Musculoskeletal System

How the Musculoskeletal System Works
The musculoskeletal system provides both structure and function for the body. The
bones protect and support the vital organs. The skeleton is divided into the axial
and appendicular areas. The axial skeleton protects the vital organs, surrounding
the central nervous system and thoracic cavity. The appendicular skeleton attaches
to the axial skeleton and consists primarily of the limbs. Bones are classified by
both their shape and their composition. Short bones (like the phalanges) are found
in the fingers and toes. Long bones (like the humerus or femur) are found in the
limbs. Irregular bones are named for their shapes and are found in the joints of
the ankle or wrist and in the middle ear. Flat bones (ribs or scapula) protect inner
organs. The outer layer of bone tissue is a dense compact bone tissue called the
cortex. Blood supply for the bone travels through small blood vessels within
haversian canals located longitudinally within the cortical bone area. The inner layer
is more spongy, cancellous tissue which has spaces filled with marrow. Production
of blood cells occurs within the red bone marrow. Yellow bone marrow is composed
primarily of fat cells. Osteoblasts (bone-building cells) and osteoclasts (bone-resorb-
ing cells) are found in the outer layer of the bone.
Joints are the areas where two or more bones come together. Joints are described
as being freely movable (synovial joints like the hip), partially movable (pelvic
bones), or immovable (suture lines in the skull). Synovial joints are lined with syn-
ovium. This membrane secretes synovial fluid to lubricate the joint and act as a
shock absorber during motion or weight bearing. Synovial joints have a variety of
range of motion including flexion, extension, rotation, circumduction, supination,
pronation, abduction, adduction, inversion, and eversion. Partially movable joints
have specific small amounts of motion that are typical of the joint space. Pelvic
bones and individual joints between the vertebral bones are partially movable. Im-
movable joints are areas where bones come together, but no movement is allowed.
Muscles work in groups, with one set of muscles relaxing as another set con-
tracts to create motion. A small amount of muscle contraction is typical to main-
tain muscle tone within the muscles. Skeletal muscle is striated and voluntary.
Connective tissues are the pieces that hold other parts together. Tendons attach
muscles to bones; ligaments attach bones to bones. Cartilage provides a smooth
surface within joints to ease movement and provide cushioning to weight-bearing
joints. Bursa are small fluid-filled sacs, within joint areas or adjacent to bone
which provide cushioning at points of friction.