ANATOMY & PHYSIOLOGY
Skeletal System

I. Introduction

The skeletal system includes connective tissues such as bone, cartilage, tendons, and ligaments. These tissues are combined with the various types of muscle tissue to form the Musculo-Skeletal System.

1) Bone itself has five functions, including:

Movement
Support
Calcium Storage
Production of Red Blood Cells
Protection

2) Cartilage serves as the fetal template for bone formation, and covers the ends of bone, most especially at the joints, or points of articulation.

3) Tendons connect muscles to bone

4) Ligaments connect bone to bone

II. Bone Biology

1) Bone is composed of organic material (mostly collagen, a spongy protein), within an inorganic matrix called hydroxyapatite (mostly calcium and potassium).

2) Bone tissue consists of three specialized cell types, osteoblasts, osteocytes, and osteoclasts

a. Osteoblasts are bone-forming cells, which line the surface of a bone’s structure.

b. Osteocytes are bone cells and are found within the bone’s structure.

c. Osteoclasts are cells that resorb bone trough a degradation process.

3) Process of Bone Formation

At birth, most of the skeletal system is composed of cartilage, which over time is replaced by bone. By the early twenties, most bone growth is complete, although bone is remodeled throughout life.

Through the process of remodeling, osteoclasts circulate throughout the bone and look for old or damaged osteocytes to break down, which are then replaced by osteoblasts which lay down new bone tissue.

This breakdown and buildup occurs throughout the bone, but is most visible at the growth plates of the bone which form at the junction of the epiphysis (bone ends), and the diaphysis (bone shaft). This junction is called the epiphyseal plate, located towards the end of the bone shaft.

Finally, some growth occurs in the periosteum, which is a thin sheaf of tissue that covers the outside of the bone surface. The periosteum also serves as an intake of nutrition and gasses.

III. Bone Anatomy

1) There are approximately 206 bones in the human body

2) They can be classified into four main classes

a. Long bones: main components of limbs, include the femur, humerus, radius and ulna, tibia and fibula.

b. Short Bones: include metacarpals of hands and metatarsals of feet

c. Flat Bones: includes cranial bones, innominates and scapula, offer protection and large muscle attachments

d. Irregular Bones: includes vertebra, carpals (hand) and tarsals (feet), many of the cranial bones. These bones are generally complex in design and serve specialized
purposes

3) The skeleton can also be divided into two parts, the axial skeleton, and the appendicular skeleton.

a. The axial skeleton includes the skull or cranium, the vertebral column, and the ribs.

b. The appendicular skeleton includes the pelvic and pectoral girdles, as well as the upper and lower limb bones.

i. The pectoral girdle includes the scapula and clavicle, and forms the shoulder

ii. The pelvic girdle includes innominate or hip bones.

iii. The upper limbs include the humerus, radius, ulna, carpals, metacarpals, and hand phalanges.

iv. The lower limbs include the femur, tibia, fibula, patella, tarsals, metatarsals, and foot phalanges.