Important Point: A whole skeletal muscle is considered an organ of the muscular system. Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.All routine functions, crucial to our daily activity, are dependent on the musculoskeletal system.
Topics in This Chapter
The musculoskeletal system consists of muscles and tendons and ligaments and bones and joints and associated tissues that move the body and maintain form. More specifically, all movements of the human body are dependent on the functioning of muscles. Muscles function to move the body and appendages; they assist with the mechanics of breathing; and they work to move body fluids within the body. When muscles are active, their cells release heat to the body. Muscles also provide form and stability to the body.
The adult human skeletal system, consisting of 206 bones (80 axial and 126 appendicular bones), provides the framework to which muscles attach/support shape. The skeleton also protects sensitive internal organs. Bones provide attachments for muscles (act as levers), which are important in body movements. The marrow from the center of bones produces blood cells. When minerals (e.g., calcium) are not readily available in sufficient amounts, they may be withdrawn from bones.
• Aponeurosis—a whitish, fibrous membrane that connects a muscle to a bone or fascia
• Appositional growth—growth accomplished by the addition of new layers on those previously formed
• Canaliculi—radiate from the lacunae into surrounding bone matrix
• Chondrocytes—produce and maintain the cartilaginous matrix
• Diaphysis—the main or mid section (shaft) of a long bone
• Endomysium—a layer of connective tissue that ensheaths a muscle fiber and is composed mostly from reticular fibers
• Epimysium—a layer of connective tissue which ensheaths the entire muscle
• Epiphyseal plate—the growing ends of young bones
• Fasciculus—a bundle of fibers
• Hematopoiesis—process by which immature precursor cells develop into mature blood cells
• Lamellae—portion of compact bone, used in anchoring
• Osteocytes—most abundant cell found in bone
• Perimysium—is a sheath of connective tissue which groups individual muscle fibers into bundles or fascicles
• Periosteum—is an envelope of fibrous connective tissue that is wrapped around the bone in all places except in joints
• Stapedium muscle—muscle of the middle ear
The human muscular system is composed of specialized cells called muscle fibers. Their predominant function is contractibility. Muscles, where attached to bones or internal organs and blood vessels, are responsible for movement. Nearly all movement in the body is the result of muscle contraction.
Obvious movements such as walking and running are the results of the integrated action of joints, bones, and skeletal muscles. Moreover, the more subtle movements that result, for example, in various facial expressions, eye movements, and respiration are produced by skeletal muscles.
In addition to movement, muscle contraction also fulfills some other important functions in the body, such as posture, joint stability, and heat production. Sitting and standing (posture) is maintained as a result of contraction. The skeletal muscles are continually making fine adjustments that hold the body in stationary positions. The tendons of any muscles extend over joints and in this way contribute to joint stability. This is particularly evident in the knee and shoulder joints, where muscle tendons are a major factor in stabilizing the joint. In maintaining body temperature, heat is an important by-product of muscle metabolism. In fact, nearly 85% of the heat produced in the body is the result of muscle contraction.
Important Point: A whole skeletal muscle is considered an organ of the muscular system. Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.
Skeletal muscles vary considerably in size, shape, and arrangement of fibers. They range from extremely tiny strands such as the stapedium muscle of the middle ear to large masses such as the muscles of the thigh. Some skeletal muscles are broad in shape and some narrow. In some muscles the fibers are parallel to the long axis of the muscle, in some they converge to a narrow attachment, and in some they are oblique.
Each skeletal muscle fiber is a single cylindrical muscle cell. An individual skeletal muscle may be made up of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering. Each muscle is surrounded by a connective tissue sheath called the epimysium. Fascia, connective tissue outside the epimysium, surrounds and separates the muscles. Portions of the epimysium project inward to divide the muscle into compartments. Each compartment contains a bundle of muscle fibers. Each bundle of muscle fiber is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. Within the fasciculus, each individual muscle cell, called a muscle fiber, is surrounded by connective tissue called the endomysium.
Skeletal muscle cells (fibers), like other body cells, are soft and fragile. The connective tissue coverings furnish support and protection for the delicate cells and allow them to withstand the forces of contraction. The coverings also provide pathways for the passage of blood vessels and nerves.
Commonly, the epimysium, perimysium, and endomysium extend beyond the fleshy part of the muscle, the belly or gaster, to form a thick ropelike tendon or a broad, flat sheet-like aponeurosis. The tendon and aponeurosis form indirect attachments from muscles to the periosteum of bones or to the connective tissue of other muscles. Typically a muscle spans a joint and is attached to bones by tendons at both ends. One of the bones remains relatively fixed or stable while the other end moves as a result of muscle contraction.
