The Musculoskeletal System
The musculoskeletal system encompasses the bones, muscles, tendons, ligaments, and fasciae. Practitioners who diagnose and treat health conditions of the musculoskeletal system are orthopedists (also called orthopedic surgeons). Orthopedists may further specialize in sports medicine or physiatry (rehabilitative medicine).
Functions of the Musculoskeletal System
The musculoskeletal system gives the body form and structure, protects the internal organs, and provides movement. It determines body height and mass. It is the foundation for facial features, hand characteristics, and athletic ability. The bones of the SKELETON form the core of the structural body; the muscles build the body’s outward appearance. In tandem, the bones and the muscles carry the body through life.
A soft start: the skeleton’s origins
The SKELETON arises from the mesoderm very early in embryonic development, taking rudimentary form at about three weeks of gestational age. Hyaline CARTILAGE, a tough, dense type of connective tissue, forms the template that will become the ossified (mineralhardened) skeleton. Though the process of ossification begins before birth, the greater percentage of the skeleton is still cartilage at birth to facilitate passage through the birth canal.
After birth an intricate, HORMONE-regulated process immediately sets about to convert cartilage cells (chondrocytes) to BONE cells (osteocytes). This process of ossification takes the first two decades of life to reach fruition. Bone tissue continues to grow and change throughout life even after bone size reaches stability through another process called bone remodeling, in which bone-building cells (osteoblasts) create new bone structure in synchronization with bone-destroying cells (osteoclasts) which remove old bone structure.
Framework: the skeleton
The 206 bones of the adult human skeleton give the body shape, protection, and mobility. There are two divisions of the skeleton:
- The axial skeleton forms the body’s central alignment; its bones are primarily those of support and shelter.
- The appendicular skeleton “hangs from” the axial skeleton; its bones are primarily those of movement.
Bones provide the structure that gives the body resistance against gravity and makes movement possible. Long bones, such as those in the arms and legs, function as levers for the skeletal muscles to generate movement and locomotion. A honeycombed structure within the long bones reduces their density and weight while increasing their STRENGTH. The compact construction of short bones, such as those in the hands and feet, supports functions that require greater strength and less leverage. Flat bones, such as the scapulae (shoulder blades) and pelvis (hip bones), provide surface area for firmly anchoring the large skeletal muscles that make movement possible.
Some bones function as armor, protecting vital structures and organs. The smooth, thick bones of the skull completely encase the BRAIN in a chamber that has few natural points of entry. Vertebrae separated by cushions of cartilage enclose the SPINAL CORD, their irregular shapes deflecting access while at the same time permitting FLEXIBILITY. The ribs form a cage that contains the HEART and LUNGS, providing a framework for the bellows-like action of the lungs with the thick sternum like a shield to shelter the heart.
Form and function: the muscles
The 650 or so muscles in the body give the body shape and make movement, including locomotion, possible. The skeletal muscles cover and protect the bones, attaching directly to them. Muscles also support and protect other structures such as BLOOD vessels and nerves. Most skeletal muscles work in opposing pairs, with one MUSCLE group contracting and the other relaxing in synchronization to permit the balanced, coordinated, and smooth movements necessary for all body mobility from sitting to running.
Muscle cells form collective structures, muscle fibers, that are the functional units of movement. NERVE impulses from motor neurons (nerve cells that direct movement) travel from the NERVOUS SYSTEM to the muscle fibers. The NEUROTRANSMITTER acetylcholine facilitates the transfer of the impulse from the NEURON to the muscle fiber. Some muscle fibers remain in a state of partial contraction, providing muscle tone that supports posture. Other muscle fibers contract and relax in rapid sequence, providing muscle strength.
Connecting structures: tendons, ligaments, and fasciae
Specialized structures of connective tissue join the bones and the muscles. Tendons, fibrous bands that arise from muscle, join muscle to bone. Ligaments are tough and sinewy; they connect bones to each other. Sheetlike FASCIA covers the muscles, connecting muscle to muscle and muscle to SKIN.
Articulating interfaces: the joints
The ends of the bones come together in various ways that facilitate their movement. Hinge joints, such as the knee and the elbow, allow flexion and extension. Ball and socket joints, such as the hips and shoulders, allow rotational movement. The joints of the cranium—called sutures—are fused, allowing no movement at all. The vertebrae—the bones of the spine—have slight movement between each but collectively allow the body to bend in half.
Most joints contain cartilage, the body’s most dense type of connective tissue, to cushion and protect the bones. Cartilage is very smooth, almost slick, permitting movement with minimal resistance. Synovial fluid lubricates larger joints, further reducing friction. A thin coat of cartilage covers the caps of the long bones in the arms and the legs. Thick pads of cartilage cushion the knees and the vertebrae, joints that bear considerable force with movement such as walking.
