Study Notes: Skin and Body Membranes, The Skeletal System, and The Muscular System

Chapter 4: Skin and Body Membranes

Functions of the Integumentary System

• Protection: The skin acts as a physical barrier against mechanical injury, pathogens, and harmful substances. It also prevents dehydration by reducing water loss.

• Synthesis of Vitamin D: When exposed to sunlight, the skin synthesizes vitamin D, which is essential for calcium absorption and bone health.

• Sensory Receptors and Communication: The skin contains sensory receptors for touch, pain, temperature, and pressure, allowing the body to interact with the environment.

• Regulation of Body Temperature: Through sweating and vasodilation or vasoconstriction of blood vessels, the skin helps maintain homeostasis of body temperature.

Layers of the Skin and Associated Tissues

• Epidermis: The outermost layer, composed of stratified squamous epithelial tissue. It is avascular and provides a protective barrier.

• Dermis: The middle layer, made of dense irregular connective tissue. It contains blood vessels, nerves, hair follicles, and glands.

• Hypodermis (Subcutaneous Layer): The deepest layer, consisting mainly of adipose tissue. It insulates the body and anchors the skin to underlying structures.

Structure of the Epidermis: Cell Types and Strata

• Stratum Corneum: Outermost layer; consists of dead, keratinized cells that provide a tough, protective barrier.

• Stratum Lucidum: Present only in thick skin (palms, soles); a thin, clear layer of dead cells.

• Stratum Granulosum: Cells contain granules of keratohyalin; involved in keratin formation.

• Stratum Spinosum: Several layers of keratinocytes; provides strength and flexibility.

• Stratum Basale: Deepest layer; contains stem cells that divide to form new keratinocytes. Also contains melanocytes (produce melanin) and Merkel cells (sensory function).

Structure of Hair

• Location: Hair is found on most body surfaces except palms, soles, lips, and certain genital areas.

• Structure:

  • Hair Root: The portion of hair embedded in the skin.

  • Hair Follicle: The sheath surrounding the hair root; extends into the dermis.

  • Hair Shaft: The visible part of the hair above the skin surface.

  • Keratin: The protein that gives hair its strength and resilience.

  • Arrector Pili Muscle: A small muscle attached to the hair follicle; contraction causes "goosebumps."

Types of Exocrine Glands

• Exocrine Glands: Glands that secrete substances onto body surfaces or into body cavities via ducts.

• Apocrine Glands: Found mainly in the axillary and genital regions; secrete a thicker, milky fluid; become active at puberty.

• Eccrine Glands: Widely distributed; produce watery sweat for thermoregulation.

• Oil (Sebaceous) Glands: Secrete sebum (an oily substance) into hair follicles; lubricates and waterproofs skin and hair.

Chapter 5: The Skeletal System

Functions of the Skeletal System

• Support: Provides a structural framework for the body.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serves as levers for muscles to act upon, enabling movement.

• Storage: Stores minerals (mainly calcium and phosphorus) and lipids in yellow marrow.

• Hematopoiesis: Blood cell formation occurs in red bone marrow.

Macroscopic Parts of a Typical Long Bone

• Diaphysis: The shaft; composed of compact bone surrounding a medullary cavity.

• Epiphysis: The ends of the bone; mostly spongy bone covered by a thin layer of compact bone.

• Epiphyseal Line and Plate: The plate is a region of cartilage in growing bones; the line is the remnant in adults after growth stops.

• Articular Cartilage: Hyaline cartilage covering the epiphyses; reduces friction at joints.

• Periosteum: Dense connective tissue membrane covering the bone except at joint surfaces; contains osteoblasts and osteoclasts.

• Marrow (Medullary) Cavity: Central cavity in the diaphysis; contains yellow marrow (fat) in adults.

• Marrow: Red marrow (hematopoietic tissue) is found in certain bones (e.g., pelvis, sternum) in adults.

Bone Tissue Composition and Types of Bone Cells

• Inorganic Compounds: Mainly hydroxyapatite (calcium phosphate crystals); provide hardness and strength.

• Organic Compounds: Collagen fibers and ground substance; provide flexibility and tensile strength.

• Bone Cells:

  • Osteocytes: Mature bone cells; maintain bone matrix.

  • Osteoblasts: Bone-forming cells; synthesize and secrete bone matrix.

  • Osteoclasts: Bone-resorbing cells; break down bone matrix for remodeling and calcium release.

