Integumentary System

Structure and Layers of the Skin

The skin is the largest organ of the body and consists of two primary layers, with an underlying subcutaneous tissue. Each layer has distinct structural and functional properties.

• Epidermis: The outermost layer, composed of epithelial tissue. It provides a protective barrier.

• Dermis: The inner layer, made of connective tissue, containing blood vessels, nerves, and lymphatics.

• Subcutaneous Tissue (Hypodermis): Located beneath the skin, mainly adipose tissue. Anchors skin to muscle, allows movement, and acts as a shock absorber and insulator.

Layers and Cells of the Epidermis

The epidermis is a keratinized stratified squamous epithelium, organized into several layers, each with specialized cells and functions.

• Keratinocytes: Produce keratin, a protective protein.

• Melanocytes: Synthesize melanin pigment, protecting against UV radiation.

• Dendritic Cells: Macrophages that activate immune responses.

• Tactile Epithelial Cells: Associated with sensory nerve endings.

• Stratum Basale: Deepest layer, site of mitosis.

• Stratum Spinosum: Contains keratinocytes, melanin granules, and dendritic cells.

• Stratum Granulosum: Keratinocytes undergo physical changes, becoming tough outer cells.

• Stratum Lucidum: Found only in thick skin, composed of dead keratinocytes.

• Stratum Corneum: Outermost layer, thick layer of dead keratinocytes.

Dermis Structure

The dermis is strong and flexible, composed of two layers:

• Papillary Dermis: Areolar connective tissue, contains dermal papillae with capillary loops and receptors.

• Reticular Dermis: Dense irregular connective tissue, contains cleavage and flexure lines.

Skin Color

Skin color is determined by three main pigments:

• Melanin: Ranges from reddish yellow to brownish black; protects DNA from UV damage.

• Carotene: Yellow-orange pigment from foods, accumulates in stratum corneum and hypodermis.

• Hemoglobin: Gives pinkish hue in fair skin, visible through transparent skin.

Changes in skin color can indicate disease states (e.g., cyanosis, jaundice).

Hair and Hair Follicles

Hair is composed of dead, keratinized cells and serves protective and sensory functions.

• Regions: Shaft (projects from skin), root (embedded in skin).

• Layers: Medulla (core), cortex (middle), cuticle (outer).

• Hair Color: Determined by melanins produced by melanocytes.

• Hair Follicle: Extends from epidermis into dermis/hypodermis; contains hair bulb, root hair plexus, connective tissue sheath, and epithelial root sheath.

• Arrector Pili Muscle: Smooth muscle causing hair to stand upright.

• Types: Vellus (fine, pale), Terminal (coarse, longer).

• Growth: Influenced by nutrition, hormones; follicles have limited cycles.

• Baldness: Genetic, sex-influenced (e.g., male pattern baldness).

Nails

Nails are scale-like modifications of the epidermis, providing protection.

• Structure: Hard keratin, free edge, nail plate, root.

• Nail Matrix: Responsible for nail growth.

Sweat and Sebaceous Glands

Glands in the skin regulate temperature and secrete protective substances.

• Eccrine Sweat Glands: Produce true sweat, abundant on palms, soles, forehead; regulated by sympathetic nervous system.

• Apocrine Sweat Glands: Located in axillary/anogenital areas; produce fat- and protein-rich sweat; function in sexual signaling.

• Ceruminous Glands: Modified apocrine glands in ear canal, secrete cerumen (earwax).

• Mammary Glands: Modified sweat glands, secrete milk.

• Sebaceous Glands: Secrete sebum (oil), found everywhere except palms/soles; holocrine glands; lubricate skin/hair, bactericidal.

Functions of the Skin

The skin serves as a barrier and performs multiple functions:

• Protection: Chemical, physical, and biological barriers.

• Temperature Regulation: Sweat production, capillary constriction.

• Cutaneous Sensation: Sensory receptors detect touch, pain, temperature.

• Metabolic Functions: Vitamin D synthesis for calcium absorption.

• Blood Reservoir: Holds up to 5% of blood supply.

