Bones and Muscles

The musculoskeletal system is an intricate structure of interconnecting parts. Every movement of the body is the result of the coordination of bones, muscles, tendons, and ligaments.

Bones

Bones are hardened masses of living tissue. They collect calcium for the entire body, storing 99 percent of the body's supply of this mineral. Within some bones is a substance called marrow, which produces red and white blood cells and platelets.

There are 206 bones in the human skeleton. The skeleton maintains the body's shape and protects internal organs. For example, the bones in the skull shield the brain, and the bones in the rib cage encircle and protect the lungs.

The place where two or more bones meet is called a joint. This juncture usually allows movement of the bones that are involved. However, movement is governed by ligaments (bands of fibrous tissue) attached to the bones and by cartilage (elastic tissue) covering the ends of the bones. Ligaments connect one bone with another.

Cartilage cushions and protects bones with the aid of various joint fluids and bursae (small sacs containing lubricating fluid that encompass the joints). Other bands of connective tissue called tendons attach bones to muscles. Muscles are specific kinds of tissues that have the ability to contract. It is this contraction that pulls on the tendons and makes movement possible.

Muscles

There are three types of muscle: striated, smooth, and cardiac. Striated, or striped, muscle consists of layers of tissue divided into bundles of interwoven fibers that run parallel to one another. These layers are attached to the skeleton, and they aid the body in voluntary movement.

Smooth, or organic, muscle is present in several of the internal organs, including the intestines and the bladder, and in the larger blood vessels. This type of muscle functions under involuntary control by the autonomic nervous system. Among its many functions, smooth muscle helps circulate blood and glandular secretions, moves material through the digestive tract, and regulates breathing. Smooth muscle contains elongated, spindly cells arranged parallel to one another that are often grouped into bundles.

Cardiac muscle is the muscle of the heart, and its job is to pump blood. The unique characteristic of the cardiac muscle is that although it is striated (like the muscles of the skeleton), it is controlled by the autonomic nervous system (like the smooth muscles of the internal organs).