Muscles ( ESSAY )

Your muscle cells is one of the places that you can find insatiable longing, forbidden love and tragic separation, and those love story is played out by a nice pretty pair of protein strands called actin and myosin. These couples love story is the reason that you can move, dance, or even stand up. It makes ALL of your motions possible, not just the voluntary stuff, but also the involuntary, like your heart pumping and your stomach digesting the food that you just gulp down. So someone better give them a movie contract cause these story is very important. The amazing thing about your muscle tissue is that they turn potential energy into mechanical energy or movement, simply by doing two things : contracting and relaxing. I hope you remember the types of muscle tissue in our bodies :

• Smooth
• Cardiac
• Skeletal

If you don't remember check out my post on this three types of muscle tissue. But right now, I'll only talk about your 640 skeletal muscles. These are mostly voluntary, meaning that you can control what they do, you have to command them to move them, unlike the other two type of muscle tissue which is involuntary. Most of your skeletal muscle tissue are attached to your bone and create movement by pulling or pushing the bone this way and that. Each one of your skeletal muscles is technically its own organ, because they contain connective tissue, blood vessels, and nerve fibers. Add one more to the list of organs that you never know about. And because your muscles are voracious energy hogs, each one is rigged up with its own personal nerve to stimulate contraction, and its own artery and vein to keep it well fed with all the blood, oxygen, and proteins they would need to operate. But to understand those operations, we need the "anatomy of the skeleton badge" under our "anatomy and physiology belt", so let's move on partner. Basically, a skeletal muscle is constructed like a really sturdy piece of rope. It contains fibers, layers and layers deep. Thousands of tiny parallel threads called myofibrils squish together to form muscle fibers, which are your actual muscle cells, which is no different from any other cell. It contains mitochondria, multiple nuclei, and a cellular membrane called a sarcolemma. Those muscle fibers then form larger, string-like bundles called fascicles, which combine to form the larger rope-like muscle organ. Overall, this bundles-within-bundles configuration makes muscle tissue fairly sturdy. But considering how much abuse your muscles take when you do something, it's no surprise that they need a little help. That's why every muscle contains a few different kinds of supportive sheaths of connective tissue, the protective reinforcements to keep that bulging muscle from bursting. Well the anatomy part of the story is complete, now we'll approach the "how" part of the story. But, there are rules before we go on :

1. Proteins like to change shape when stuff binds to them
2. Changing shapes can allow proteins to bind or unbind with other stuff