Important Point: Each muscle fiber is surrounded by endomysium. The fibers are collected into bundles covered by perimysium. Many bundles, or fasciculi, are wrapped together by the epimysium to form a whole muscle.
Skeletal muscles have an abundant supply of blood vessels and nerves. This is directly related to the primary function of skeletal muscle, contraction. Before a skeletal muscle fiber can contract, it has to receive an impulse from a nerve cell. Generally, an artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle. Branches of the nerve and blood vessels follow the connective tissue components of the muscle of a nerve cell and with one or more minute blood vessels called capillaries.
Important Point: One of the most predominant characteristics of skeletal muscle tissue is its contractility and nearly all movement in the body is the result of muscle contraction.
In the body, there are 3 types of muscle: skeletal (striated), smooth, and cardiac:
• Skeletal muscle—attached to bones, is responsible for skeletal movements. The peripheral portion of the central nervous system (CNS) controls the skeletal muscles. Thus, these muscles are under conscious, or voluntary, control. The basic unit is the muscle fiber with many nuclei. These muscle fibers are striated (having transverse streaks) and each acts independently of neighboring muscle fibers.
• Smooth muscle—found in the walls of the hollow internal organs such as blood vessels, the gastrointestinal tract, bladder, and uterus; is under control of the autonomic nervous system. Smooth muscle cannot be controlled consciously and thus acts involuntarily. The nonstriated (smooth) muscle cell is spindle-shaped and has 1 central nucleus. Smooth muscle contracts slowly and rhythmically.
• Cardiac muscle—found in the walls for the heart, is also under control of the autonomic nervous system. The cardiac muscle cell has 1 central nucleus, like smooth muscle, but also is striated, like skeletal muscle. The cardiac muscle cell is rectangular in shape. The contraction of cardiac muscle is involuntary, strong, and rhythmical.
Important Point: Four functions of muscle contraction are movement, posture, joint stability, and heat production.
The more than 600 muscles in the human body account for about 40% of a person’s weight. Most skeletal muscles have names that describe some feature of the muscle. Often several criteria are combined into one name. The following are some terms relating to muscle features that are used in naming muscles:
• Size—vastus (huge); maximus (large); longus (long); minimus (small); brevis (short)
• Shape—deltoid (triangular); rhomboid (like a rhombus with equal and parallel sides); latissimus (wide); teres (round); trapezius (like a trapezoid, a 4-sided figure with 2 sides parallel)
• Direction of fibers—rectus (straight); transverse (across); oblique (diagonally); orbicularis (circular)
• Location—pectorals (chest); gluteus (buttock or rump); brachii (arm); supra- (above); infra- (below); sub- (under or beneath); lateralis (lateral)
• Number of origins—biceps (2 heads); triceps (3 heads); quadriceps (4 heads)
• Origin and insertion—sternocleidomastoideus (origin on the sternum and clavicle, insertion on the mastoid process); brachioradialis (origin on the brachium or arm, insertion on the radius)
• Action—abductor (to abduct a structure); adductor (to adduct a structure); flexor (to flex a structure); extensor (to extend a structure); levator (to lift or elevate a structure); masseter (a chewer)
Important Point: Muscles are attached to bones by tendons.
Numerous and assorted muscle disorders can occur in the human muscular system. In this text, for illustrative purpose only, 2 of the best-known disorders are discussed.
According to the National Institutes of Health (2005), fibromyalgia is a disorder that causes muscle pain and fatigue (feeling tired). People with fibromyalgia have “tender points” on the body. Tender points are specific places on the neck, shoulders, back, hips, arms, and legs. These points hurt when pressure is put on them. The causes of fibromyalgia are unknown.
Important Point: Fibromyalgia affects as many as 1 in 50 Americans. Most people with fibromyalgia are women. However, men and children also can have the disorder. Most people are diagnosed during middle age.
The muscular dystrophies (MD) are a group of more than 30 genetic diseases characterized by progressive weakness and degeneration of the skeletal muscles that control movement. Some forms of MD are seen in infancy or childhood, while others may not appear until middle age or later. The disorders differ in terms of the distribution and extent of muscle weakness (some form of MD also affect cardiac muscle), age of onset, rate of progression, and pattern of inheritance.
Important Point: There is no specific treatment to stop or reverse any form of MD. Treatment may include physical therapy, respiratory therapy, speech therapy, orthopedic appliances used for support and corrective orthopedic surgery.
