Skin Structure and Function

Overview of Skin Structure

• Skin is the largest organ of the human body, serving as a protective barrier and playing roles in sensation, thermoregulation, and metabolic functions.

• It consists of three main layers: epidermis, dermis, and hypodermis (subcutaneous tissue).

Layers of the Epidermis

• The epidermis is a stratified squamous epithelium with several layers:

  • Stratum basale: Deepest layer, site of cell division.

  • Stratum spinosum: Provides strength and flexibility.

  • Stratum granulosum: Contains granules for waterproofing.

  • Stratum lucidum: Present only in thick skin (palms, soles).

  • Stratum corneum: Outermost, dead keratinized cells.

• Thick skin (palms, soles) has all five layers; thin skin lacks the stratum lucidum.

Cells of the Epidermis

• Keratinocytes: Produce keratin, a protein that provides strength and waterproofing.

• Melanocytes: Produce melanin, which protects against UV radiation.

• Langerhans cells: Immune cells that detect pathogens.

Skin Components

• Glands: Sweat (eccrine and apocrine) and sebaceous glands.

• Receptors: Detect touch, pressure, pain, and temperature.

Factors Affecting Skin

• Wounds, chemicals, UV radiation, and burns can damage skin structure and function.

• The rule of nines is used to estimate the percentage of body surface area affected by burns.

Skeletal System

Cartilage and Bone Structure

• Cartilage: Flexible connective tissue found in joints, ear, nose, and respiratory tract.

• Bone: Rigid connective tissue providing support, protection, and mineral storage.

• Osteon: The structural unit of compact bone, consisting of concentric lamellae around a central canal.

Types of Bones

• Membranous bones: Formed directly from mesenchyme (e.g., flat bones of the skull).

• Endochondral bones: Develop from cartilage templates (e.g., long bones).

• Long bones: Longer than wide (e.g., femur).

• Short bones: Cube-shaped (e.g., carpals).

• Flat bones: Thin, flattened (e.g., sternum).

• Irregular bones: Complex shapes (e.g., vertebrae).

Structure of Long Bones

• Diaphysis: Shaft, composed of compact bone.

• Epiphyses: Ends, contain spongy bone and red marrow.

• Medullary cavity: Central cavity containing yellow marrow.

• Periosteum: Outer fibrous covering.

Ossification (Endochondral)

• Process by which cartilage is replaced by bone during development.

• Key steps:

  1. Bone collar formation around diaphysis.

  2. Cavitation of cartilage shaft.

  3. Primary ossification center forms in diaphysis.

  4. Secondary ossification centers form in epiphyses.

  5. Epiphyseal plate (growth plate) allows for longitudinal growth.

Bone Cells and Remodeling

• Osteoblasts: Build new bone matrix.

• Osteoclasts: Break down bone matrix.

• Osteocytes: Mature bone cells maintaining bone tissue.

Joints

Joint Categories

• Structural classification:

  • Fibrous (e.g., sutures of the skull)

  • Cartilaginous (e.g., intervertebral discs)

  • Synovial (e.g., knee, shoulder)

• Functional classification:

  • Synarthrosis: Immovable (e.g., skull sutures)

  • Amphiarthrosis: Slightly movable (e.g., pubic symphysis)

  • Diarthrosis: Freely movable (e.g., shoulder joint)

Synovial Joints

• Components: Articular cartilage, joint cavity, synovial fluid, articular capsule, ligaments, nerves, and blood vessels.

• Special features of select joints:

  • Shoulder: Ball-and-socket, greatest range of motion.

  • Elbow: Hinge joint, allows flexion and extension.

  • Hip: Ball-and-socket, stable and weight-bearing.

  • Knee: Modified hinge, contains menisci and cruciate ligaments.

Types of Joint Movement

• Flexion: Decreases angle between bones.

• Extension: Increases angle between bones.

• Hyperextension: Extension beyond anatomical position.

• Abduction: Movement away from midline.

• Adduction: Movement toward midline.

• Rotation: Bone turns around its own long axis.

Muscular System

Types of Muscle Tissue

• Skeletal muscle: Voluntary, striated, attached to bones.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

• Cardiac muscle: Involuntary, striated, found only in the heart.

Muscle Fiber Structure

• Sarcoplasmic reticulum: Stores and releases calcium ions.

• Triad: T tubule flanked by two terminal cisternae of the sarcoplasmic reticulum.

• T tubule: Invagination of the sarcolemma, conducts impulses.

• Actin: Thin filament, interacts with myosin for contraction.

• Myosin: Thick filament, forms crossbridges with actin.

• ATPase: Enzyme on myosin head, hydrolyzes ATP for energy.

• Crossbridges: Connections formed between actin and myosin during contraction.

Muscle Organization Levels

• Myofibril: Contractile threads within muscle fiber.

• Muscle fiber: Single muscle cell.

• Fascicle: Bundle of muscle fibers.

• Muscle: Bundle of fascicles.

• Connective tissue layers: Endomysium (around fiber), Perimysium (around fascicle), Epimysium (around muscle).

Sliding Filament Mechanism

• Muscle contraction occurs as myosin heads pull actin filaments toward the center of the sarcomere.

• Bands and zones:

  • A band: Length of myosin filaments (remains constant).

  • I band: Region with only actin (shortens during contraction).

  • H zone: Center of A band with only myosin (shortens).

  • M line: Center of sarcomere, holds myosin in place.

Types of Skeletal Muscle Fibers

• Slow oxidative fibers: Endurance, posture muscles.

• Fast glycolytic fibers: Short bursts, rapid fatigue.

• Fast oxidative fibers: Intermediate properties.

Aging and Exercise Effects on Muscle

• Aging leads to decreased muscle mass and strength (sarcopenia).

• Regular exercise can maintain or increase muscle mass and function.