Muscular System Definition

The muscular system is a set of tissues in the body with the ability to change shape. Muscle cells contain a variety of proteins which help them contract in size. The proteins form fibers, which connect various parts of the cells. The main proteins used are actin and myosin. As seen in the graphic below, the muscular system contracts when energy from ATP is applied to the myosin heads of the myosin protein filament. The head releases the actin, reaches forward, and reattaches to the actin. This moves the protein filaments and contracts the fiber. Depending on the muscle cell, different formation of actin and myosin can be used, and in some organisms completely different proteins are used.

Skeletal Muscle Contraction

Functions of the Muscular System


The most obvious function of the muscular system is movement. Organisms have adopted a variety of methods to use the contractile function of the muscular system to move themselves through the environment. The most basic movements of fish include contracting muscles on opposite sides of the body in succession to propel themselves through the water. In organisms with limbs, tendons and other connective tissues are used to secure muscles to the joints and skeleton, which may be internal like human skeletons, or external like the exoskeleton of crabs. The nervous system coordinates the contraction of the muscular system to synchronize the movement of the limbs. Animals like the cheetah, swordfish, and bat have obtained speeds above 60 miles per hour through the power of their muscles alone.


The second and less obvious function of the muscular system is to assist with circulation. Visceral and cardiac muscle tissues surround the blood vessels and lymph vessels that carry crucial nutrients to the cells of the body. Cardiac muscle makes up the heart, and supplies the main force for blood traveling through the body. Large arteries and veins have associated muscles which can contract or relax to control the pressure of the blood. The actions of large skeletal muscles also help pump the blood and lymph through the body. While you exercise and contract large and small muscles, they push vessels aside, which works like a pump to move fluids around your body.


Much like its ability to move fluids through vessels in the circulatory system, the muscular system also aids in moving food through the digestive system. Most digestive organs are surrounded by visceral, or smooth muscle tissue. Although the tissue cannot be voluntarily contracted like skeletal muscles, it is controlled subconsciously. When food needs to be moved through the gut, the muscles contract in a synchronized fashion in a wave through the digestive system.

Muscular System Organs

Unlike other organ systems, the muscular system is divided into different types of tissues, which are incorporated into various organs in the body. Skeletal muscle is the tissue most commonly associate with the muscular system. Skeletal muscle attaches to the skeleton, and moves the limbs and body of an organism. Skeletal muscle systems are composed of striated muscle which has distinct bands of proteins called actin and myosin in each myofibril. When these proteins are given energy, they slide past each other, pulling the ends of each muscle cell together. The sarcomeres, or function units of actin and myosin, produce the banding that can be seen in striated muscle. This can be seen in the image below.

Skeletal muscle

Visceral muscle cells do not contain these stark bands of protein, and the actin and myosin fibers work differently. Instead of think fibers than run through the cell, visceral muscle is surrounded by a net of actin and myosin fibers, which squeezes the cell when contracted. This can be seen in the image below. Visceral muscle is also known as smooth muscle for this reason. Cardiac muscle, which surround the chambers of the heart, is striated like skeletal muscle, but the cells are connected to adjacent cells, which creates more of a contractile motion to pump the blood.

Smooth Muscle Contraction

Skeletal muscle is connected mostly to the somatic nervous system which is controlled by voluntary impulses from the brain. The cardiac and visceral muscles, on the other hand, are controlled largely by the autonomic nervous system, which controls the subconscious actions of an organism. The separation of these nerve systems ensures that the autonomic functions like breathing and digestion continue to take place as an animal moves about and searches for more food. The difference in muscular system tissues is due to their very different uses. Skeletal muscles must be able to do large amount of work quickly, therefore they consist of striated muscle cells, which can contract voluntarily. The smooth muscle tissue found in visceral tissues have fewer energy producing mitochondria and are simply used to contract hollow organs and move the fluid inside. The stomach, intestines, and blood vessels are lined with visceral muscles. Cardiac muscle is striated, because it needs to produce lots of force, although it is not controlled voluntarily.

Related Biology Terms

  • Visceral Muscle – Muscle surrounding or attached to the circulatory and digestive systems, which helps circulate fluid and food through the body.
  • Striated Muscle – Muscles with distinct stripped cause by bands of actin and myosin, which are often part of the voluntary, or somatic nervous system.
  • Cardiac Muscle – Specialized muscle tissue of the heart, which is striated and connected to adjacent cells.


1. Lymph is fluid in your body that helps circulate important immune cells which help protect you from bacteria and viruses. A common remedy to the common cold is to get up and exercise a little. Why can this be helpful?
A. It will help you breath more, clearing out your lungs
B. Running in the cold is really good for your sickness
C. It helps circulate white blood cells, which fight disease

Answer to Question #1

2. The muscle that controls your lungs is called the diaphragm. The diaphragm is a thin sheet of muscle that extends across the bottom of the chest cavity. While you can hold your breath for a while, once you pass out your body will resume breathing for you. Is the diaphragm connected to the autonomic or somatic nervous systems?
A. Autonomic
B. Somatic
C. Both

Answer to Question #2

3. Why are there no distinct organs in the muscular system?
A. The muscular tissues span most of the other organ systems
B. Muscle tissues are basically all the same
C. The heart is a distinct organ

Answer to Question #3

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