As mentioned, the human skeleton usually consists of 206 named bones. These bones can be grouped in 2 divisions: axial skeleton and appendicular skeleton. The 80 bones of the axial skeleton form the vertical axis of the body. They include the bones of the head, vertebral column, ribs, and breastbone or sternum. The appendicular skeleton consists of 126 bones and includes the free appendages and their attachments to the axial skeleton. The free appendages are the upper and lower extremities, or limbs, and their attachments (called girdles). The named bones of the body are listed below by category.
Cranial Bone
• Parietal (2)
• Temporal (2)
• Frontal (1)
• Occipital (1)
• Ethmoid (1)
• Sphenoid (1)
Facial Bones
• Maxilla (2)
• Zygomatic (2)
• Mandible (1)
• Nasal (2)
• Platine (2)
• Inferior nasal concha (2)
• Lacrimal (2)
• Vomer (1)
Auditory Ossicles
• Malleus (2)
• Incus (2)
• Stapes (2)
Vertebral Column (26)
• Cervical vertebrae (7)
• Thoracic vertebrate (12)
• Sacrum (1)
• Coccyx (1)
Thoracic Cage (25)
• Sternum (1)
• Ribs (24)
• Clavicle (2)
• Scapula (2)
• Humerus (2)
• Radius (2)
• Ulna (2)
• Carpals (16)
• Metacarpals (10)
• Phalanges (28)
• Coxal, innominate, or hip bones (2)
• Femur (2)
• Tibia (2)
• Fibula (2)
• Patella (2)
• Tarsals (14)
• Metatarsals (10)
• Phalanges (28)
Humans rely on a sturdy internal frame that is centered on a prominent spine. The human skeletal system consists of bones, cartilage, ligaments, and tendons, and accounts for about 20% of the body weight. The living bones in the body use oxygen and give off waste products in metabolism. They contain active tissues that consume nutrients, require a blood supply, and change shape or remodel in response to variations in mechanical stress.
Bones provide a rigid framework (the skeleton) that supports and protects the soft organs of the body. The skeleton supports the body against the pull of gravity. The large bones of the lower limbs support the trunk when standing. The fused bones of the cranium surround the brain to make it less vulnerable to injury. Vertebrae surround and protect the spinal cord and bones of the rib cage help protect the heart and lungs of the thorax.
Important Point: Bones work together with muscles as simple mechanical lever systems to produce body movement.
Bones contain more calcium than any other organ. The intercellular matrix of bone contains large amounts of calcium salts, the most important being calcium phosphate. When the blood calcium levels decrease below normal, calcium is released from the bones so that there will be an adequate supply for metabolic needs. When blood calcium levels are increased, the excess calcium is stored in the bone matrix. The dynamic process of releasing and storing calcium goes on almost continuously.
Important Point: Hematopoiesis, the formation of blood cells, mostly takes place in the red marrow of the bones.
In infants, red marrow is found in the bone cavities. With age, it is largely replaced by yellow marrow for fat storage. In adults, red marrow is limited to the spongy bone in the skull, ribs, sternum, clavicles, vertebrae, and pelvis. Red marrow functions in the formation of red blood cells, white blood cells, and blood platelets.
Important Point: The human skeleton is well adapted for the functions it must perform. Functions of bones include support, protection, movement, mineral storage, and formation of blood cells.
There are 2 types of bone tissue: compact and spongy. The names imply that the 2 types differ in density, or how tightly the tissue is packed together. There are 3 types of cells that contribute to bone homoeostasis. Osteoblasts are bone-forming cells, osteoclasts break down bone, and osteocytes are mature bone cells. Equilibrium between osteoblasts and osteoclasts maintains bone tissue.
• Compact bone—consists of closely packed osteons or haversian systems. The osteon consists of a central canal called the osteonic (haversian) canal, which is surrounded by concentric rings (lamellae) of matrix. Between the rings of matrix, the cone cells (osteocytes) are located in spaces called lacunae. Small channels (canaliculi) radiate from the lacunae to the osteonic (haversian) canal to provide passageways through the hard matrix. In compact bone, the haversian systems are packed tightly together to form what appears to be a solid mass. The osteonic canals contain blood vessels that are parallel to the long axis of the bone. These blood vessels interconnect, by way of perforating canals, with vessels on the surface of the bone.