• Excretion: Eliminates nitrogenous wastes.

Skin Disorders: Cancer and Burns

Skin is susceptible to cancer and burns, which can be life-threatening.

• Basal Cell Carcinoma: Least malignant, most common; arises from stratum basale.

• Squamous Cell Carcinoma: Arises from stratum spinosum; can metastasize.

• Melanoma: Cancer of melanocytes; highly metastatic, dangerous.

• ABCD Rule: Asymmetry, Border Irregularity, Color, Diameter for evaluating moles.

• Burns: Classified as first-, second-, or third-degree; risks include dehydration, infection.

• Rule of Nines: Used to estimate fluid loss by dividing body into regions.

Bones and Skeletal Tissue

Types and Properties of Cartilage

Cartilage forms much of the skeleton in early development and persists in certain adult structures.

• Hyaline Cartilage: Most abundant; found in articular, costal, respiratory, and nasal cartilages.

• Elastic Cartilage: Flexible; found in external ear and epiglottis.

• Fibrocartilage: Withstands pressure/stretch; found in knee, intervertebral discs.

Cartilage grows by appositional (outer expansion) and interstitial (internal expansion) mechanisms.

Functions of Bones

Bones perform several essential functions:

• Support: Framework for the body.

• Protection: Shields vital organs.

• Anchorage: Attachment for muscles.

• Mineral Storage: Stores calcium and phosphate.

• Blood Cell Formation: Hematopoiesis in red marrow.

• Triglyceride Storage: Fat in yellow marrow.

• Hormone Production: Osteocalcin regulates insulin.

Classification of Bones

Bones are classified by location and shape.

• Axial Skeleton: Skull, vertebral column, rib cage; protection/support.

• Appendicular Skeleton: Limbs and girdles; movement/manipulation.

• Long Bones: Longer than wide; e.g., femur.

• Short Bones: Cube-shaped; e.g., carpals.

• Flat Bones: Thin, curved; e.g., sternum.

• Irregular Bones: Complex shapes; e.g., vertebrae.

Gross and Microscopic Structure of Bones

Bones have a complex structure, both grossly and microscopically.

• Compact Bone: Dense outer layer.

• Spongy Bone: Internal, with trabeculae.

• Long Bone Structure: Diaphysis (shaft), epiphyses (ends), medullary cavity (yellow marrow), articular cartilage, periosteum (outer), endosteum (inner).

• Red Bone Marrow: Hematopoietic tissue in spongy bone and flat bones.

• Bone Markings: Projections (attachment), surfaces (joints), depressions/openings (vessels/nerves).

• Bone Cells: Osteoprogenitor (stem), osteoblasts (build), osteocytes (maintain), bone lining cells, osteoclasts (resorb).

• Osteon: Structural unit of compact bone; concentric lamellae, central canal, lacunae, canaliculi.

• Spongy Bone: Trabeculae with irregular lamellae.

• Organic Components: Cells, osteoid (collagen, ground substance); flexibility/tensile strength.

• Inorganic Components: Hydroxyapatites (calcium phosphates); hardness/compression resistance.

Bone Development and Growth

Bones develop by two processes and grow in length and width.

• Endochondral Ossification: Replaces hyaline cartilage; forms most bones below skull.

• Intramembranous Ossification: Forms membrane bone from fibrous tissue; cranial bones, clavicles.

• Epiphyseal Plate: Site of longitudinal growth; four zones (proliferation, hypertrophic, calcification, ossification).

• Appositional Growth: Increases thickness.

• Hormonal Regulation: Growth hormone, thyroid hormone, sex hormones.

Bone Remodeling and Repair

Bone is constantly remodeled and repaired after injury.

• Remodeling: Osteoclasts resorb, osteoblasts deposit bone; regulated by hormones (PTH, calcitonin) and mechanical stress (Wolff's law).

• Repair: Four stages: hematoma formation, fibrocartilaginous callus, bony callus, remodeling.

Bone Disorders

Abnormal bone deposition and resorption lead to disorders.

• Osteomalacia/Rickets: Inadequate mineralization; soft bones.

