Anatomy & Physiology Study Guide: Skeletal and Muscular Systems

Chapter 6: The Skeletal System

Overview and Functions of the Skeleton

The skeletal system provides the structural framework for the body, supports movement, protects internal organs, stores minerals, and houses bone marrow for blood cell production.

• Support: Provides shape and support for the body.

• Protection: Shields vital organs (e.g., skull protects the brain).

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

• Mineral Storage: Reservoir for calcium and phosphate.

• Blood Cell Formation: Hematopoiesis occurs in red marrow.

Skeletal Tissues

• Fibrous Connective Tissue (CT): Forms periosteum (outer covering of bone) and ligaments.

• Periosteum: Outer fibrous layer (dense irregular CT) and inner osteogenic layer (contains bone-forming cells).

• Ligaments: Connect bone to bone, composed of dense regular CT.

• Cartilage Types:

  • Hyaline Cartilage: Most abundant; found at articular surfaces, costal cartilages.

  • Articular Cartilage: Specialized hyaline cartilage at joint surfaces.

  • Fibrocartilage: Strong, resists compression; found in intervertebral discs, menisci.

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

Bone Matrix Components

• Osteoid (Organic): Makes up ~1/3 of bone matrix; composed of collagen fibers and ground substance, produced by osteoblasts. Provides tensile strength and flexibility.

• Hydroxyapatites (Inorganic): Makes up ~2/3 of bone matrix; mainly calcium phosphate crystals. Provides hardness and resistance to compression.

Architectural Anatomy & Location

• Spongy Bone (Cancellous): Porous, trabecular structure; found at epiphyses of long bones and inside flat bones. Contains red marrow.

• Compact Bone: Dense, solid; forms outer layer of all bones and diaphysis of long bones. Organized into osteons (Haversian systems).

Skeletal Tissue Cell Types

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; secrete osteoid.

• Osteocytes: Mature bone cells; maintain bone matrix.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

• Chondroblasts: Cartilage-forming cells.

• Fibroblasts: Produce collagen fibers in connective tissue.

Development of Bones (Osteogenesis)

• Intramembranous Ossification: Bone develops from fibrous membrane; forms flat bones (e.g., skull, clavicle).

• Endochondral Ossification: Bone replaces hyaline cartilage; forms most bones of the body, especially long bones.

Growth & Remodeling of Bones

• Epiphyseal Plate Zones: Regions of cartilage where bone growth occurs in length. Zones: resting, proliferation, hypertrophic, calcification, ossification.

• Hormonal Regulation: Growth hormone (GH), thyroid hormone, and sex hormones regulate bone growth and maturation.

• Nutrition: Adequate calcium and protein intake are essential for bone growth and remodeling.

• Remodeling: Continuous process of bone resorption and formation.

  • Positive Remodeling: Strengthens bone in response to stress.

  • Negative Remodeling: Bone loss due to lack of stress or hormonal imbalance.

• Mechanical Stress: Bone adapts to mechanical load (Wolff's Law); lack of stress leads to bone loss.

• Hormonal Regulation of Calcium Balance:

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

  • Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

Healing Fractures

• Stages of Healing:

  1. Hematoma Formation: Blood clot forms at fracture site.

  2. Fibrocartilage Callus Formation: Soft callus of collagen and cartilage forms.

  3. Bony Callus Formation: New bone replaces soft callus.

  4. Bone Remodeling: Bone is reshaped to original form.

Chapter 9: The Muscular System

Muscle Tissue Types

Muscle tissue is specialized for contraction and movement. There are three main types, each with unique anatomical and functional characteristics.

• Skeletal Muscle: Voluntary, striated, multinucleated; attached to bones for movement.

• Cardiac Muscle: Involuntary, striated, branched, single nucleus; found in heart, contains intercalated discs.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped; found in walls of hollow organs.

• Similarities between Skeletal and Cardiac Muscle: Both are striated and use sliding filament mechanism for contraction.

Gross Anatomy of Skeletal Muscle

• Connective Tissue Layers:

  • Endomysium: Surrounds individual muscle fibers.

  • Perimysium: Surrounds fascicles (bundles of fibers).

  • Epimysium: Surrounds entire muscle.

• Fascicles: Bundles of muscle fibers within a muscle.

• Muscle Fibers: Also called muscle cells; long, cylindrical, multinucleated.

Growth & Development of Skeletal Muscle

• Myoblasts: Embryonic cells that fuse to form muscle fibers.

• Myotubes: Immature multinucleated muscle cells formed from myoblasts.