Biomechanics of movement
Movement is a function of leverage and resistance that represents a complex and intricate interaction among the nerves, muscles, connective tissues, and bones. The cerebral cortex coordinates the numerous processes that make movement possible, integrating external sensory data with internal messages. A specialized sensory process, PROPRIOCEPTION, establishes unconscious awareness of the body’s location within its physical environment. Proprioception helps the brain interpret and respond to the myriad messages about the body’s relation to gravity and speed.
Harder than bone: the teeth
The TEETH are the hardest structures in the body, formed of calcium and other minerals with a nearly impermeable enamel coating. The jaw bones—the maxilla (upper jaw) and the mandible (lower jaw) anchor the teeth. Like most mammals, humans have two sets of teeth, the deciduous (sometimes called primary, milk, or baby teeth) and the permanent. Deciduous teeth begin to erupt through the gum line at about age four months; they drop out and permanent teeth replace them starting at about age six or seven years. The adult mouth contains 32 permanent teeth, generally occurring in pairs on each side of the mouth. They are of three major structures:
- Incisors and cuspids have sharp surfaces for cutting; these teeth are in the front of the mouth.
- Molars have flat surfaces for grinding and crushing; these teeth are in the back of the mouth.
- Bicuspids, sometimes called premolars, function somewhat as transitional structures, capable of secondary biting and preliminary chewing; they are in the middle of the jaw line.
Within the calcium cap is the tooth’s living tissue, the pulp. Hollow extensions penetrate deep into the bones of the jaw, their protective canals encasing the nerves and blood vessels that supply the pulp. Chips and cracks in the enamel occur over time, weakening its protection and allowing BACTERIA to penetrate and begin to destroy the calcium cap, exposing the inner pulp. This kind of damage—dental caries (cavities)—is the leading oral health challenge. The teeth also facilitate speech, functioning like reflective walls to amplify sound and providing resistance for the tongue as it reshapes sound into words.
Health and Disorders of the Musculoskeletal System
The musculoskeletal system carries the human body hundreds of thousands of miles in the course of a typical lifetime. Trauma notwithstanding, it does so with few “maintenance” requirements and little complaining. Proper nutrition and regular physical exercise are about all the bones, muscles, and connective structures require for most of life. However, the musculoskeletal system is vulnerable to numerous hereditary, congenital, and acquired health conditions. Trauma is the most significant risk to the musculoskeletal structures, particularly the limbs and joints. Sprains, strains, and fractures are common injuries. Over time, the repeated trauma of daily function also takes its toll. OSTEOARTHRITIS, the consequence of degenerative damage to the joints, is the most common musculoskeletal ailment, affecting as many as 60 million Americans.
Hereditary and congenital disorders can affect both the structure and function of the musculoskeletal system—and by extension, of other systems of the body as well. Connective tissue, the foundation of the musculoskeletal system, exists in nearly every body structure. Disorders of connective tissue such as MARFAN SYNDROME affect not only the skeleton and muscles but the walls of the arteries and the structure of organs. Though many movement disorders are neurologic in origin, disorders of muscle function such as MUSCULAR DYSTROPHY also affect mobility and motor function.
Traditions in Medical History
Not until the end of the Renaissance did physicians and scientists fully understand the structure of the human musculoskeletal system. The skeleton represented death; only without flesh was it visible. Seeing a bone, even in life, was never a good thing. Fractures, particularly compound fractures in which the bone ends broke through the surface of the skin, were frequently fatal. INFECTION was nearly inescapable. Fractures that did not kill often maimed; ancient doctors had little knowledge of biomechanics and without guidance from technology commonplace today, setting a fracture was at best an imprecise art.
The discovery of the X-RAY—electromagnetic energy capable of penetrating soft tissue—in the late 19th century finally gave doctors a means to examine the bones of living people. With X-ray doctors could see the bone ends of fractures and realign those ends for proper HEALING, and orthopedic medicine was born. Today X-ray remains the quintessential diagnostic tool for skeletal injuries.
Breakthrough Research and Treatment Advances
Today’s technology allows incredible visualization of musculoskeletal structures, well beyond the black-and-white X-ray, and of musculoskeletal functions—with minimal intrusion into the body. Nucleotide bone scans, MAGNETIC RESONANCE IMAGING (MRI), ULTRASOUND, and COMPUTED TOMOGRAPHY (CT) SCAN allow doctors to “see” injuries such as torn ligaments, ruptured tendons, and stress fractures. Arthroscopy uses fiberoptic technology to view the inside of a JOINT, providing a method of minimally invasive visualization for diagnostic and therapeutic purposes.
As in most areas of health and medicine, the most significant breakthroughs in research and treatment for musculoskeletal conditions comes from new discoveries in genetics. Researchers have identified many of the GENE mutations responsible for muscular dystrophy, for example. Though GENE THERAPY as treatment for genetically based musculoskeletal conditions remains experimental, the potential is great for treatments that can reverse the effects of gene mutations to halt and correct disease processes.
