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, supporting movement, protecting organs, and serving as sites for muscle attachment. Joints, or articulations, connect bones and allow for a range of movements essential for daily activities.

Learning Objectives

• Differentiate between the axial and appendicular skeleton.

• Understand the components of each skeletal region.

• Know the structural and functional classifications of joints.

• Describe the structure of synovial joints.

• Relate musculoskeletal anatomy to function and basic joint movements.

Structure of the Skeletal System and Skeletal Cartilage

The adult human skeleton consists of approximately 206 bones and 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 vital organs.

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

• Vertebral Column: Consists of 33 bones; 24 are individual vertebrae protecting the spinal cord, while the remaining vertebrae are fused to form the sacrum and coccyx.

• Thoracic Cage: Includes 12 pairs of ribs, the sternum, and part of the vertebral column. It encases and protects the lungs, heart, and other thoracic organs.

Appendicular Skeleton

The appendicular skeleton consists of the bones of the limbs and girdles, primarily facilitating movement and muscle attachment.

• Pectoral Girdle: Clavicle and scapula, which attach the upper limbs to the trunk.

• Upper Limb: Arm (humerus), forearm (radius and ulna), 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), and foot (tarsals, metatarsals, phalanges).

Joints (Articulations)

Joints are sites where two or more bones meet. They provide mobility to the skeleton and hold bones 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. They are all 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 for strength.

  • Inner Synovial Membrane: Loose connective tissue that produces synovial fluid.

• Synovial Fluid: Viscous, slippery filtrate of plasma and hyaluronic acid. It lubricates and nourishes articular cartilage and contains phagocytic cells to remove microbes and debris.

Movements Allowed by Synovial Joints

Synovial joints allow a wide range of movements, classified as angular, rotational, or special movements.

Angular Movements

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

• Extension: Increases the angle between two bones (e.g., straightening the knee).

• Hyperextension: Extension beyond the anatomical position.

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

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

• Circumduction: Circular movement combining flexion, abduction, extension, and adduction, so 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	Protection of brain, spinal cord, and thoracic organs
Appendicular Skeleton	Pectoral girdle, upper limb, pelvic girdle, lower limb	Movement, support, muscle attachment

Example: Application of Synovial Joint Movements

• Flexion at the elbow joint allows you to bring food to your mouth.

• Abduction at the shoulder joint allows you to raise your arm to the side.

• Supination and pronation of the forearm are essential for turning a doorknob.