• Spongy (cancellous) bone—is lighter and less dense than compact bone. Spongy bone consists of plates (trabeculae) and bars of bone adjacent to small, irregular cavities that contain red bone marrow. The canaliculi connect to the adjacent cavities, instead of a central haversian canal, to receive their blood supply. It may appear that the trabeculae are arranged in a haphazard manner, but they are organized to provide maximum strength, similar to braces that are used to support a building. The trabeculae of spongy bone follow the lines of stress and can realign if the direction of stress changes.
The terms osteogenesis and ossification are often used synonymously to indicate the process of bone formation. Parts of the skeleton form during the first few weeks after conception. By the end of the 8th week after conception, the skeletal pattern is formed in cartilage and connective tissue membranes and ossification begins.
Bone development continues throughout adulthood. Even after adult stature is attained, bone development continues for repair of fractures and for remodeling to meet changing lifestyles. Osteoblasts, osteocytes and osteoclasts are the 3 cell types involved in the development, growth and remodeling of bones. Osteoblasts are bone-forming cells, osteocytes are mature bone cells and osteoclasts break down and reabsorb bone.
There are 2 types of ossification, intramembranous and endochondral:
• Intramembranous ossification—involves the replacement of sheet-like connective tissue membranes with bony tissue. Bones formed in this manner are called intramembranous bones. They include certain flat bones of the skull and some of the irregular bones.
• Endochondral ossification—involves the replacement of hyaline cartilage with bony tissue. Most of the bones of the skeleton are formed in this manner.
Bones grow in length at the epiphyseal plate by a process that is similar to endochondral ossification. The cartilage in the region of the epiphyseal plate next to the epiphysis continues to grow by mitosis. The chondrocytes, the region next to the diaphysis, age and degenerate. Osteoblasts move in and ossify the matrix to form bone. This process continues throughout childhood and the adolescent years until the cartilage growth slows and finally stops. When cartilage growth ceases, usually in the early 20s, the epiphyseal plate completely ossifies so that only a thin epiphyseal line remains and the bones can no longer grow in length. Bone growth is under the influence of growth hormone from the anterior pituitary gland and sex hormones from the ovaries and testes.
Important Point: Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or to weight. The increase in diameter is called appositional growth.
The bones of the human body come in a variety of sizes and shapes. The 4 principal types of bones are long, short, flat, and irregular:
• Long bones—are longer than they are wide. They consist of a long shaft with 2 bulky ends or extremities. They are primarily compact bone but may have a large amount of spongy bone at the ends or extremities. Long bones include bones of the thigh, leg, arm, and forearm.
• Short bones—are roughly cube shaped with vertical and horizontal dimensions approximately equal. They consist primarily of spongy bone, which is covered by a thin layer of compact bone. Short bones include the bones of the wrist and ankle.
• Flat bones—are thin, flattened, and usually curved. Most of the bones of the cranium are flat bones.
• Irregular bones—includes those bones not listed in the 3 categories above. They are primarily spongy bone that is covered with a thin layer of compact bone. The vertebrae and some of the bones in the skull are irregular bones.
Important Point: All bones have surface markings and characteristics that make a specific bone unique. There are holes, depressions, smooth facets, lines, projections, and other markings. These usually represent passageways for vessels and nerves, points of articulation with other bones, or points of attachment for tendons and ligaments.
An articulation, or joint, is where 2 bones come together. In terms of the amount of movement they allow, there are 3 types of joints: immovable, slightly movable and freely movable:
• Immovable joints (synarthroses; singular, synarthrosis)—in these joints, the bones come in very close contact and are separated only by a thin layer of fibrous connective tissue. The sutures in the skull are examples of immovable joints.
• Slightly movable joints (amphiarthroses; singular, amphiarthrosis)—in this type of joint, the bones are connected by hyaline cartilage or fibrocartilage. The ribs connecte to the sternum by costal cartilages are slightly movable joints connected by hyaline cartilage. The symphysis pubis is a slightly movable joint in which there is a fibrocartilage pad between the 2 bones. The joints between the vertebrae and the intervertebral disks are also of this type.
• Freely movable joints (diarthroses; singular, diarthrosis)—most joints in the adult human body are of this type. In this type of joint, the ends of the opposing bones are covered with hyaline cartilage, the articular cartilage, and they are separated by a space called the joint cavity. The components of the joints are enclosed in a dense fibrous joint capsule. The outer layer of the capsule consists of the ligaments that hold the bones together. The inner layer is the synovial membrane that secretes synovial fluid into the joint cavity for lubrication. Because all of these joints have a synovial membrane, they are sometimes called synovial joints.
