Musculoskeletal System: Anatomy and Physiology, BSc Nursing 1st year

The musculoskeletal system consists of bones and muscles that work together to provide shape, support, and movement to the human body. In this session, the focus is on the skeletal system, which forms the body's rigid framework and plays a vital role in protecting organs such as the brain, heart, and lungs. 

The skeletal system also enables movement by acting as a support structure for muscles and joints. A clear understanding of the structure, functions, and components of the skeletal system is essential for nursing students, as it helps them develop a strong foundation for learning anatomy, physiology, and clinical practice.

Skeletal System: Foundational Terminology

Two key branches of study define the musculoskeletal system:

• Myology: The study of muscles.

• Osteology: The study of bones.
  This lecture focuses entirely on Osteology.

Development of Bone from Germ Layers

Bones originate from primary germ layers formed after fertilization. The three primary germ layers are the Ectoderm, Mesoderm, and Endoderm.

Comparative Structure: Germ Layer Derivatives

The Skeletal System primarily develops from the Mesoderm.

The Cellular Pathway of Bone Formation

Bone and cartilage development from the mesoderm follows a specific cellular lineage:

1. Mesoderm (Germ Layer)

2. Gives rise to Mesenchymal Cells.

3. Differentiate into Chondroblasts (cartilage-forming cells) and Osteoblasts (bone-forming cells).

4. These immature cells mature into Chondrocytes (mature cartilage cells) and Osteocytes (mature bone cells).

This pathway shows Osteoblasts create bone tissue, which then matures into Osteocytes.

General Skeletal System Terminology

• Function: The primary role of the skeletal system is to provide the body with structure and support.

• Joint: A junction where two or more bones or cartilages meet. Joints are crucial as they permit movement.

• Muscle: A contractile tissue that generates force for movement. Muscles are often referred to as the "motors of the body."

• Periosteum: The outer covering of the bone, a fibrous and osteogenic (bone-forming) layer containing blood vessels and nerves.

Composition of Bone

Bone tissue is 90% solid substances and 10% liquid. The solid component comprises organic and inorganic materials.

1. Organic Material:

• Collagen Fibers

• Extracellular Matrix

• Chondroitin Sulfate: The main substance responsible for bone formation.

2. Inorganic Material:

• Calcium

• Phosphate

• Magnesium

Adequate calcium intake is vital for bone strength. The principal ions in bone are Calcium (first) and Phosphate (second).

The Cells of Bone

There are three primary types of bone cells, each with a distinct function.

(Memory Tip: To remember the functions of the three main bone cells, use the following analogy: Osteoblasts are the "Builders" that construct new bone. Osteocytes are the "Managers" that maintain mature bone. Osteoclasts are the "Crushers" that break down old bone.)

Comparative Structure: Bone Cell Functions

Types of Bones

Bones are classified based on their shape, structure, and development.

Divisions of the Skeletal System

The adult human skeleton has 206 bones, divided into two main parts:

1. Axial Skeleton (80 bones)

• Forms the central, vertical axis of the body.

• Function: Protects vital organs (brain, spinal cord, heart, lungs) and supports body weight.

• Components: Skull (22 bones), Vertebral Column (33 vertebrae at birth), Rib Cage (24 ribs and 1 sternum), Hyoid Bone, and Ear Ossicles.

2. Appendicular Skeleton (126 bones)

• Consists of the limbs (appendages) and their girdles.

• Function: Primarily responsible for movement and locomotion.

• Components: Pectoral (Shoulder) Girdle (4 bones: Scapula and Clavicle), Upper Limbs, Pelvic (Hip) Girdle (2 hip bones), and Lower Limbs.

Classification of Bones by Structure

Bones are also classified by their macroscopic structure:

1. Compact Bone

• A strong, dense, hard outer layer in the shaft of long bones.

• Structural unit: osteon (Haversian system) with a central canal for vessels and nerves.

• Function: Provides protection, support, and resists weight/movement stress.

2. Spongy (Cancellous) Bone

• Porous, honeycomb-like internal structure, found at long bone ends and within flat/vertebral bones.

• Function: Acts as a shock absorber, distributes forces, reduces bone weight, and contains red bone marrow.

Anatomy of a Long Bone

A growing long bone (e.g., the femur) has distinct regions vital for growth and function.

Regions of a Long Bone

• Diaphysis: The main, long shaft; thick compact bone surrounding the medullary cavity.

• Epiphysis: Expanded ends of the bone (proximal and distal); mainly spongy bone, covered with articular cartilage.

• Metaphysis: Narrow region where diaphysis meets epiphysis.

• Epiphyseal Plate (Growth Plate): Cartilage layer in the metaphysis, responsible for increasing bone length.

