Skin Anatomy and Physiology

Layers of the Skin

The skin is composed of three main layers, each with distinct functions and structures.

• Epidermis (superficial to deep):

  • Stratum Corneum: Dead, keratinized cells; provides protection.

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

  • Stratum Granulosum: Cells begin to die; waterproofing layer.

  • Stratum Spinosum: Provides strength; contains immune cells.

  • Stratum Basale: Site of mitosis; melanocytes produce melanin.

• Dermis:

  • Papillary layer: Loose connective tissue; capillaries nourish epidermis; contains touch receptors.

  • Reticular layer: Dense connective tissue; houses hair follicles, sweat glands, sebaceous glands, blood vessels, and nerves.

• Hypodermis (Subcutaneous layer):

  • Mostly adipose tissue (fat).

  • Provides insulation, energy storage, cushions organs, and anchors skin.

Tactile Receptors

Specialized sensory structures in the skin detect various stimuli.

• Meissner’s corpuscles: Detect light touch.

• Merkel discs: Sense steady touch and pressure.

• Pacinian corpuscles: Detect deep pressure and vibration.

• Ruffini endings: Respond to skin stretch.

• Free nerve endings: Sense pain and temperature.

Sebaceous Glands

• Produce sebum (oil).

• Lubricate skin and hair, prevent drying, and inhibit bacterial growth.

Sweat Glands

• Eccrine glands: Found almost everywhere; produce watery sweat for cooling.

• Apocrine glands: Located in axilla and groin; empty into hair follicles; active at puberty; produce thicker sweat (odor caused by bacteria).

Functions of the Skin

• Protection

• Temperature regulation

• Sensation

• Vitamin D production

• Excretion (water, salts)

• Blood reservoir

Rule of Nines (Burns)

Used to estimate the extent of burns on the body.

Skeletal System

Functions of the Skeletal System

• Support

• Protection

• Movement

• Mineral storage (Calcium & Phosphorus)

• Blood cell formation (hematopoiesis)

• Fat storage (yellow marrow)

Axial vs. Appendicular Skeleton

Bone Matrix

• Organic matrix: Collagen fibers; provides flexibility and tensile strength.

• Inorganic matrix: Calcium phosphate crystals; provides hardness and strength.

Compact vs. Spongy Bone

Yellow Bone Marrow

• Stores fat.

• Can convert to red marrow during severe blood loss.

Bone Cells

• Osteoblasts: Build bone.

• Osteocytes: Maintain bone.

• Osteoclasts: Break down bone.

Memory trick: Blast = Build; Clast = Crush.

Parts of a Long Bone

• Epiphysis: Ends of the bone.

• Diaphysis: Shaft of the bone.

• Epiphyseal line: Remnant of the growth plate.

Bone Markings

• Ramus: Arm-like bar.

• Tuberosity: Large rough projection.

• Foramen: Hole for nerves/blood vessels.

• Epicondyle: Projection above a condyle.

Growth Hormone vs. Parathyroid Hormone

Appositional Growth

• Bones grow wider, not longer.

• Osteoblasts add bone to the outside; osteoclasts remove bone from the inside.

Joints and Movements

Structure of a Synovial Joint

• Articular cartilage

• Joint cavity

• Synovial fluid

• Synovial membrane

• Fibrous capsule

• Ligaments

Function of Synovial Fluid

• Lubricates joint

• Reduces friction

• Nourishes cartilage

• Absorbs shock

Six Types of Synovial Joints

Synchondroses

• Cartilage joints (hyaline cartilage).

• Allow little or no movement.

• Examples: Epiphyseal plate, first rib to sternum.

Movements

• Flexion: Decrease joint angle.

• Extension: Increase joint angle.

• Hyperextension: Extension beyond normal position.

• Abduction: Away from midline.

• Adduction: Toward midline.

• Circumduction: Circular movement.

• Opposition: Thumb touches fingertips.

Muscle Anatomy and Physiology

Stages of Skeletal Muscle Contraction

Skeletal muscle contraction involves a series of steps:

1. Nerve Stimulation: Motor neuron releases acetylcholine (ACh); ACh binds to muscle fiber receptors; action potential travels along sarcolemma and T-tubules.

