Skeletal System: Anatomy & Physiology Practice Exam Study Guide

Skeletal System

Overview of Bone Structure and Function

The skeletal system provides structural support, protection, movement, mineral storage, and blood cell production. Understanding the anatomy and physiology of bones is essential for comprehending how the body maintains homeostasis and responds to injury.

• Long Bones: Characterized by a shaft (diaphysis) and two ends (epiphyses). Contain a medullary cavity filled with bone marrow.

• Compact Bone: Dense outer layer providing strength and protection.

• Spongy Bone: Porous inner layer containing trabeculae and red marrow.

• Bone Marrow: Red marrow produces blood cells; yellow marrow stores fat.

• Periosteum: Membrane covering bone surface, involved in growth and repair.

Bone Growth and Development

Bone growth occurs through two main processes: intramembranous ossification and endochondral ossification. Growth can be appositional (increasing thickness) or interstitial (increasing length).

• Appositional Growth: Bone increases in diameter by adding new bone tissue at the surface.

• Interstitial Growth: Bone lengthens at the epiphyseal (growth) plates.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage models.

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull).

Bone Cells and Their Functions

Bone tissue is maintained and remodeled by several specialized cell types.

• Osteoblasts: Bone-forming cells that secrete bone matrix.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoclasts: Large cells that resorb or break down bone matrix.

• Chondrocytes: Cartilage cells involved in endochondral ossification.

Classification of Joints

Joints (articulations) are classified by structure and function, determining their range of movement.

• Structural Classes:

  • Fibrous Joints: Bones joined by dense connective tissue; little or no movement (e.g., sutures of the skull).

  • Cartilaginous Joints: Bones joined by cartilage; limited movement (e.g., intervertebral discs).

  • Synovial Joints: Bones separated by a fluid-filled cavity; freely movable (e.g., shoulder, knee).

• Functional Classes:

  • Synarthrosis: Immovable joints.

  • Amphiarthrosis: Slightly movable joints.

  • Diarthrosis: Freely movable joints (all synovial joints).

Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and the movements they allow.

• Ball-and-Socket: Allow movement in all axes (e.g., shoulder, hip).

• Hinge: Permit flexion and extension (e.g., elbow, knee).

• Pivot: Allow rotation (e.g., atlas and axis of the neck).

• Condyloid, Saddle, Gliding: Permit various combinations of movement.

Bone Remodeling and Repair

Bone is a dynamic tissue that undergoes continuous remodeling and can repair itself after injury.

• Bone Remodeling: Ongoing replacement of old bone tissue by new bone tissue, involving osteoclasts and osteoblasts.

• Bone Repair: Involves hematoma formation, soft callus formation, hard callus formation, and bone remodeling.

Steps in Bone Repair:

1. Fracture hematoma forms (blood clot).

2. Soft callus of cartilage forms.

3. Hard callus of bone replaces cartilage.

4. Bone is remodeled to original shape.

Calcium Homeostasis and Hormonal Regulation

Calcium levels in the blood are tightly regulated by hormones, primarily parathyroid hormone (PTH) and calcitonin.

• PTH: Increases blood calcium by stimulating osteoclast activity and increasing calcium reabsorption in kidneys.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

Key Equation:

$\text{Ca}^{2+}_{\text{blood}} \uparrow \Rightarrow \text{Calcitonin released}$ $\text{Ca}^{2+}_{\text{blood}} \downarrow \Rightarrow \text{PTH released}$

Bone Disorders and Diseases

Several conditions can affect bone health and function.

• Osteoporosis: Severe loss of bone density, increasing fracture risk.

• Osteoarthritis: Degeneration of joint cartilage and underlying bone.

• Rheumatoid Arthritis: Autoimmune disorder causing joint inflammation.

• Fractures: Breaks in bone, classified as simple (closed) or compound (open).

Bone Markings and Features

Bones have various markings that serve as attachment points for muscles, passageways for nerves and blood vessels, and articulation surfaces.

• Foramen: Opening for blood vessels or nerves.

• Fossa: Shallow depression.

• Tuberosity: Large, rounded projection.

• Condyle: Rounded articular surface.

Bone Matrix Composition

The bone matrix consists of organic and inorganic components.

• Organic: Collagen fibers provide flexibility and tensile strength.

• Inorganic: Hydroxyapatite (calcium phosphate crystals) provides hardness.

Key Formula:

$\text{Hydroxyapatite:} \ \text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$

Bone Marrow Types and Functions

Bone marrow is found in the medullary cavities of long bones and in the spaces of spongy bone.

• Red Marrow: Site of hematopoiesis (blood cell formation); abundant in children.

• Yellow Marrow: Stores fat; increases with age.

Accessory Structures of Joints

Synovial joints often have accessory structures that provide stability and reduce friction.

• Bursae: Fluid-filled sacs that cushion movement.

• Ligaments: Connect bone to bone, providing stability.

• Tendons: Connect muscle to bone.

• Menisci: Pads of cartilage that improve fit between bones.

Movements at Joints

Joints allow various types of movement, classified by direction and plane.

• Flexion: Decreases the angle between bones (e.g., bending the elbow).

• Extension: Increases the angle between bones.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

Table: Comparison of Endochondral and Intramembranous Ossification

Feature	Endochondral Ossification	Intramembranous Ossification
Starting Tissue	Hyaline cartilage model	Mesenchymal (fibrous) tissue
Location	Most bones (e.g., long bones)	Flat bones (e.g., skull, clavicle)
Process	Cartilage replaced by bone	Bone develops directly from mesenchyme
Primary Ossification Center	Diaphysis	Multiple centers in flat bones

Table: Types of Bone Cells

Cell Type	Function
Osteoblast	Builds bone matrix
Osteocyte	Maintains bone tissue
Osteoclast	Resorbs bone matrix
Chondrocyte	Maintains cartilage

Additional info:

• Some questions and fill-in-the-blank items in the exam refer to specific anatomical features (e.g., "sutures," "fontanelles," "bursae") and clinical conditions (e.g., "osteoporosis," "rickets").

• Students should be familiar with the terminology for bone markings, types of fractures, and the stages of bone healing.

• Understanding the differences between the axial and appendicular skeleton is essential for classification questions.