Taking a stroll through the park with your child in a stroller seems pretty simple. But in reality, every single movement is very complex. Muscle movement is dependent upon the nervous system and many basic ions, such as sodium, potassium, and calcium. Let us explore the anatomy of muscles and muscle movement. There are three kinds of muscles in the body, skeletal, cardiac, and smooth, but we will focus on skeletal.
What constitutes a muscle? There are hundreds of different muscles in the body. But each one has the same basic structure. In order from largest to smallest part, the muscle consists of fascicles, muscle fibers, myofibrils, sarcomeres, and myofilaments. Here is a great chart which describes them:
Let us examine each component. A muscle is an organ. It is surrounded by epimysium. Within each muscle are bundles of muscle cells called fascicles, with each being surrounded by perimysium. Each fascicle has many many cells inside of it, with each being surrounded by endomysium. It is within these cells that incredible action takes place.
In order to understand the mechanism of muscle movement, a microscopic view of muscles must be examined. Each cell contains thousands of myofibrils, which contain sarcomeres, which are the actual contractile unit. A sarcomere has many different components. Here are diagrams of the microscopic anatomy of the sarcomere:
As can be seen from part c, each sarcomere goes from Z disc to Z disc. In the outer edge of the A band are thick and thin filaments. The thick filaments are made of myosin, and thin filaments are made of actin. In the middle of the A band is the H zone, the light zone, as it consists of only thick filaments. Between A bands are the I bands, consisting of only thin filaments. In the middle of the I bands is the Z disc. Because of the light zones and dark zones, a skeletal muscle has striations, as can be seen from part a in the above diagram.
In order to understand the mechanism of contraction, let us look closely at the composition of thin and thick filaments:
Skeletal muscle contains two sets of intracellular tubules which are important in contraction: sarcoplasmic reticulum (SR), whose major rule is to regulate intracellular levels of calcium. It stores calcium and releases it on demand when the muscle needs it. The other is the T tubule system. At each A band-I band junction, the sarcolemma of the muscle cell protrudes deep into the cell interior. The T tubules are continuous with extracellular space. Because of this, the T tubules conduct nervous impulses to the deepest regions of the muscle cell and to every sarcomere. The nervous impulse is required for the release of calcium. Here is a diagram of the SR and T tubule system:
Now that we have a picture of the anatomy of the muscles, we can take a look at how contraction occurs. In order for a muscle to contract, it must be activated by stimulus from a nerve ending at the neuromuscular joint. Then, it must generate and spread an electrical current, called an action potential, across the cell membrane, or sarcolemma. The final step is a short rise in intracellular calcium which triggers contraction. Here is a fascinating diagram of what a nervous impulse triggers at the neuromuscular junction:
Although this is really a discussion for a different time, I just want to point out that sodium and potassium in particular are vital for the nervous system action potential, as well as for a membrane potential in muscle cells. Sodium and potassium are crucial!!
Because sodium moves into the cell membrane at a faster rate than potassium moves out, the inside of the cell becomes less negative. This is called depolarization. This depolarization occurs at the end of a cell, but it spreads to the rest of it. Once the action potential is over, there is a stage called repolarization, in which sodium channels close and potassium channels open, leading to an increase in negative charge of the interior of the cell. This whole process can be seen in the following diagram:
Transmission of an action potential along the T tubules changes the shape of voltage-sensitive proteins which are in the T tubules, stimulating SR release of calcium into the cytosol (cellular fluid).
Now we come to the actual movement of the muscle, which happens through cross bridge activity. The whole process is called excitation-contraction coupling. Please take a moment to study the following diagram, as it is fascinating:
Here is a detailed view of the cross bridge cycle:
This is how your muscles move, every single time. When you lift a finger, or turn your head, or smile, there is so much incredible activity going on. Each movement requires sodium, potassium, and calcium in particular, so make sure you get your daily amounts of all of these ions. All three are vital in other areas as well.
So next time you take a stroll with your child in the stroller, appreciate how much incredible activity is taking place throughout your body.