So keep those rules in mind and you should be okay. Now let's go back to those myofibrils that forms kinda like the base of the muscle cell. It is divided lengthwise into segments called sarcomeres, which contain two even tinier strands of protein -- two different kind of myofilaments called actin and myosin, the lovebirds. A sarcomere contains both thin filaments, made up mostly of two light and twisty actin strands, and thick filaments, composed of thicker, lumpy-looking myosin strands. Each sarcomere is separated by what's known as a Z line at either end, which is just a border formed by alternating thin filaments in a kind of zig zag pattern. When you are at rest, your actin and myosin strands are not touching, but would really, really like to touch one another. Specifically, those club-headed myosin wants to get up-close-and-personal with the actin strand. But just like a good human love story, this love story also have some obstacles that wanted to keep them separated. And this "obstacle" is a protein called tropomyosin and troponin. But luckily these bodyguards can be bought off by Adenosine Triphosphate, or better known as ATP, and some calcium. ATP is like our bodies main body of currency that is welcomed in all part of your body, not like different country have different currencies. ATP contains chemical energy, and your muscles are all about converting chemical energy to motion, so they are always hungry for more ATP. Say I want to move my arm. My brain then sends action potentials along the motor neuron until it synapses with a muscle cell in my arm. Then the motor neuron releases acetylcholine into the synapse, the channels open up, and then create a rush of sodium into the cell as a graded potential, which, if it's strong enough, causes nearby voltage-gated sodium channels to open. So that action potential zips along a muscle cell's membrane, the sarcolemma, which has lots of tubes that run deep inside the cell, which are called T-Tubules. When the action potential travels down one of those tubes, it eventually triggers the voltage-sensitive proteins that are linked to those calcium channels on the cells sarcoplasmic reticulum. When those channels are thrown open, the calcium stored inside rushes into the rest of the cell. The protein troponin just loves to bind with calcium, and remember rule #1. So the calcium latches on to the troponin which then pulls on the other bodyguard, the tropomyosin away from the sites on the actin strands that the myosin really wants to get a hand on. But the only myosin heads that can bind to those newly opened sites are the ones that are ready for action. That is, the one that have already grabbed a molecule of ATP that's been floating around, and broken it down into ADP and the leftover phosphate. When a myosin head does that, it changes shape in to an extended position, kinda like a stretched spring, still holding on to the ADP and phosphate, and still storing the energy that was released when they were broken apart. When that energy was released, the myosin finally "kiss" her beloved boy. So after all that, the myosin finally binds with the actin strands and what a beautiful scene. When they bind, the myosin tugs and pulls on the muscle fibers and that is what makes your muscles stretched and contracts. Now, with it's energy spend, that little head has no use for the ADP and the phosphate. So they un-bind with it, because remember rule number 2, that when proteins changes shape it encourages the protein to bind or unbind with stuff. That created a slight change in the myosin strand, which lets a fresh ATP come and bind with it which created another change, but this time it is what makes the myosin releases on the actin and fall back to its resting stage, contracting the muscle in it's wake. But fear not! This epic is not about to end. This one is very similar to most of your body's processes, it forms a cycle. When it go back to its resting stage, it again turns those ATP into ADP and a leftover phosphate, making it move into the stretched position again, and yes the cycle continues.
Your 640 skeletal muscle comes in different shape and sizes, from the longest (the Sartorius in your upper thigh) to the biggest (the gluteus maximus in your butt), to the tiniest (the stapedius in your middle ear). These organs are capable of a whole range of power and duration, as well as surprising and delicate subtlety. The same muscle that you would use to pluck an eyebrow growing in the wrong place, or catch a mosquito, or hug a baby; those same muscle could be use to crush cans, punch hole through walls like an angry Mama Hulk when her Baby Hulk are being teased by an Evil Brother Hulk, and do push-ups. How can that be possible? Well stick around to find out.
Now when you look at how the muscular system moves, you got to keep two things in mind:

1. Muscles never push. They always pull
2. Whatever one muscle does, another muscle can undo.

Wait. How can muscles not able to push? Let's remember that skeletal muscles, well most of them, extends across the joint over to connect at least two bones together. When a muscle contracts, the bone that moves is called the muscle's insertion point. And the muscle brings the insertion point closer to the bone that doesn't move, or at least moves less, and that is called the muscle's origin. And that movement is always a pull, always a muscle tugging on the insertion point to get closer to the origin. And when you think about it, it has to be that way. Muscles cannot, like extends themselves more than their resting point length to push a bone away from the origin after pulling it closer. Every single movement that your skeleton makes uses the very same principal, whether you are doing exercise or writing on a piece of paper, all of your possible movement uses the very same principal, that your muscle are pulling on an insertion point to move it closer to the origin. Keep in mind here that you cannot just say to one of your bone that it is an insertion point. You have to look at the thing that its doing. One bone can be an insertion point when you do this, but it will be the origin when your other muscle counteract what your other muscle has done. I know, its complicated.....

You can generally classify skeletal muscles into four functional groups depending on the movement being performed :