• Osteoporosis: Resorption exceeds formation; porous, fragile bones.

• Paget's Disease: Excessive, disorganized bone remodeling; deformity.

Joints

Classification of Joints

Joints, or articulations, are classified structurally and functionally.

• Structural: Fibrous, cartilaginous, synovial (based on binding material and cavity presence).

• Functional: Synarthroses (immovable), amphiarthroses (slightly movable), diarthroses (freely movable).

Fibrous Joints

Bones are connected by fibrous tissue; little to no movement.

• Sutures: Short fibers; e.g., skull.

• Syndesmoses: Ligament connection; e.g., distal tibiofibular joint.

• Gomphoses: Peg-in-socket; e.g., teeth.

Cartilaginous Joints

Bones are connected by cartilage; limited mobility.

• Synchondroses: Plate of hyaline cartilage; e.g., epiphyseal plate.

• Symphyses: Articular cartilage fused to fibrocartilage; e.g., intervertebral discs.

Synovial Joints

Freely movable joints with a fluid-filled cavity.

• Articular Cartilage: Covers bone ends.

• Joint Cavity: Contains synovial fluid.

• Articular Capsule: Fibrous layer and synovial membrane.

• Synovial Fluid: Lubricates, nourishes.

• Reinforcing Ligaments: Strengthen joint.

• Nerves and Blood Vessels: Detect pain/stretch, supply nutrients.

• Bursae/Tendon Sheaths: Reduce friction.

• Stability Factors: Articular surface shape, ligaments, muscle tone.

Types and Examples of Synovial Joints

Synovial joints allow various movements and are classified by shape and movement.

• Jaw (Temporomandibular): Hinge and lateral movement; articular disc divides cavity.

• Shoulder (Glenohumeral): Ball-and-socket; most freely moving, least stable; rotator cuff stabilizes.

• Elbow: Hinge; flexion/extension; strong ligaments restrict side movement.

• Hip (Coxal): Ball-and-socket; deep socket and ligaments provide stability.

• Knee: Three joints in one; hinge and plane; stabilized by ligaments and tendons.

Joint Injuries and Disorders

Joints are prone to injury and disease.

• Cartilage Tears: Meniscus damage.

• Sprains: Ligament stretching/tearing.

• Dislocations: Bones forced out of alignment.

• Bursitis/Tendonitis: Inflammation due to injury or overuse.

• Arthritis: Osteoarthritis (degenerative), rheumatoid (autoimmune), gouty (uric acid crystals).

• Lyme Disease: Bacterial infection from ticks; joint inflammation.

Muscles and Muscle Tissue

Types and Functions of Muscle Tissue

Muscle tissue is classified into three types, each with unique properties and functions.

• Skeletal Muscle: Striated, voluntary, attached to bones.

• Cardiac Muscle: Striated, involuntary, found in heart.

• Smooth Muscle: Non-striated, involuntary, walls of hollow organs.

• Excitability: Responds to stimuli.

• Contractility: Contracts forcibly.

• Extensibility: Stretches.

• Elasticity: Returns to original length.

• Functions: Movement, posture, joint stabilization, heat generation.

Structure of Skeletal Muscle

Skeletal muscle is a complex organ composed of muscle fibers, nerves, blood vessels, and connective tissue.

• Connective Tissue Sheaths: Endomysium (fiber), perimysium (fascicle), epimysium (muscle).

• Attachments: Direct (epimysium to bone), indirect (tendons/aponeuroses).

Microscopic Anatomy of Muscle Fibers

Skeletal muscle fibers contain specialized structures for contraction.

• Myofibrils: Contractile elements; striations due to A and I bands.

• Myofilaments: Thick (myosin) and thin (actin) filaments.

• Regulatory Proteins: Tropomyosin (blocks sites), troponin (binds Ca2+).

• Sarcoplasmic Reticulum: Stores/releases Ca2+.

• T Tubules: Conduct impulses, trigger Ca2+ release.

• Sliding Filament Model: Thin filaments slide past thick, shortening sarcomere.