As with muscular disorders, skeletal disorders are numerous and assorted. For illustrative purposes, 3 of the most common and well-known disorders, bone fracture, osteoporosis, and vitamin D deficiency, are discussed in the following.
A bone fracture (broken bone) occurs when a force exerted against bone is stronger than the bone can structurally withstand. Bones are a form of connective issue, reinforced with calcium and bone cells. Bones have a softer center, called marrow, where blood cells are made. As mentioned, the main functions of the skeleton include support, movement and protection of vulnerable internal organs. There are different types of bone fractures that vary in severity. Factors that influence severity include the degree and direction of the forces, the particular bone involved, and the person’s age and general health. Common sites for bone fractures include the wrist, ankle and hip. Hip fractures occur most often in elderly people. Broken bones take around 4 to 8 weeks to heal, depending on the age, health of the individual, and the type of break.
The different types of bone fracture include:
• Greenstick fracture—the bone sustains a small, slender crack. This type of fracture is more common in children, due to the comparative flexibility of their bones.
• Comminuted fracture—the bone is shattered into small pieces. This type of complicated fracture tends to heal at a slower rate.
• Simple fracture (or closed fracture)—the broken bone has pierced the skin.
• Compound fracture (or open fracture)—the broken bone juts through the skin, or a wound leads to the fracture site. The risk of infection is higher with this type of fracture.
• Pathological fracture—bones weakened by various diseases (such as osteoporosis or cancer) tend to break with very little force.
• Avulsion fracture—muscles are anchored to bone with tendons, a type of connective tissue. Powerful muscle contractions can wrench the tendon free, and pull out pieces of bone. This type of fracture is more common in the knee and shoulder joints.
• Compression fracture—occurs when 2 bones are forced against each other. The bones of the spine, called vertebrae, are prone to this type of fracture. Elderly people, particularly those with osteoporosis, are at increased risk.
Vitamin D is a fat-soluble vitamin that is found in food and can also be made in your body after exposure to ultraviolet (UV) rays from the sun. Sunshine is a significant source of vitamin D because UV rays from sunlight trigger vitamin D synthesis in the skin.
The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. By promoting calcium absorption, vitamin D helps to form and maintain strong bones. Vitamin D also works in concert with a number of other vitamins, minerals, and hormones to promote bone mineralization. Without vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults, 2 forms of skeletal diseases that weaken bones.
Important Point: Nutrient deficiencies are usually the result of dietary inadequacy, impaired absorption and utilization, increased requirement, or increased excretion (loss).
1. Most movement in the human body is the result of _____________.
2. In addition to movement, muscle contraction also fulfills some other important functions in the body, such as _____________, _____________, and _____________.
3. Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.
• True
• False
4. Tendon is formed by _____________, _____________, and _____________.
5. Cardiac muscle, found in the walls of the heart, is under control of the _____________ nervous system.
6. The skeleton protects the _____________.
7. When blood calcium levels decrease below normal, _____________ is released from the bones so that there will be an adequate supply for metabolic needs.
8. The osteon consists of a central canal called the _____________, which is surrounded by concentric rings of matrix.
9. Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or weight.
• True
• False
10. Short bones consist primarily of _____________, which is covered by a thin layer of compact bone.
BetterHealthChannel. 2006. Bone Fractures. Victoria, Australia. Accessed September 9, 2006, at www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Bone_fractures?open.
Chesney, R. 2003. Rickets: An Old Form for a New Century. Pediatrics International 45:509–11.
Gaby, A.R.1994. Preventing and Reversing Osteoporosis. New York: Random House.
NIH. 2005. What Is Fibromyalgia? National Institutes of Health. Accessed September 2006, at www.niams.nih.gov/hi/topics/fibromyalgia/fffibro.htm.
NIH. 2006. Dietary Supplement Use & Safety. Accessed September 9, 2006, at http://dietarysupplements.info.nih.gov.
SEER. 2000. Anatomy & Physiology: Skeletal and Muscular System. Accessed September 6, 2006, at http://training.seer.cancer.gov/module.
Van den Berg, H. 1997. Bioavailability of Vitamin D. European Journal of Clinical Nutrition 51 Suppl 1:S76-9.
U.S. Department of Agriculture, Agricultural Research Service. 2003. USDA Nutrient
Database for Standard Reference, Release 16. Nutrient Data Laboratory Home Page. Accessed August 18, 2006, at www.nal.usda.gov/fnic.foodcomp.