2. Calcium Release: Sarcoplasmic reticulum (SR) releases Ca2+.

3. Troponin & Tropomyosin: Calcium binds to troponin; troponin changes shape, moving tropomyosin away from actin’s binding sites.

4. Cross-Bridge Formation: Myosin heads bind to actin.

5. Power Stroke: Myosin pulls actin toward the center of the sarcomere; ADP and Pi are released.

6. Detachment: ATP binds to myosin; myosin releases actin.

7. Reactivation: ATP is split into ADP + Pi; myosin head is re-cocked.

8. Relaxation: Calcium is pumped back into SR; tropomyosin covers binding sites; muscle relaxes.

Role of Tropomyosin

• At rest: Covers myosin-binding sites on actin, preventing contraction.

• During contraction: Calcium binds troponin, tropomyosin moves away, allowing myosin to bind actin.

Function of the Sarcoplasmic Reticulum

• Stores calcium.

• Releases Ca2+ to initiate contraction; pumps Ca2+ back in to stop contraction.

Three Phases of a Muscle Twitch

Importance of Actin & Myosin

• Actin: Thin filament; contains binding sites for myosin.

• Myosin: Thick filament; pulls actin during contraction; uses ATP.

Muscle Organization

• Epimysium: Surrounds entire muscle.

• Perimysium: Surrounds fascicles.

• Endomysium: Surrounds each muscle fiber.

Rigor Mortis

• Occurs after death due to cessation of ATP production.

• Calcium leaks from SR; myosin binds actin; without ATP, myosin cannot detach, causing stiffness.

Sarcomere

• Functional unit of skeletal muscle (Z disc to Z disc).

• Contains thick (myosin) and thin (actin) filaments.

Isotonic vs. Isometric Contractions

Sliding Filament Theory

• Actin slides over myosin during contraction.

• Sarcomere shortens; I band and H zone shorten; A band remains the same.

Skeletal vs. Cardiac vs. Smooth Muscle

Muscle Fiber Types

Muscle Roles

• Prime Mover (Agonist): Main muscle producing movement (e.g., biceps brachii during elbow flexion).

• Antagonist: Opposes the movement (e.g., triceps brachii during elbow flexion).

• Synergist: Helps the prime mover (e.g., brachialis assisting the biceps).

• Fixator: Stabilizes the origin (e.g., shoulder muscles stabilize the shoulder while the biceps flexes the elbow).

High-Yield Questions & Answers

• Which epidermal layer undergoes mitosis? Stratum basale

• Which sweat gland cools the body? Eccrine

• Which bone cells build bone? Osteoblasts

• Which hormone increases blood calcium? Parathyroid hormone

• Which joint allows the greatest range of motion? Ball-and-socket

• What does synovial fluid do? Lubricates, nourishes cartilage, reduces friction

• Compact bone contains what structural unit? Osteon

• Which bone contains yellow marrow? Medullary cavity of long bones

• What is the function of the papillary dermis? Supplies nutrients to the epidermis and contains touch receptors

• Rule of nines: one entire leg equals what percentage? 18%

• What does calcium bind to? Troponin

• What blocks the myosin-binding sites? Tropomyosin

• What stores calcium? Sarcoplasmic reticulum

• What provides energy for contraction? ATP

• Functional unit of muscle? Sarcomere

• What causes rigor mortis? Lack of ATP prevents myosin from releasing actin

• Which contraction changes muscle length? Isotonic

• Which contraction produces tension without movement? Isometric

• Which muscle fiber is most fatigue resistant? Slow Oxidative (SO)

• During contraction, which band stays the same length? A band

Quick Memory Tricks

• Blast = Build bone; Clast = Chew up bone.

• SO = Slow & Stays (endurance).

• FG = Fast & Gone (quick fatigue).

• Epi → Peri → Endo = Whole muscle → Fascicle → Fiber.

• Troponin grabs Calcium; Tropomyosin gets out of the way.

• A Band = Always the same.

Additional info: These notes cover key concepts from Chapters 5–10, including skin anatomy, bone structure, joint types, and muscle physiology, as emphasized in college-level Anatomy & Physiology courses.