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The Muscular System

The Muscular System is the organ system which allows us to move and stay balanced. Because our bodies are always doing one of the two, we have muscles everywhere. These come in three types to generate or stop different kinds of movement. Yet, all of them have the same basic function: Muscles use energy to control motion.

What is a muscle?

When you think about muscles, you might picture gyms or sports tournaments. Or, you might remember specific muscles like the calves, used for running. This makes sense. Athletes use these muscles to move things (including themselves).
But, these are only one type of muscle, called skeletal muscle. Skeletal muscles are the muscles that allow us to run, lift things, or do other strength-based work. They also help us to balance.
Diagram of muscle use.

A cartoon character using their skeletal muscles to do exercises.

There are also two other types of muscle: smooth muscle and cardiac muscle. We’ll talk more about these in a bit. But, it’s important to remember that muscle does more than make us strong. Some types are important just for living!
With that, we’re going to examine muscle structure. We’ll start with skeletal muscles because their structure is the most complex. Additionally, they share many features with other types of muscle. If you understand how skeletal muscle works, learning about the others will be easy.

Skeletal Muscle Structure

The basic unit of skeletal muscles are cells called muscle fibers. Muscle fibers are special cells that are very long and have lots of mitochondria. These extra mitochondria let them make lots of energy to help us move. But even with all that energy, one cell can’t move the whole body. So, muscle fibers form groups with each other, called fascicles. 
Wires to describe muscles.

Muscles look like bundles of cords. Instead of plastic, muscles are made of long fascicles and fibers.

Still, even a fascicle is not enough to generate a lot of motion! As a result, they continue to bundle up into a large cord. We call this cord skeletal muscle. 

How does skeletal muscle work?

The basic function of any muscle cell is contraction. This is when the long cell shortens. Skeletal muscles are connected to the skeletal system. So, their contraction pulls bones. Remember that bones make-up the structure of our body. Skeletal muscles work to pull that structure in different directions, generating motion.
Note that this motion is voluntary. This seems obvious, but it’s not the case for cardiac or smooth muscles.

What are the other kinds of muscles?

Different kinds of muscle.

The different kinds of muscle.


Smooth muscles are much less obvious to us than skeletal muscles. But, they’re also very common. They are a sort of muscular tissue that forms a lining for many areas of our body. For example, the wall of the stomach contains many smooth muscle cells. This allows the organ to contract and expand to break up food.
Smooth muscles differ from skeletal muscles by how they contract. They aren’t striated or made of cord-like fibers. Instead, smooth muscle is a clump of cells that shrinks and expands all at once. This is good for non-linear movement, like when the stomach rumbles.
Another very important function for smooth muscles is to make our blood flow. Blood vessels going away from the heart, called arteries, are lined with smooth muscle. The muscle contracts in a ring around the artery to increase the pressure inside. This higher pressure pushes the blood to areas of low pressure, where more blood is needed


Now, it might seem strange that the muscles which push veins aren’t cardiac muscles. But, cardiac muscles are found only in the heart. They’re very common there. In fact, the heart is pretty much one big muscle.
Illustration of the human heart.

Cardiac muscles are only found in the heart. They must work every second of the day to make sure your body gets the blood it needs.

It’s a very important one, too! When the heart beats, it’s because cardiac muscles have contracted. This increases pressure in the heart and pushes blood out to the arteries. 
Cardiac muscles are different from the other types of muscles. This is because they are independent. That is, they’re not linked in cords or bunches. In fact, cardiac muscle cells are able to beat on their own! Of course, that wouldn’t be very useful. So, the heart coordinates their beating to produce a single, strong beat.

Other Great Resources:

Bill Nye on the Muscular System: 

 Written by: Noah Louis-Ferdinand.