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 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. Reviewed by: KidsHealth Medical Experts. Larger text size Large text size Regular text size.
Bones are made up of two types of bone tissues: Compact bone is the solid, hard outside part of the bone. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves. Cancellous pronounced: KAN-suh-lus bone , which looks like a sponge, is inside compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae pronounced: truh-BEH-kyoo-lee. This is where bone marrow is found. How Do Bones Grow? Bone contains three types of cells: osteoblasts pronounced: AHS-tee-uh-blastz , which make new bone and help repair damage osteocytes pronounced: AHS-tee-o-sites , mature bone cells which help continue new born formation osteoclasts pronounced: AHS-tee-o-klasts , which break down bone and help to sculpt and shape it What Are Muscles and What Do They Do?
Humans have three different kinds of muscle: Skeletal muscle is attached by cord-like tendons to bone, such as in the legs, arms, and face. Skeletal muscles are called striated pronounced: 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 their movement.
They can contract shorten or tighten quickly and powerfully, but they tire easily. By the contraction and extension of your skeletal muscles. Notice how the muscles are attached to the bones. The muscles pull on the bones, causing movement. There are well over skeletal muscles in the human body, some of which are identified in Figure below.
Skeletal muscles vary considerably in size, from tiny muscles inside the middle ear to very large muscles in the upper leg. Each skeletal muscle consists of hundreds or even thousands of skeletal muscle fibers. The fibers are bundled together and wrapped in connective tissue, as shown Figure below. The connective tissue supports and protects the delicate muscle cells and allows them to withstand the forces of contraction.
It also provides pathways for nerves and blood vessels to reach the muscles. Skeletal muscles work hard to move body parts. They need a rich blood supply to provide them with nutrients and oxygen and to carry away their wastes. Skeletal Muscle Structure. Skeletal muscles are attached to the skeleton by tough connective tissues called tendons see Figure above. The balance makes movements smooth, which helps prevent damage to the musculoskeletal system.
Skeletal muscles are controlled by the brain and are considered voluntary muscles because they operate with a person's conscious control. The size and strength of skeletal muscles are maintained or increased by regular exercise.
In addition, growth hormone and testosterone help muscles grow in childhood and maintain their size in adulthood. Smooth muscles control certain bodily functions that are not readily under a person's control. Smooth muscle surrounds many arteries and contracts to adjust blood flow. It surrounds the intestines and contracts to move food and feces along the digestive tract. Smooth muscle also is controlled by the brain but not voluntarily.
The triggers for contracting and relaxing smooth muscles are controlled by the body's needs, so smooth muscles are considered involuntary muscle because they operate without a person's conscious control.
Cardiac muscle forms the heart and is not part of the musculoskeletal system. Like skeletal muscle, cardiac muscle has a regular pattern of fibers that also appear as stripes under a microscope. However, cardiac muscle contracts and relaxes rhythmically without a person's awareness.
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