11.9: Muscles

Does the heart have muscles?

When you think of muscles, you might think of biceps and the external muscles you see in a bodybuilder. However, some muscles are found deep inside your body. The heart, for example, is a very muscular organ. It has to pump blood all around your body.

Types of Muscles

The muscular system consists of all the muscles in the body. This is the body system that allows us to move. You also depend on many muscles to keep you alive. Your heart, which is mostly muscle, pumps blood around your body.

Each muscle in the body is made up of cells called muscle fibers. Muscle fibers are long, thin cells that can do something that other cells cannot do—they are able to get shorter. Shortening of muscle fibers is called contraction. Muscle fibers can contract because they are made of proteins, called actin and myosin, that form long filaments (or fibers). When muscles contract, these protein filaments slide or glide past one another, shortening the length of the cell. When your muscles relax, the length extends back to the previous position. Nearly all movement in the body is the result of muscle contraction.

You can control some muscle movements. However, certain muscle movements happen without you thinking about them. Muscles that are under your conscious control are called voluntary muscles. Muscles that are not under your conscious control are called involuntary muscles.

Muscle tissue is one of the four types of tissue found in animals. There are three different types of muscle in the body (Figure below):

1. Skeletal muscle is made up of voluntary muscles, usually attached to the skeleton. Skeletal muscles move the body. They can also contract involuntarily by reflexes. For example, you can choose to move your arm, but your arm would move automatically if you were to burn your finger on a stove top. This voluntary contraction begins with a thought process. A signal from your brain tells your muscles to contract or relax. Quickly contract and relax the muscles in your fingers a few times. Think about how quickly these signals must travel throughout your body to make this happen.
2. Smooth muscle is composed of involuntary muscles found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. These muscles push materials like food or blood through organs. Unlike skeletal muscle, smooth muscle can never be under your control.
3. Cardiac muscle is also an involuntary muscle, found only in the heart. The cardiac muscle fibers all contract together, generating enough force to push blood throughout the body. What would happen if this muscle was under conscious or voluntary control?

Figure \(\PageIndex{1}\): There are three types of muscles in the body: cardiac, skeletal, and smooth.

Summary

• Muscles that are under your conscious control are called voluntary muscles, while muscles that are not under your conscious control are called involuntary muscles.
• The three types of muscles in the body include skeletal muscle, smooth muscle, and cardiac muscle.

Review

1. What are muscle fibers? What is unique about these fibers?
2. Describe voluntary muscle.
3. Which two of the three types of muscles in the body are involuntary?
4. Distinguish between skeletal muscle and smooth muscle.