Introduction to the Skeletal System and Joints

Introduction to the Skeletal System and Joints

The skeletal system provides the structural framework for the human body, supports movement, and protects vital organs. Joints, or articulations, connect bones and allow for a range of movements essential for daily activities.

Learning Objectives

• Differentiation between the axial and appendicular skeleton

• Understanding the components of each skeletal region

• Knowledge of the structural and functional classification of joints

• Understanding the structure of synovial joints

• Relating musculoskeletal anatomy to function, including basic angular and special movements

Structure of the Skeletal System and Skeletal Cartilage

The adult human skeleton is composed of approximately 206 bones and their associated cartilages. The skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton.

Axial Skeleton

The axial skeleton forms the longitudinal axis of the body and is primarily responsible for protecting the central nervous system and vital organs.

• Skull: 22 bones (8 cranial and 14 facial bones) that encase the brain and form the face.

• Vertebral column: 33 bones; 24 individual vertebrae protect the spinal cord, while the remaining vertebrae are fused to form the sacrum and coccyx.

• Thoracic cage: 12 pairs of ribs, the sternum, and part of the vertebral column; encases and protects the lungs, heart, and other delicate contents of the thoracic cavity.

Appendicular Skeleton

The appendicular skeleton consists of the bones of the limbs and girdles, which are primarily suited for movement, support, and muscle attachment.

• Pectoral girdle: Clavicle and scapula

• Upper limb: Arm (humerus), forearm (radius and ulna), wrist and hand (carpals, metacarpals, phalanges)

• Pelvic girdle: Two pelvic bones and the sacrum, collectively called the pelvis

• Lower limb: Thigh (femur), lower leg (tibia and fibula), ankle and foot (tarsals, metatarsals, phalanges)

Joints (Articulations)

Joints are sites where two or more bones meet. They provide the skeleton with mobility and hold the skeleton together.

Classification of Joints

• Structural classification: Based on the material binding the bones and the presence or absence of a joint cavity. The three types are:

  • Fibrous joints

  • Cartilaginous joints

  • Synovial joints

• Functional classification: Based on the amount of movement allowed by the joint. The three types are:

  • Synarthrosis (immovable)

  • Amphiarthrosis (slightly movable)

  • Diarthrosis (freely movable)

Synovial Joints

Synovial joints are characterized by bones separated by a fluid-filled joint cavity. These joints are diarthrotic (freely movable) and include almost all limb joints.

General Features of Synovial Joints

• Articular cartilage: Hyaline cartilage covering the ends of bones, preventing crushing of bone ends.

• Joint (synovial) cavity: A small, fluid-filled potential space unique to synovial joints.

• Articular (joint) capsule: Two layers thick:

  • External fibrous layer: Dense irregular connective tissue

  • Inner synovial membrane: Loose connective tissue that produces synovial fluid

• Synovial fluid: Viscous, slippery filtrate of plasma and hyaluronic acid that lubricates and nourishes articular cartilage. Contains phagocytic cells to remove microbes and debris.

Movements Allowed by Synovial Joints

Synovial joints allow a variety of movements, which can be classified as angular, rotational, or special movements.

Angular Movements

• Flexion: Decreases the angle between two bones (movement along the sagittal plane).

• Extension: Increases the angle between two bones.

• Hyperextension: Movement beyond the anatomical position.

• Abduction: Movement along the frontal plane, away from the midline.

• Adduction: Movement along the frontal plane, toward the midline.

• Circumduction: Involves flexion, abduction, extension, and adduction in succession; the limb describes a cone in space.

Rotational Movements

• Rotation: Turning of a bone around its own long axis.

  • Medial rotation: Toward the midline

  • Lateral rotation: Away from the midline

Special Movements

• Supination: Rotation of the forearm so the palm faces anteriorly; radius and ulna are parallel.

• Pronation: Rotation of the forearm so the palm faces posteriorly; radius rotates over ulna.

• Dorsiflexion: Bending the foot toward the shin.

• Plantar flexion: Pointing the toes downward.

Summary Table: Major Components of the Human Skeleton

Region	Main Bones	Function
Axial Skeleton	Skull, vertebral column, thoracic cage (ribs, sternum)	Protection of CNS and vital organs, support
Appendicular Skeleton	Pectoral girdle, upper limb, pelvic girdle, lower limb	Movement, support, muscle attachment

Example: Application of Synovial Joint Movements

• Flexion and extension at the elbow joint allow for lifting and lowering objects.

• Abduction and adduction at the shoulder joint enable arm movements away from and toward the body.

• Rotation at the hip joint allows the leg to turn inward or outward.