Microscopic Structure of Compact and Spongy Bone

• Compact Bone:

  • Central (Haversian) Canal: Contains blood vessels and nerves.

  • Osteon (Haversian System): The structural unit of compact bone; consists of concentric lamellae around a central canal.

  • Osteocytes: Located in lacunae between lamellae.

  • Matrix Rings (Lamellae): Layers of bone matrix.

  • Lacunae: Small spaces housing osteocytes.

• Spongy Bone: Composed of trabeculae (lattice-like network); spaces filled with red marrow; lighter and less dense than compact bone.

Bone Growth: Endochondral Ossification and Appositional Growth

• Endochondral Ossification: Process by which most bones form from a cartilage template; cartilage is replaced by bone tissue.

• Appositional Growth: Increase in bone diameter by addition of new bone tissue at the surface.

Hormonal Influence on Bone Growth and Remodeling

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclast activity.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts and stimulating calcium deposition in bone.

• Growth Hormone: Stimulates bone growth, especially during childhood and adolescence.

Chapter 6: The Muscular System

Primary Functions of Muscles

• Movement: Muscles contract to produce movement of the body and its parts.

• Posture: Muscles maintain body posture and position.

• Stabilize Joints: Muscles reinforce and stabilize joints.

• Generate Heat: Muscle contractions produce heat, helping to maintain body temperature.

Microscopic Components of Skeletal Muscle Tissue and Cells

• Sarcolemma: The plasma membrane of a muscle fiber (cell).

• Sarcoplasm: The cytoplasm of a muscle fiber; contains organelles and myofibrils.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores and releases calcium ions.

• Myofibrils: Long, rod-like structures within muscle fibers; composed of repeating units called sarcomeres.

  • A Band: Dark region; contains thick (myosin) filaments.

  • I Band: Light region; contains thin (actin) filaments only.

  • Z Line (Disc): Defines the boundaries of a sarcomere; anchors thin filaments.

  • Sarcomere: The functional contractile unit of muscle; extends from Z line to Z line.

  • H Zone: Central region of the A band; contains only thick filaments.

  • Myosin (Thick) Filaments and Myosin Heads: Myosin heads bind to actin and use ATP to generate force for contraction.

  • Actin (Thin) Filaments and Active Sites: Actin contains binding sites for myosin heads; regulated by troponin and tropomyosin.

Components of the Motor Unit and Neuromuscular Junction

• Axon Terminal: The end of a motor neuron; releases neurotransmitters.

• Synaptic Cleft: The small gap between the axon terminal and the muscle fiber's sarcolemma.

• Sarcolemma of Muscle Cell: Receives the neurotransmitter signal and initiates an action potential.

• Neurotransmitter: Acetylcholine (ACh) is released from the axon terminal to stimulate muscle contraction.

Excitation-Contraction Mechanism and Sliding Filament Theory

• Role of Myosin Heads: Myosin heads attach to actin, pivot, and pull the thin filaments toward the center of the sarcomere, using energy from ATP hydrolysis.

• Sarcomere Shortening: As thin filaments slide past thick filaments, the sarcomere shortens, resulting in muscle contraction.

Sliding Filament Theory Equation:

$\text{Muscle Contraction} = \text{Sliding of Actin Filaments over Myosin Filaments}$

Events Leading to Muscle Contraction (Proper Order)

1. Nerve impulse reaches the axon terminal of the motor neuron.

2. Acetylcholine is released into the synaptic cleft.

3. Acetylcholine binds to receptors on the sarcolemma, generating an action potential.

4. The action potential travels along the sarcolemma and down T-tubules.

5. Sarcoplasmic reticulum releases calcium ions into the sarcoplasm.

6. Calcium binds to troponin, causing tropomyosin to move and expose active sites on actin.

7. Myosin heads bind to actin, forming cross-bridges.

8. Myosin heads pivot, pulling actin filaments toward the center of the sarcomere (power stroke).

9. ATP binds to myosin heads, causing them to detach from actin and re-cock for another cycle.

10. Muscle contraction continues as long as calcium and ATP are present.

Example: When you pick up a heavy object, your skeletal muscles contract as described above, shortening sarcomeres and generating the force needed for movement.

Additional info: The above notes expand on the study guide outline by providing definitions, explanations, and examples for each key point, ensuring a comprehensive review for exam preparation.