For the purposes of this workbook the skeletal system consists of four components: bones, cartilage, tendons, and ligaments, the latter three may also be considered part of the articular system. There are 206 bones in the body creating the skeletal structural framework. This framework gives the body its shape, provides support and protection for tissues and organs, and serves as a storage site for minerals such as calcium and phosphorus. Bone tissue is also a site for blood cell production (hematopoiesis), and supplies the levers for movement, or kinesiology.

Figure 1

The bones of the body are divided into two main categories (figure 1). The axial (consisting of approximately 80 bones of the head, thorax, and trunk) and the appendicular (consisting of approximately 126 bones of the upper and lower extremities).

Axial Skeleton (80 bones)

• Cranium (8) – Frontal, Sphenoid, Ethmoid, Occipital, Parietal (2), Temporal (2)
• Face (14) – Mandible, Vomer, Maxilla (2), Zygomatic (2), Lacrimal (2), Inferior Concha (2), Palatine (2), Nasal (2)
• Other (7) – Hyoid, Ear ossicles (3)
• Vertebral column (26) – Cervical (7), Thoracic (12), Lumbar (5), Sacrum (5 fused), Coccyx (1 – 3 fused)
• Thorax (25) – Sternum, ribs (12 pair -  7 true, 3 false, 2 floating)

Appendicular skeleton (126 bones)

• Upper Extremity (64) – Scapula (2), Clavicle (2), Humerus (2), Radius (2),    
• Ulna (2), Carpals (16), Metacarpals (10), Phalanges (28)
• Lower Extremity (62) – Os Coxae (2 – 6 fused together), Femur (2), Tibia (2),
• Fibula (2), Patella (2), Tarsals (14), Metatarsals (10), Phalanges (28)

Figure 2 

Bone Structure - Classification & Function

Joint Complex Connective Tissues

There are three basic materials that make up the connective tissues throughout the body, 1) fibrous proteins (collagen and elastin), 2) ground substance, and 3) cells. Each will be described in more detail. The proportion and arrangement of these three materials determine whether the connective tissue forms articular cartilage, periarticular structures – ligament, tendon, bursa, joint capsule, meniscus, or bone.  

1. Fibrous Proteins:

• Collagen: provides significant tensile strength or the ability to withstand tension or pulling forces.  Type I collagen is found primarily in ligaments, tendons, fascia, labra, menisci, fibrocartilage, joint capsule, and skin. Type II collagen is thinner with less tensile strength but provides a framework that helps maintain the shape of a structure as seen in hyaline cartilage and intervertebral discs.
• Elastin: provides the ability of a tissue to deform and then return to its original shape (extensibility) after being stretched and is found in joint and periarticular structures, skin, arterial walls, and the tracheobronchial tree.

2. Ground Substance: a water filled matrix that surrounds and contains collagen and elastin fibers composing connective tissue. It is composed mainly of glycosaminoglycans and water. This combination of fibrous proteins and the hydrophilic glycosaminoglycans allow tissues to expand and form a semifluid tissue that can greatly resist compression. However, through intermittent compression that occurs during movement, the fluid component of the tissues transports gasses, nutrients, and waste products back and forth between the connective tissue cells and the surrounding nutrient-rich synovial fluid. This is one reason why movement is so very important to the health and integrity of tissues. The absence of movement, for example immobilization of a joint, is detrimental to joint tissue health.
   
   
3. Cells: the cells composing most connective tissues are called fibroblasts (synthesize mostly type I collagen) and these cells can differentiate into chondroblasts that produce mostly type II collagen. Fibroblasts can also become tenoblasts (tendon cells) and osteoblasts (bone cells). As these cells mature they are referred to as fibrocytes, chondrocytes, tenocytes, and osteocytes.

Bone Tissue and Wolff’s Law

Bone tissue is a type of connective tissue composed of bone cells and matrix.

