Every time you walk, settle into a chair, or hug your child, you're using your bones, muscles, and joints. Without these important body parts, we wouldn't be able to stand, walk, run, or even sit.
From our head to our toes, bones provide support for our bodies and help form our shape. The skull protects the brain and forms the shape of our 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, lungs, liver, and spleen, and the pelvis helps protect the bladder, intestines, and in women, the reproductive organs.
Although they're very light, bones are strong enough to support our entire weight.
The human skeleton has 206 bones, which begin to develop before birth. When the skeleton first forms, it is made of flexible cartilage, but within a few weeks it begins the process of ossification. Ossification is when the cartilage is replaced by hard deposits of calcium phosphate and stretchy collagen, the two main components of bone. It takes about 20 years for this process to be completed.
The bones of kids and young teens are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of columns of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age.
Bone-building continues throughout life, as a body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells: osteoblasts, which make new bone and help repair damage; osteocytes, mature bone cells which help continue new born formation; and osteoclasts, which break down bone and help to sculpt and shape it.
Osteoclasts are very active in kids and teens, working on bone as it is remodeled during growth. They also play an important role in the repair of fractures.
Bones are made up of calcium, phosphorus, sodium, and other minerals, as well as the protein collagen. Calcium is needed to make bones hard, which allows them to support body weight. Bones also store calcium and release some into the bloodstream when it's needed by other parts of the body. The amounts of certain vitamins and minerals that you eat, especially vitamin D and calcium, directly affects how much calcium is stored in the bones.
The soft bone marrow inside many of the bones is where most of the blood cells are made. The bone marrow contains stem cells, which produce the body's red blood cells and platelets, and some types of white blood cells. Red blood cells carry oxygen to the body's tissues, and platelets help with blood clotting when someone has a cut or wound. White blood cells help the body fight infection.
Bones are made up of two types of bone tissues:
Bones are fastened to other bones by long, fibrous straps called ligaments. Cartilage, a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other.
Bones don't work alone — they need help from the muscles and joints. Muscles pull on the joints, allowing us to move. They also help your body perform other functions so you can grow and remain strong, such as chewing food and then moving it through the digestive system.
The human body has more than 650 muscles, which make up half of a person's body weight. They are connected to bones by tough, cord-like tissues called tendons, which allow the muscles to pull on bones. If you wiggle your fingers, you can see the tendons on the back of your hand move as they do their work.
Humans have three different kinds of muscle:
Even when we sit perfectly still, muscles throughout the body are constantly moving. Muscles enable the heart to beat, the chest to rise and fall during breathing, and blood vessels to help regulate the pressure and flow of blood through the body. When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy.
The movements your muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum.
When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.
The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm.
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.
Joints occur 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.
Joints are classified by their range of movement. Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which must be immovable to protect the brain. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone.
Partially movable, or cartilaginous, joints move a little. They are linked by cartilage, as in the spine. Each of the vertebrae in the spine moves in relation to the one above and below it, and together these movements give the spine its flexibility.
Freely movable, or synovial, joints move in many directions. The main joints of the body — found at the hip, shoulders, elbows, knees, wrists, and ankles — are freely movable. They are filled with synovial fluid, which acts as a lubricant to help the joints move easily.
Three kinds of freely movable joints play a big part in voluntary movement:
As strong as bones are, they can break. Muscles can weaken, and joints (as well as tendons, ligaments, and cartilage) can be damaged by injury or disease.
Problems that can affect the bones, muscles, and joints include:
Reviewed by: Yamini Durani, MD
Date reviewed: October 2012
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