, the proximal attachment), and the insertion is the attachment further from the center of the body. Every muscle is enclosed by the epimysium, which is essentially the skin of a muscle.

Muscle cells (aka muscle fibers) are very long and have many nuclei located on the outer edges of the cell. I feel a little progression will make the most sense for the makeup of a muscle, so hang with me. Every muscle is made up of bundles of muscle fibers called “fasciculi,” which are surrounded by a connective tissue called the perimysium. Each fasciculi is made up of up to 150 muscle fibers, each of which is surrounded by endomysium, yet another connective tissue. So many Mysiums. And then of course each individual fiber is encircled by it’s own membrane, called the sarcolemma. All of these connective tissues – the epimysium, perimysium, and endomysium – are connected directly to the tendon. This means that when tension is developed in a muscle cell, it’s transmitted straight to the tendon through these tissues. 

Let’s review the progressive dive into a muscle, and let’s do ourselves a favor and skip out on the connective tissues. So we have (each arrow means “is made up of”): 

Muscle --> muscle fiber bundles (fasciculi) --> individual muscle fibers --> myofibrils --> myofilaments (actin (thin) and myosin (thick)). You follow? No? Relax; that’s just because we haven’t discussed the myofibril or myofilaments yet. But holy cow, that was a lot. Here's a picture. Way simpler. 

In terms of the connective tissues, the epimysium surrounds the entire muscle, the perimysium surrounds the fasciculi, and the endomysium surrounds the individual muscle fibers. This may be easier to remember if we recall that “epi” means “over” (over everything muscley), “peri-“ means around (around the bundles), and “endo-“ means “within,” meaning it’s the innermost connective tissue.

Let’s jump back into the individual muscle fiber. Within an individual muscle fiber is the sarcoplasm, which is the cytoplasm (or the particle-filled, fluidy mess) of the fiber. The sarcoplasm contains contractile components, enzymes, and specialized organelles such as the mitochondria (the POWERHOUSE OF THE CELL! Still remember that from like kindergarten or whenever), and the sarcoplasmic reticulum. Within the muscle cell are hundreds of myofibrils, which contain the apparatus that contracts the muscle cell. So finally after digging and digging into the muscle, we find what makes it work.

Within this “apparatus” are two types of myofilament: myosin and actin. Myosin filaments are the thicker of the two, and have globular heads called “cross-bridges” that stick out like coat hangers at regular intervals. The actin filaments are thinner and consist of two strands arranged in a double helix. The myosin and actin filaments are arranged to form the smallest contractile unit of the skeletal muscle, the sarcomere, which is a building block of the myofibril. Countless sarcomeres lined end-to-end make up a myofibril, but is that as micro is we’re going? Nope, of course not.

Visualize with me: we’re looking straight at a sarcomere from it’s side, and from this view we can see its individual sections. On the outer ends are the Z-lines made up of Z-disks. From one Z-line to the next Z-line is the length of 1 single sarcomere. In between these Z-lines is the M-line, a region made dark by the sole presence of myosin filaments (which are dark) anchored to one another. The sarcomere is made up of 2 bands: the I-band and the A-band. The I-band is a light region around 2 joining sarcomeres made up of only actin filaments, which are lighter in color. The A-band is darker in color and corresponds with the alignment of the myosin filaments. Within the A-band is the H-zone (which contains the M-line), where only myosin filaments are present. The picture below is a good simplification of all the mess I just described. Amazing how simply picture put things.

The next picture is a little more complex, but gives a much better depiction of what’s really going on. Note the globular heads (the hook-of-the-hanger looking things) on the thick filament (the myosin). Those will play an important role in muscle contraction, which I’ll discuss in greater depth in part 2 of this section tomorrow. But for starters, when a muscle contracts, the H-zone and I-band decrease as the Z-lines are pulled closer in to one another. You can almost see how it would happen looking at the pictures below. Imagine that titan filament (the white coils) as suspension, and when a muscle contracts, the Z-disks are squeezed in as the thin filament overlaps the thick filament. Pretty crazy stuff. More on that on another day.

And bam. We need to zoom back out a bit. These individual sarcomeres make up the myofibril, so back yourself out and let’s talk about the myofibril again. A complex system of tubules, called the sarcoplasmic reticulum, surrounds every myofibril, and is the storehouse for calcium ions. “Sarcoplasm”—you recognized that word, yes? We discussed it earlier, as it’s the cytoplasm of the muscle fiber. The sarcoplasmic reticulum is just an entity within that cytoplasm, which I also mentioned in what seems like ages ago.  So a storehouse for calcium ions, huh? Doesn’t sound that important. False. Believe it or not, the regulation of calcium within a muscle fiber is what controls muscular contraction! Insane. My mind has been melted.

Of course, there’s more. But we’re almost done, I promise. Also within the sarcoplasm running perpendicular to the sarcoplasmic reticulum are the T-tubules, or transverse tubules. These guys are essentially one with the sarcoplasmic reticulum. They’re kind of like the Z-line of the sarcomere, as they create sections within the SR. These groups of 2 SR sections split by a T-tubule is called a triad. Not sure why that’s important, but it’s a bolded word, so there you go.

These T-tubules are interconnected between myofibrils, which is kind of big deal. It’s this connection that guarantees that an action potential (an electrical nerve impulse created by a release increased concentration of calcium (from the sarcoplasmic reticulum)) will reach all depths of the muscle fiber at nearly the exact same instant, creating a coordinated muscle contraction. Powerful.

Oh my lanta, what just happened. If you made it this far, please let me know so I can applaud you. To be honest, I'm guessing that I alone will be reading this text, so blah blah blah pew pew pew bing bang bam. Anywho, if you actually did read through this, mad props to you, and I hope you learned something! I know I certainly did. Believe it or not, there's always more to learn, so part 2 of the muscular system will be rolling in tomorrow. 

Get excited. 

Thanks for hanging with me, you guys!

The large majority of this post came straight out of "Essentials of Strength Training and Conditioning" (Third Edition) from the National Strength and Conditioning Association, edited by Thomas R. Baechle and Roger W. Earle. The random comments were my own, but again, most of this came straight out of the text. I didn't bother putting quotes in an effort to increase readability. Sorry, English teachers everywhere. 

The schematic muscle diagram was attained from: http://www.msdlatinamerica.com/ebooks/PracticalOrthopaedicSportsMedicineArthrocopy/sid137425.html

Basic sarcomere picture was attained from wikipedia. 

Detailed sarcomere picture was attained from: http://www.baileybio.com/plogger/images/anatomy___physiology/05._powerpoint_-_muscular_system/sarcomere.jpg