The Musculoskeletal System - What Is It?

Muscles, Bones, Joints, Ligaments, Tendons

Shoulder anatomy
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Musculoskeletal is a general term which, as its name suggests, relates to the muscles and the skeleton of the body. More specifically, the musculoskeletal system includes bones, muscles, joints, cartilage, ligaments, tendons, and bursae. The musculoskeletal system provides stability and also allows for movement of the body.

Anatomy of the Musculoskeletal System

Bones - There are 206 bones in the adult human body.

The structure of bone consists of a hard outer part made of proteins (mostly collagen) and hydroxyapatite (mostly calcium and other minerals). The inner portion of bone, called trabecular bone, is softer than the hard outer cortical bone, but it still is necessary for maintaining bone strength. While the structure of all bone is the same, the bones perform various functions in the body:

  • Bones provide structural support for the body (i.e., a skeletal frame for attachment of organs and tissues) and protect certain organs (e.g., rib cage protects the heart).
  • Bones store the majority of calcium in the body.
  • Bones have an inner cavity which contains the bone marrow where red blood cells, white blood cells, and other components of blood are produced.

Bones undergo a process that is known as remodeling. Bone remodeling is a continuous process whereby old bone is gradually replaced by new bone. Every bone is completely reformed over a period of about 10 years.

Each year, 20% of the body's bone is replaced.

Muscles - There are two kinds of muscle that are part of the musculoskeletal system - skeletal and smooth. The third type of muscle, cardiac, is not part of the musculoskeletal system. Skeletal muscles are bundles of contractile fibers. The action of contracting muscle is what moves various parts of the body.

Skeletal muscles are attached to bones and positioned in opposing groups around the joints (e.g., muscles that bend the elbow are positioned opposite muscles that straighten the elbow). Skeletal muscles are controlled by the brain and they operate voluntarily under a person's conscious direction. Smooth muscles play a role in certain bodily functions that are not under a person's control. Smooth muscle is located around some of the arteries, contracting to adjust blood flow. Smooth muscle is also around the intestines, contracting to move food and feces along the tract. While smooth muscle is also controlled by the brain, it is not voluntary. The engagement of smooth muscle is based on bodily needs -- not conscious control.

Joints - The joints are where the ends of two or more bones come together. While there are joints that do not move (e.g., between the plates of the skull), most joints are capable of facilitating movement. There are two types of joints that facilitate movement: cartilaginous and synovial.

Synovial joints are the type that is familiar to most people. Human synovial joints come in several varieties: ball-and socket, condyloid, gliding, hinge, pivot, and saddle joints. The ends of the bones in this type of joint are covered with cartilage. Joints are enclosed in a joint capsule which has a lining (synovium). Cells of the synovium produce synovial fluid which nourishes the cartilage and helps to reduce friction during movement.

Cartilage - The ends of the bone that form a joint are covered with cartilage. Normal cartilage is smooth, tough, and protective of the ends of the bone. Cartilage is composed of collagen, water, and proteoglycans. Cartilage serves as a shock absorber and reduces friction with movement of a joint.

Ligaments - Ligaments are tough, fibrous cords or bands of tissue that connect bone to bone. Ligaments are composed of collagen and elastic fibers. The elastic fibers allow ligaments to have some stretchability. Ligaments surround and support the joints, allowing movement in specific directions.

Tendons - Tendons are tough, fibrous bands of tissue that connect muscle to bone. Tendons are primarily made of collagen. Tendons are usually found within a sheath (i.e., the tendon sheath) which allows tendons to move friction-free. A tendon sheath has two layers: a synovial sheath and a fibrous tendon sheath.

Bursae - Bursae are small, fluid-filled sacs that serve as a cushion and low-friction gliding surface between adjacent moving body parts like bone, muscles, tendons, and skin. Bursae are found throughout the body. Bursae vary in size depending on their location in the body. There are approximately 160 bursae found throughout the body.

Musculoskeletal Diseases

Musculoskeletal diseases include arthritis, bursitis, and tendinitis, among others. Primary symptoms of musculoskeletal disease include pain, stiffness, swelling, limited range of motion, weakness, fatigue, and decreased physical function. A rheumatologist is a specialist in arthritis and rheumatic diseases. Orthopedic doctors also treat musculoskeletal conditions.

Sources:

Merck Manuals. Bones. Alexandra Villa-Forte, MD. Accessed 11/28/2015.
http://www.merckmanuals.com/home/bone-joint-and-muscle-disorders/biology-of-the-musculoskeletal-system/bones

Merck Manuals. Muscles. Alexandra Villa-Forte, MD. Accessed 11/28/2015.
http://www.merckmanuals.com/home/bone-joint-and-muscle-disorders/biology-of-the-musculoskeletal-system/muscles

Merck Manuals. Ligaments. Alexandra Villa-Forte, MD. Accessed 11/28/2015.
http://www.merckmanuals.com/home/bone-joint-and-muscle-disorders/biology-of-the-musculoskeletal-system/ligaments

Kelley's Textbook of Rheumatology. Ninth edition. Elsevier.

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