• Bone Cells consist of osteoblasts that build up bone by secreting matrix, osteoclasts that break down matrix by resorbing the mineral matrix, and osteocytes that are mature bone cells fully surrounded by matrix.
• Matrix of a connective tissue is defined as everything other than the cells and can be divided into the fiber component (collagen fibers that add tensile strength), the ground substance component (proteoglycans -glucosamine and chondroitin sulfate molecules that trap fluid) and the inorganic mineral content (calcium-phosphate salts) that give bone its rigidity or a state of being rigid, stiff, inflexible.

Figure 3  

Bone as a Living, Dynamic Tissue 

Because bone responds to the physical demands placed upon it, it is subject to Wolff’s Law. Wolff’s Law states that the structural formation of bone (its size, mass, and shape),grows according to the direction and magnitude of stresses and strains, habitually applied to it.

As such, if increased physical stress is placed on the bone, then the bone responds by gaining a bony matrix (function of osteoblasts) and increases in density and strength. If decreased physical stress is applied to the bone, then the bone responds by losing the bony matrix (function of osteoclasts) and decreases in density and strength. This principle allows the bone to adapt to the changing demands that are repetitively placed upon it. 

Wolff’s Law and the Piezoelectric Effect

When mechanical stress/pressure – compression via impact (e.g., plyometrics) or pulling via muscular contraction (resistance training) is placed on a tissue, a slight electric charge is produced in the tissue. This is known as the piezoelectric effect (“Piezo” means pressure). This causes a transient depolarization in the membranes of the osteocytes that are embedded in the bone matrix through stretch activated Ca++ channels. This depolarization causes the release of growth factors that stimulate nearby osteoblasts to produce matrix.

Osteoblasts can lay down bone matrix in any tissue, while osteoclasts are unable to resorb (break down) bone matrix from bone in piezoelectrically charged tissue. As a result, greater bone mass forms in regions of a bone that are under greater pressure producing stronger bones more capable of withstanding forces applied to them. On the other hand, mass is diminished in bone where there is a lesser stress applied. As such, there is a greater risk for bone damage from trauma and from the degenerative bone disease of osteoporosis.

Wolff’s Law Gone Bad

Unfortunately, chronic unrelieved and unbalanced stress/pressure placed on a bone may result in excessive matrix being deposited in the bone. As the bone tissue becomes denser and denser the body starts to place calcium along the outer margins of the bone in an abnormal manner. This can result in bone spurs, degenerative joint disease, osteoarthritis, or abnormal formation of the bone, etc. Below are some examples of the effects of abnormal/unbalanced stress placed on bone tissue.

Figures 4

Osteophyte formation

Figure 5

Torsion

Figure 6

Scoliosis

Soft Connective Tissue

There are several different types of soft tissues in the body. Joints, the union formed between bones where body movement takes place, have various anatomical structures depending on the type or functional classification of the joint. The components of the periarticular soft connective tissue that forms many joints are the joint capsule, ligaments, tendons, articular cartilage, bursae, and fibrocartilage. Other soft connective tissues associated with the musculoskeletal system include retinaculum, tendon sheaths, aponeurosis, and fascia. 

Properties of Soft Connective Tissues

Extensibility: the ability of a tissue to be stretched, becoming longer without injury or damage

Weight Bearing: the ability of a tissue to bear the compressive force of the weight of the body located above it, without injury or damage

Tensile Strength: the ability of a tissue to withstand a pulling force without injury or damage

Elasticity: the ability of a tissue to return to its normal resting length after being stretched

Plasticity: the ability of a tissue to have its shape altered or molded, and then to retain the new shape (elongation)

Elasticity and Plasticity Compared

Whenever a soft tissue of the body has been altered or deformed because a force has been applied to it (compressed or pulled) the tissue has a certain elastic ability to return to its original shape. If that elasticity is exceeded, then plasticity describes the fact that the shape of the tissue will stay permanently (or relatively permanently) altered or deformed to some degree. An example is a ligament that has been stretched. If it is only slightly stretched, its elastic ability allows it to return to its original length and provide the appropriate joint stability. However, if it is stretched to a greater degree (or repeatedly stretched) and its elastic ability is exceeded, the tissue will enter its plastic range and will become permanently (or relatively permanently) overstretched. This increased laxity results in a decrease in stability where the ligament is located. 

Figure 12