Muscle Contraction: Neuromuscular Junction and Cross Bridge Cycle

Muscle contraction is initiated by motor neuron stimulation and involves a series of steps.

• Neuromuscular Junction: Axon terminal releases acetylcholine (ACh), binds to receptors, generates action potential.

• Excitation-Contraction Coupling: Action potential triggers Ca2+ release, leading to cross bridge formation.

• Cross Bridge Cycle: Ca2+ binds troponin, tropomyosin moves, myosin binds actin, power stroke occurs.

Muscle Twitch, Summation, and Contraction Types

Muscle contractions can be graded and occur in different forms.

• Motor Unit: Motor neuron and all fibers it innervates.

• Muscle Twitch: Response to single stimulus; latent, contraction, relaxation phases.

• Temporal Summation: Rapid stimuli, sustained contraction (tetanus).

• Recruitment: Increasing stimulus activates more motor units.

• Isotonic Contraction: Muscle changes length; concentric (shortens), eccentric (lengthens).

• Isometric Contraction: Tension increases, length unchanged.

ATP Production and Muscle Fatigue

Muscle contraction requires ATP, produced by several mechanisms.

• Creatine Phosphate: Rapid ATP regeneration.

• Anaerobic Glycolysis: Glucose breakdown, lactic acid production.

• Aerobic Respiration: Most ATP during moderate activity; produces CO2 and H2O.

• Muscle Fatigue: Inability to contract; caused by ionic imbalances, decreased ATP, increased phosphate, decreased glycogen.

• EPOC: Excess postexercise oxygen consumption; restores reserves.

Force, Velocity, and Muscle Fiber Types

Several factors influence muscle contraction characteristics.

• Force: Number of fibers, size, stimulation rate, length-tension relationship.

• Velocity/Duration: Fiber type, load, recruitment.

• Fiber Types: Slow oxidative (endurance), fast glycolytic (power, fatigue), fast oxidative (intermediate).

Effects of Exercise

Exercise induces adaptations in muscle tissue.

• Aerobic Exercise: Increases capillaries, mitochondria, myoglobin; endurance.

• Resistance Exercise: Increases myofilaments, myofibrils, glycogen; hypertrophy.

Smooth Muscle

Smooth muscle differs from skeletal muscle in structure and function.

• Structure: Small, spindle-shaped, single nucleus, no striations, arranged in sheets.

• Contraction: Slow, synchronized; triggered by Ca2+ binding to calmodulin, activating myosin light chain kinase.

• Regulation: Autonomic nerves, hormones, local factors.

• Types: Unitary (visceral, coupled, rhythmic), multi-unit (independent, graded).

The Muscular System

Muscle Roles and Naming

Muscles act as prime movers, antagonists, or synergists, and are named by various criteria.

• Prime Mover (Agonist): Main force for movement.

• Antagonist: Opposes/reverses movement.

• Synergist: Assists agonist, reduces unwanted movement; fixators stabilize.

• Naming Criteria: Location, shape, size, fiber direction, number of origins, attachment sites, action.

• Examples: Temporalis (location), deltoid (shape), biceps (number of origins), sternocleidomastoid (attachments), flexor (action).

Fascicle Arrangement and Muscle Shape

Fascicle patterns determine muscle shape and force generation.

• Circular: Concentric rings.

• Convergent: Broad origin, converges to tendon.

• Parallel: Fascicles run parallel; fusiform is spindle-shaped.

• Pennate: Short fascicles attach obliquely; unipennate (one side), bipennate (both sides), multipennate (multiple feathers).

Lever Systems

Muscles and bones form lever systems to move loads.

• Lever: Rigid bar (bone) moves on fulcrum (joint) by effort (muscle contraction).

• Power Lever: Load close to fulcrum, effort far; moves heavy load with little effort.

• Speed Lever: Load far from fulcrum, effort near; moves load rapidly over distance.

• Types: First-class (fulcrum between effort/load), second-class (load between fulcrum/effort), third-class (effort between load/fulcrum).

Example: The biceps brachii acts as a third-class lever during elbow flexion, providing rapid movement.