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               Muscular System


                       Almost half the weight of your body

                       is in the form of muscles. The

muscular  system allows you to move. There are

many movements in the body that you may not

even notice, but if it is moving, it’s a muscle.


   There are two types of muscle movements

in your body. Voluntary muscles are muscles that you can control. You

can choose to move your arms and legs. You can choose to smile with muscles in your face and you choose to play the piano with muscles in your hands.

  Involuntary muscles are muscles that you cannot consciously control. You don’t have to think about moving these muscles to get important jobs done. Food gets moved through your digestive tract, blood moves through blood vessels, and your diaphragm allows you to breathe. Think about how difficult it would be if you had to think about these movements all the time!

Learning Objectives   

By the end of this lesson, students should be able to:

  • Describe the structure and function of muscles.

  • Compare and contrast three types of muscles.

  • Explain how muscles and bones work together in movement.                                    


cardiac muscle—muscle found only in the heart.

involuntary muscle—muscles that are not consciously controlled.

ligament—a strong band of tissue that connects bones to each other at a joint.

muscle—an organ that contracts and gets shorter to move body parts.

skeletal muscle—a type of muscle that you can control.

smooth muscle—a type of muscle that you cannot control.

tendon—a tough band of tissue that connects muscles to bones.

voluntary muscle—any muscle that can be controlled.


   There are three types of muscle tissue—smooth, skeletal, and cardiac.  Each type of muscle has its own function and each has a different structure, but all muscle allow movement.

Skeletal Muscle

   Skeletal muscle is called voluntary because you can control this type of muscle. You can voluntarily choose to move them.  These muscles attach to your bones and move the skeleton. They are found in the arms, legs, neck, or anywhere you can choose to move a body part.

  Look at the microscope slide of skeletal muscle

tissues at the right. Do you see the stripes in the

cells? These muscle fibers appear striated (striped)

and have more than one nucleus. These fibers may

up to 30 cm long in humans. Look at the table in this

lesson that compares the features of the three type of muscle fibers.


Smooth Muscle

  Smooth muscle is called involuntary muscle because you cannot control the movement. These muscles line internal organs, blood vessels and organs found in your digestive system.

                                       Look at the microscope slide of smooth muscle at                                    the left. Do you see how this muscle does not have                                  the striped appearance that skeletal muscle has?                                      Smooth muscle has only one nucleus per cell and                                    are usually shorter in length than skeletal muscles.                                    They produce weaker contractions.


Cardiac Muscle

   Cardiac muscle tissue is involuntary muscle and is found only in your heart. You cannot control your heart muscle; it automatically works for you.

   These muscle fibers also appear striated when

magnified and have more than one nuclei per cell.

A major difference between cardiac muscle and the

other types of muscle is that cardiac muscle cells

are branched.


 Fast Fact: Everyone has the same number of muscles. When you see someone who lifts weights and has bulging muscles, it just means that the muscle tissue enlarges.  The number of muscle in the human body is the same for everyone.


   Let’s consider how connected the body systems are to each other. The muscular system works with the nervous system to produce a contraction that moves muscles. Our muscles are connected to the skeletal system to allow for strength and movement. Our circulatory system must transfer the nutrients necessary for the processes to function. Our digestive system allows us to process the food we eat into nutrients that are available to our cells.

   Each our of body systems work together to allow you, the complete organism to function.


Major Muscles of the Human Body

The diagrams of the major muscles in your body gives you an idea of how muscles are grouped and layered in your body to allow you to move in many different ways.


Anterior View                                                Posterior View





















Check for Understanding

1. Compare and contrast three types of muscles.




2. What is the difference between voluntary and involuntary muscles?



3. Why is it a benefit that we have muscles that we cannot control?



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anterior view of human muscles on torso and head
microscope slide showing striped appearance of skeletal muscle tissue
microscope slide showing absence of striations in smooth muscle tissue
microscope slide showing striation and branching of cardiac muscle tissue
anterior view of human muscles
posterior view of human muscles
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