Musculoskeletal System: Normal Structure & Function
What should I know about the musculoskeletal system?
The skeletal system includes the bones of the skeleton and the cartilages, ligaments, and other connective tissue that stabilize or connect the bones. In addition to supporting the weight of the body, bones work together with muscles to maintain the body’s position and to produce controlled, precise movements. Without the skeleton to pull against, contracting (tightened) muscle fibers could not make us sit, stand, walk, or run.
There are 206 bones in the adult body. The bones perform five main functions for the body:
- Provide support: The skeletal system provides structural support for the entire body. Individual bones or groups of bones provide a framework for the attachment of soft tissues and organs.
- Store minerals and lipids: Calcium is the most abundant mineral in the body. (Ninety-nine percent of the body's calcium is found in the skeleton.) The calcium salts of bone are a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids. The bones of the skeleton also store energy reserves as lipids (fats) in areas filled with yellow marrow.
- Produce blood cells: Red blood cells, white blood cells, and other blood elements are produced in the red marrow, which fills the internal cavities of many bones.
- Protect body organs: Many soft tissues and organs are surrounded by skeletal elements. For example, the rib cage protects the heart and lungs, the skull protects the brain, the vertebrae protect the spinal cord, and the pelvis protects the delicate reproductive organs.
- Provide leverage and movement: Many bones function as levers that can change the magnitude (strength) and direction of the forces generated by muscles.
Each bone in the skeleton contains two forms of tissue: compact (dense) bone that is relatively solid, and spongy (cancellous) bone that forms an open network of struts and plates. Compact bone is found on the external surface of the bone; spongy bone is located inside the bone.
The amount of compact and spongy bone depends on the shape of the bone. Compact bone is thickest where stresses arrive from a limited range of directions. Spongy bone is located where bones are not heavily stressed or where stresses arrive from many directions. Spongy bone is much lighter than compact bone, which helps reduce the weight of the skeleton and makes it easier for muscles to move the bones.
Bone development and growth
The growth of the skeleton determines the size and proportions of the body. Bones begin to form in a fetus about six weeks after fertilization, and portions of the skeleton do not stop growing until a person is about 25 years old.
Most bones begin as hyaline cartilage. The cartilage is gradually converted to bone through a process called ossification. Bone growth begins at the center of the cartilage. As bones enlarge, bone growth activity shifts to the ends of the bones (an area commonly called the growth plate), which causes the bones to grow longer.
Bone growth "factoids"
- Twenty percent of the adult skeleton is replaced each year.
- Moderate amounts of physical activity and weight-bearing activities are essential to stimulate bone maintenance and to maintain adequate bone strength.
Other elements of the musculoskeletal system
- Joints: These are where two bones interconnect. Each joint reflects a compromise between stability and range of motion. For example, the bones of the skull are very stable but with little motion, whereas the shoulder joint allows for a full range of motion but is a relatively unstable joint.
- Tendons: These attach muscle to bone.
- Ligaments: These attach bone to bone.
- Skeletal muscles: These muscles contract to pull on tendons and move the bones of the skeleton. Skeletal muscles also:
- Maintain posture and body position;
- Support soft tissues;
- Guard entrances and exits to the digestive and urinary tracts; and, Maintain body temperature.
- Nerves: Nerves control the contraction of skeletal muscles, interpret sensory information, and coordinate the activities of the body's organ systems.
- Cartilage: This is a type of connective tissue. It is a firm gel-like substance. The body contains three major types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage.
- Hyaline cartilage is the most common type of cartilage. It provides stiff but somewhat flexible support. Examples in adults include the tips of ribs (where they meet the sternum [breastbone]) and part of the nasal septum. Another example is articular cartilage, which covers the ends of bones within a joint. The surfaces of articular cartilage are slick and smooth, which reduces friction during joint movement.
- Elastic cartilage provides support but can tolerate distortion without damage and return to its original shape. Elastic cartilage can be found in the external flap of the ear, among other places.
- Fibrocartilage resists compression, prevents bone-to-bone contact, and limits relative movement. Fibrocartilage can be found within the knee joint, between the pubic bones of the pelvis, and between the spinal vertebrae.
Cartilage heals poorly, and damaged fibrocartilage in joints such as the knee can interfere with normal movements. The knee contains both hyaline cartilage and fibrocartilage. The hyaline cartilage covers bony surfaces; fibrocartilage pads in the joint prevent contact between bones during movement. Injuries to the joints can produce tears in the fibrocartilage pads, and the tears do not heal. Eventually, joint mobility is severely reduced.