• Prime Movers
• Antagonists
• Synergists
• Fixators

For example, the muscles that are mainly responsible for a certain movement are called those motion's Prime Movers or agonist muscle. Take an example when you do a series of jumping jacks. You are using those pectorals located in your chest and latissimus dorsi on your back to adduct your arms back down to your sides. Then those muscle are your bodies Prime Movers muscle for adduction. Well referring to the second rule, there would be another muscle to counteract what the Prime Movers do. That's where the Antagonists came. It counteracts what the Prime Movers has done. And one particular muscle could be a Prime Mover when your body are doing this, and can be an Antagonist when your body do the opposite thing. The third functional muscle group that you have is called the Synergists. And like its name it help the prime movers by giving them extra energy, and also to stabilize joints from dislocating from their position, which will be painful, believe me on that one.
Now back to the question of how can you hug a baby in one time and the next minute crush a can. I got two words for you : Motor Units. A motor unit is a group of muscle fibers that all get their signals from the same and single motor neuron. Since all of those fibers listen to only one motor neuron, they act together as one unit. In a big power-generating muscle like your rectus femoris in your quad, each of a thousand or so motor neurons may synapse with, and innervate, with a thousand muscle fibers. Those thousand fibers together form a "large motor unit". And big units are typically found in muscles that perform big, not very delicate movements, like running, kicking, or jumping. But other muscles, the one that control your eyes and fingers, which exert fine motor control may have only a handful of muscle fibers all connected to a single motor neuron. Those relationships are "small motor unit". And when a motor unit, no matter how large or small, responds to an action potential, those fibers quickly contracts and release, in which we call a twitch. The fact is, our muscular movement are pretty smooth. That's because one muscle can produce a variation of smooth forces, called "graded muscle response". And they're generally affected by both the frequency and strength with which they're stimulated. Let's say you want to lift something heavy, like a bucket full of water. Your brain tells your muscle to increase their force, by increasing the frequency with which your motor neurons are firing. And the faster these nerve impulses fire, the stronger each successive twitch gets. In this way, twitches end up adding to each other as they got closer together in time. We call these as temporal summation. At some point though, almost all actin binding sites are exposed, so all of the myosin heads can work through their cycle of ATP and ADP, and the muscle force can't increase anymore, even with faster action potentials and more calcium. It's just that none more myosin are doing nothing, they are all busy, kissing the actin and releasing for a brief short unhappy moment. When all those little twitches blend together until they feel like one mammoth contraction, that's called tetanus. At that point, all human being on planet Earth will hit a ceiling of maximum tension, where there are just no more myosin and actin to bind.     

Joints ( ESSAY )

Joints are the meeting places between two or more bones. And even though it sounds mathematically possible, you actually have more joints than bones, which is weird. In a lot of places, like your hands and feet, each individual bone is part of more than one or two joints. And then what is the job of those joints? To help you move. Body movements happen when muscle contracts across joints, moving one bone toward another. As is often the case in anatomy, we classify joints both by what they do and by what they're made of. Because form follows function, we can't really talk one without talking the other. So the structural classification of your joints is all about what kind of material make up those joints, and it is made of three categories :

• Fibrous joint  
• Cartilaginous joint
• Fluid-Filled or synovial joint

While the functional classification of your joints focuses on how much that particular joint can move, and it is divided into several more groups :

• Synarthroses; non moving joints
• Amphiarthroses; partly-moving joints, like shock-absorbers.
• Diarthroses; fully movable

Fibrous joint connects bones with dense, fibrous, connective tissue and are mostly immovable, so they are mostly classified synarthroses joints.

Cartilaginous joints unite bones using cartilage, they don't move much, and therefore classified as ampiarthroses joint. These come in two types : synchondroses and symphyses.

Synovial joint are freely movable, so they are in the class of diarthroses. Most of your joint fall into this category. Although they do make use of cartilage and fibrous connective tissue to connect bones, they're different in that the bones that they joint are separated by a fluid-filled joint cavity, which is good, because they move a lot, and if all our joints use cartilage or fibrous connective tissue to connect all your bones, a walk down the block would make your joints so hot that it would essentially cook the surrounding tissue and leave your legs smoking like a desperate Looney Tunes character. They also have six special features :

• band-like ligaments
• articular cartilage that covers the opposing bone surface
• a joint cavity
• synovial fluid lubricant
• a fibrous joint capsule
• sensory nerve fibers and blood vessels.

These synovial joints have six different configurations that allow you do all the things that you are capable of doing, like a subtle head nod to a vigorous-whole-body shake :

• Plane
• Hinge
• Condylar
• Pivot
• Ball-and-socket
• Saddle

So that is pretty much the anatomy and physiology of your joints, and Matthew is signing off.