FreeSchool 101!
There's more to the human muscular system than the muscles you can see and feel.
In fact, you may be surprised to learn that there are over 600 muscles in the human body.
Your muscles make up about 40 percent of your total body weight and while they allow your
body to move, they also maintain posture and stability, circulate blood, allow you to eat,
breathe, digest food, and much more.
The movement of muscles is essential for your survival.
Muscles are organs, made of special cells called muscle fibers.
These fibers are long and thin and have a unique ability: they can contract.
By contracting, or shortening, they can create movement.
Any movement you can think of - inside or outside your body - from the blink of an eye,
to a heartbeat, and even that growling in your stomach is caused by muscles.
Now, there are three types of muscle tissue in your body.
Skeletal muscle, which you can remember as the muscle that moves your skeleton around,
smooth muscle, which is the type in some internal organs, and cardiac muscle, which you can
probably guess from the name is only found in your heart.
Let's start with skeletal muscle.
The most common type of muscle in your body, skeletal muscles are muscles that are attached
to bones by tough connective tissue called tendons.
Skeletal muscle is also sometimes called voluntary muscle, because it is the only type of muscle
you can move voluntarily.
You can choose to move your leg, you can choose to move your hand, but you can't tell your
heart to beat, or stop beating, or control the speed of your digestion.
Skeletal muscles can be tiny, like a muscle in your inner ear that's less than 1mm in
length, or quite large.
The largest muscle in the body, the gluteus maximus, is responsible for helping you stand
upright and climb stairs.
Each muscle is made of hundreds, or thousands, of individual muscle fibers, bundled and wrapped
together by connective tissue.
These bundles give the muscles a striped or banded appearance, called striations.
In between the bundles are blood vessels.
A steady supply of blood provides the muscles with nutrients, oxygen, and a way to get rid
of waste products.
Skeletal muscles are frequently attached to the bone near a joint.
When the muscle contracts, it pulls on the bone, causing it to move.
Muscles cannot lengthen, they can only shorten.
In order for the bones to move in opposite directions, it requires a pair of muscles
working in opposition to each other.
In other words, one set of muscles pulls the arm to bend it, and another pulls in the opposite
direction to straighten it.
The second type of muscle is smooth muscle.
Unlike striated muscle tissue, which is arranged in bundles, smooth muscle fibers are arranged
in sheets.
Smooth muscle cells can be found in the walls of hollow organs such as the bladder, intestines,
and stomach.
They also line blood vessels and the walls of your respiratory tract, and control the
contraction of your iris, among many other things.
Smooth muscle is a type of involuntary muscle, because the action of these muscles are not
under your conscious control.
Even though you don't tell them to, smooth muscles are hard at work around the clock
to keep your body running.
Smooth muscles in arteries and veins control blood flow and blood pressure.
More smooth muscle moves food through your digestive tract and helps your airways expand
and contract to bring in air.
Without the involuntary action of smooth muscles, your body would rapidly stop working.
The third type of muscle is cardiac muscle.
Cardiac muscle is found exclusively in the walls of your heart.
The contraction of cardiac muscle is what causes your heart to beat and pump blood.
Like smooth muscle, cardiac muscle is a type of involuntary muscle, because it is not under
your control.
Like skeletal muscle, cardiac muscle fibers are arranged in bundles, giving it a striated
appearance.
Unlike other types of muscle, cardiac muscle has one job and one job only: keeping your
heart beating.
No matter which type, muscles can't move on their own.
All three types of muscle cells require stimulus from nerve cells to signal them to contract.
Your brain sends electrical impulses through your nervous system and stimulates the muscle
fibers.
In addition, cardiac muscle is controlled by specialized cells called pacemaker cells.
These cells receive signals from your nervous system and regulate the speed at which your
heart beats.
So let's review: the muscular system consists of hundreds of muscles, as well as muscle
cells in various organs.
Muscle fibers are the only cells in the body that can contract, allowing them to move.
The three types of muscles are skeletal muscles (which let you move), smooth muscles (which
control most involuntary physical processes), and cardiac muscles (which keep your heart
beating).
Without your muscles, you wouldn't be able to move, eat, breathe, or live.
Muscles are amazing!