• Satellite Cells: Stem cells involved in muscle repair and growth.

Cellular Anatomy of Skeletal Muscle Fibers

• Sarcolemma: Plasma membrane of muscle fiber.

• T-Tubules (Transverse Tubules): Invaginations of sarcolemma; conduct action potentials into fiber.

• Motor End-Plate: Specialized region of sarcolemma at neuromuscular junction.

• Myofibrils: Rod-like structures containing contractile proteins.

• Sarcoplasmic Reticulum (SR): Stores and releases calcium ions.

• Terminal Cisternae: Enlarged SR regions adjacent to T-tubules; release Ca2+ during contraction.

• Sarcomeres: Functional contractile units; composed of:

  • A-Bands: Dark bands; contain thick filaments (myosin).

  • I-Bands: Light bands; contain thin filaments (actin).

  • Z Lines: Define boundaries of sarcomere.

  • M Lines: Center of sarcomere; holds thick filaments together.

  • H Zone: Central region of A-band with only thick filaments.

• Myofilaments:

  • Myosin: Thick filament; motor protein with heads for cross-bridge formation.

  • Actin: Thin filament; binding site for myosin.

  • Troponin and Tropomyosin: Regulatory proteins controlling actin-myosin interaction.

Skeletal Muscle Fiber Types

• Slow Oxidative (Type I): Fatigue-resistant, high endurance, use aerobic metabolism.

• Fast Glycolytic (Type IIb): Fatigue quickly, generate powerful contractions, use anaerobic metabolism.

Excitation-Contraction Coupling

• Events of Excitation:

  • Neuromuscular Junction: Site where motor neuron communicates with muscle fiber via acetylcholine (ACh).

  • Ion Movements: Na+ influx causes depolarization; K+ efflux causes repolarization.

  • Acetylcholinesterase: Enzyme that breaks down ACh, ending signal.

  • Triads: T-tubule flanked by two terminal cisternae; coordinate Ca2+ release.

• Events of Contraction:

  • Ca2+ binds to troponin, shifting tropomyosin and exposing actin binding sites.

  • Myosin heads form cross-bridges with actin.

  • ATP binding and hydrolysis power cross-bridge cycling.

  • ATP is split: $\mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i}$

"All or Nothing" Principle

• Individual Muscle Fiber: Contracts maximally if threshold is reached.

• Motor Unit: A motor neuron and all muscle fibers it innervates; all fibers contract together.

Muscle Twitch

• Latent Phase: Time between stimulus and contraction; excitation-contraction coupling occurs.

• Contraction Phase: Cross-bridge cycling and tension development.

• Relaxation Phase: Ca2+ reuptake, cross-bridges detach, tension decreases.

Increasing Muscle Force/Tension

• Relationship: Tension is proportional to cross-sectional area of muscle.

• Unit of Tension: The sarcomere is the fundamental unit of tension generation.

• Three Means to Increase Tension:

  • Wave Summation: Increased frequency of stimulation increases force.

  • Motor Unit Recruitment: More motor units activated for greater force; recruitment follows size principle (small to large units).

  • Sarcomere Length-Tension Relationship: Optimal overlap of actin and myosin maximizes force.

Types of Muscle Contraction

• Isometric: Muscle tension increases, but length does not change; no movement.

• Isotonic: Muscle changes length to move load.

  • Concentric: Muscle shortens while generating force.

  • Eccentric: Muscle lengthens while maintaining force.

Muscle Metabolism

• Creatine Phosphate Mechanism: Rapid ATP regeneration.

  • $\mathrm{Creatine\ Phosphate} + \mathrm{ADP} \rightarrow \mathrm{Creatine} + \mathrm{ATP}$

• Glycolysis: Breakdown of glucose to pyruvate.

  • With O2 (Aerobic): Pyruvate enters mitochondria for further oxidation.

  • Without O2 (Anaerobic): Pyruvate converted to lactic acid.

• Aerobic Metabolism: Complete oxidation of glucose in mitochondria.

  • Krebs Cycle: Produces NADH, FADH2, and CO2.

  • Electron Transport Chain (ETC): Produces most ATP via oxidative phosphorylation.

Muscle Type	Striations	Control	Location
Skeletal	Yes	Voluntary	Attached to bones
Cardiac	Yes	Involuntary	Heart
Smooth	No	Involuntary	Walls of hollow organs

Example: During weightlifting, skeletal muscles contract concentrically to lift the weight and eccentrically to lower it. Bone remodeling occurs in response to the increased mechanical load, strengthening the bones involved.