Bones, Muscles, and Joints

Bones, Muscles, and Joints

What Are Bones and What Do They Do?

Bones provide support for our bodies and help form our shape. Although they're very light, bones are strong enough to support our entire weight.

Bones also protect the body's organs. The skull protects the brain and forms the shape of the face.

The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal column.

The ribs form a cage that shelters the heart and lungs.

What Are Muscles and What Do They Do?

Muscles pull on the joints, allowing us to move. They also help the body do such things as chewing food and then moving it through the digestive system.

Even when we sit perfectly still, muscles throughout the body are constantly moving.

Muscles help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood.

When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy.

Humans have three different kinds of muscle:

Skeletal Muscle

Skeletal muscle is attached by cord-like tendons to bone, such as in the legs, arms, and face.

Skeletal muscles are called striated (STRY-ay-ted) because they are made up of fibers that have horizontal stripes when viewed under a microscope.

These muscles help hold the skeleton together, give the body shape, and help it with everyday movements (known as voluntary muscles because you can control them). They can contract (shorten or tighten) quickly and powerfully, but they tire easily.

Smooth Muscle

Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated.

We can't consciously control our smooth muscles; rather, they're controlled by the nervous system automatically (which is why they're also called involuntary).

Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system.

Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure.

Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't tire easily.

Cardiac Muscle

Cardiac muscle is found in the heart.

The walls of the heart's chambers are composed almost entirely of muscle fibers.

Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats.

How Do Muscles Work?

Muscles move body parts by contracting and then relaxing.

Muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors.

The flexor contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint.

 For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow.

What Are Joints and What Do They Do?

Joints are where two bones meet. They make the skeleton flexible — without them, movement would be impossible.

Joints allow our bodies to move in many ways. Some joints open and close like a hinge (such as knees and elbows), whereas others allow for more complicated movement — a shoulder or hip joint, for example, allows for backward, forward, sideways, and rotating movement.

Three kinds of freely movable joints play a big part in voluntary movement:

Hinge Joints

Hinge joints allow movement in one direction, as seen in the knees and elbows.

Pivot Joints

Pivot joints allow a rotating or twisting motion, like that of the head moving from side to side.

Gliding Joints

Ball-and-Socket Joints

Ball-and-socket joints allow the greatest freedom of movement. The hips and shoulders have this type of joint, in which the round end of a long bone fits into the hollow of another bone.