Internal Structures

• Medullary Cavity: Hollow space in the diaphysis. In children, it has red bone marrow; in adults, largely yellow bone marrow (fat storage).

• Spongy Bone: In the epiphyses, contains red bone marrow for blood cell production.

• Compact Bone: Dense wall of the diaphysis.

Growth and Maturation

The epiphyseal plate produces new cartilage that converts to bone, lengthening the bone. After puberty, the growth plate ossifies and fuses, becoming the epiphyseal line, marking the end of lengthwise growth.

Metaphysis and Growth Plate

The metaphysis is the epiphyseal end of the diaphysis. Until puberty, it's distinct for growth. After puberty, it fuses and becomes part of the diaphysis.

The growth plate is a layer of hyaline cartilage active during bone growth, increasing bone length. Post-puberty, it ossifies and becomes the epiphyseal line.

Comparative Structure: Growth Plate vs. Epiphyseal Line

Review of Long Bone Anatomy

A complete long bone displays:

• Articular Cartilage: Covers bone ends at joints.

• Epiphysis: Ends of the long bone (proximal closer to torso, distal farther). (Memory Tip: P for Proximal is like "Paas" (Hindi for 'near'); D for Distal is like "Dur" (Hindi for 'far').)

• Diaphysis: Main shaft.

• Metaphysis: Region between epiphysis and diaphysis where growth occurs.

• Epiphyseal Line: Indication of fused epiphysis and diaphysis post-growth.

• Spongy Bone: Porous, inner tissue in epiphyses.

• Compact Bone: Dense, hard outer layer of diaphysis.

• Endosteum: Inner membrane lining the medullary cavity.

• Periosteum: Hard, outer membrane covering bone, with arteries and veins.

• Medullary Cavity: Central cavity in diaphysis, containing yellow bone marrow.

Joints

A joint is an articulation point where one or more bones meet, permitting varying degrees of movement.

Classification of Joints

Joints are classified by structure, function, region, and number of articulating bones.

1. Classification by Structure (Uniting Tissue)

• Fibrous Joints: Bones united by fibrous tissue (e.g., Sutures of the skull).

• Cartilaginous Joints: Bones united by cartilage (e.g., Symphysis, Synchondrosis).

• Synovial Joints: Bones separated by a fluid-filled joint cavity, allowing free movement (e.g., Ball and Socket, Hinge).

2. Classification by Function (Degree of Mobility)

• Synarthrosis: Immovable joints.

• Amphiarthrosis: Slightly movable joints.

• Diarthrosis: Freely movable joints (e.g., elbow).

3. Classification by Region

• Skull Type: Immovable joints.

• Vertebral Type: Slightly movable joints.

• Limb Type: Freely movable joints.

4. Classification by Articulation (Number of Bones)

• Simple Joint: Articulation between two bones.

• Compound Joint: Articulation involving more than two bones.

• Complex Joint: Joint including an articular disc or meniscus.

Examples of Synovial Joints

• Pivot Joint: Vertebrae.

• Hinge Joint: Elbow.

• Saddle Joint: Hand (between trapezium and first metacarpal).

• Ball and Socket Joint: Hip.

• Condyloid (Ellipsoid) Joint: Wrist (between radius and carpal bones).

• Plane Joint: Foot (between tarsal bones).

The Synovial Joint: Features and Components

Synovial joints are the most common type of joint, allowing free movement, and are lined by a synovial membrane containing synovial fluid. (Memory Tip: The synovial fluid acts as a lubricant. The common complaint of "the grease in the knees is finished" during knee pain refers to the reduction or degradation of this synovial fluid.)

Key Components of a Synovial Joint:

1. Articular Surface: Articulating bones covered with hyaline cartilage. Provides a smooth, gliding surface and absorbs synovial fluid for nutrition. It has no perichondrium and no regenerative power.

2. Synovial Fluid: Clear, viscous, alkaline fluid, a filtrate of blood plasma with hyaluronic acid.

• Functions: Nutrition for cartilage, Lubrication to reduce friction, Protection against wear and tear.

3. Synovial Membrane: Cellular connective tissue lining the joint capsule. Secretes synovial fluid and removes debris.

4. Joint Cavity: Potential space between bones, enclosed by the capsule and lined by the synovial membrane, filled with synovial fluid.

5. Articular Capsule: Fibrous capsule of collagen fibers enclosing the joint.

6. Articular Disc or Meniscus: Pad of fibrocartilage between articular surfaces, increasing range of motion.

7. Labrum: Ring of fibrocartilage (e.g., shoulder, hip) that increases joint socket depth, enhancing joint stability.

8. Ligaments: Bands of fibrous connective tissue connecting bone to bone, providing joint stability.

Classification of Synovial Joints by Axes of Movement

Synovial joints are classified by movement axes: