Joints and Muscle Tissue: Study Guide for Anatomy & Physiology I (Chapters 8 & 9)

Joints (Articulations)

Definition and Classification of Joints

Joints, or articulations, are sites where two or more bones meet. They provide mobility and hold the skeleton together.

• Structural Classification: Based on the material binding bones and presence of a joint cavity.

  • Fibrous Joints: Bones joined by dense fibrous connective tissue; no joint cavity; mostly immovable. Examples: Sutures (skull), syndesmoses (distal tibiofibular joint), gomphoses (teeth in sockets).

  • Cartilaginous Joints: Bones united by cartilage; no joint cavity.

    • Synchondroses: Hyaline cartilage unites bones (e.g., epiphyseal plates, first rib-sternum joint).

    • Symphyses: Fibrocartilage unites bones (e.g., intervertebral discs, pubic symphysis).

  • Synovial Joints: Bones separated by a fluid-filled joint cavity; freely movable. Examples: Most limb joints (shoulder, knee, hip).

• Functional Classification: Based on the amount of movement allowed.

  • Synarthroses: Immovable joints (e.g., sutures).

  • Amphiarthroses: Slightly movable joints (e.g., intervertebral discs).

  • Diarthroses: Freely movable joints (e.g., synovial joints).

General Structure of Cartilaginous Joints

• Cartilaginous joints lack a joint cavity and are united by cartilage.

• Two main types: synchondroses (hyaline cartilage) and symphyses (fibrocartilage).

Structural Characteristics of Synovial Joints

Synovial joints are the most common and movable type of joint in the body.

• Six General Features:

  1. Articular cartilage: Hyaline cartilage covering bone ends.

  2. Joint (synovial) cavity: Space containing synovial fluid.

  3. Articular capsule: Two-layered capsule enclosing the joint cavity.

  4. Synovial fluid: Lubricates and nourishes articular cartilage.

  5. Reinforcing ligaments: Strengthen the joint.

  6. Nerves and blood vessels: Detect pain, monitor joint position, and supply nutrients.

Bursae and Tendon Sheaths

• Bursae: Flattened sacs filled with synovial fluid; reduce friction between tissues.

• Tendon Sheaths: Elongated bursae wrapped around tendons; reduce friction during movement.

• Comparison: Both reduce friction, but tendon sheaths specifically wrap around tendons.

Stabilization of Synovial Joints

• Three natural stabilizing factors:

  1. Shape of articular surfaces (minor role)

  2. Ligament number and location (limited role)

  3. Muscle tone (most important)

Body Movements at Synovial Joints

• Gliding: Flat bone surfaces slide past each other (e.g., intercarpal joints).

• Angular Movements: Increase or decrease the angle between bones (flexion, extension, abduction, adduction, circumduction).

• Rotation: Bone turns around its own long axis (e.g., atlas-axis, shoulder, hip).

• Special Movements: Supination, pronation, dorsiflexion, plantar flexion, inversion, eversion, protraction, retraction, elevation, depression, opposition.

Types of Synovial Joints (by Movement)

Type	Movement	Example
Plane	Gliding	Intercarpal joints
Hinge	Flexion/Extension	Elbow, knee
Pivot	Rotation	Proximal radioulnar joint
Flexion/Extension, Abduction/Adduction	Metacarpophalangeal joints
Saddle	Flexion/Extension, Abduction/Adduction	Thumb carpometacarpal joint
Ball-and-socket	All movements	Shoulder, hip

Major Synovial Joints: Structure, Movements, and Injuries

• Shoulder (Glenohumeral Joint): Head of humerus and glenoid cavity of scapula; most mobile, least stable; allows all movements; prone to dislocation.

• Elbow: Humerus, radius, ulna; hinge joint; flexion/extension; stable due to strong ligaments.

• Hip: Head of femur and acetabulum; ball-and-socket; stable, less mobile than shoulder.

• Temporomandibular Joint (TMJ): Mandibular condyle and temporal bone; hinge and gliding movements; prone to dislocation.

• Knee: Femur, tibia, patella; largest, most complex; hinge with some rotation; stabilized by ligaments and menisci; common injuries include "3 C's": cartilage (meniscus), cruciate ligaments, collateral ligaments.

Common Joint Injuries

• Sprains: Ligament stretching or tearing.

• Dislocations: Bones forced out of alignment.

• Torn Cartilage: Common in knee menisci.

• 3 C's of Knee Injury: Collateral ligaments, Cruciate ligaments, Cartilage (meniscus).

Types of Arthritis

Type	Cause	Features
Osteoarthritis	Wear and tear	Degeneration of articular cartilage
Rheumatoid Arthritis	Autoimmune	Chronic inflammation, joint deformity
Gouty Arthritis	Uric acid crystals	Sudden, severe pain, often in big toe

Lyme Disease

• Cause: Bacterial infection (Borrelia burgdorferi) from tick bites.

• Consequences: Joint pain, arthritis, neurological symptoms if untreated.

• Geography: Less common on the West Coast due to lower prevalence of the tick vector.

Muscles and Muscle Tissue

Types of Muscle Tissue

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

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

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs, single nucleus.

Muscle Terminology

• myo-, mys-: Prefixes referring to muscle.

• sarco-: Prefix referring to flesh or muscle (e.g., sarcolemma, sarcoplasm).

Characteristics and Functions of Muscle Tissue

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length.

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

Gross Structure of Skeletal Muscle

• Composed of muscle fibers (cells), connective tissue, blood vessels, and nerves.

• Connective Tissue Sheaths:

  • Epimysium: Surrounds entire muscle.

  • Perimysium: Surrounds fascicles (bundles of fibers).

  • Endomysium: Surrounds individual muscle fibers.

Microscopic Structure of Muscle Fibers

• Myofibrils: Rod-like elements containing contractile proteins (myofilaments).

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

• T Tubules: Invaginations of sarcolemma; transmit action potentials.

Sliding Filament Model of Contraction

• Muscle contraction occurs as thin filaments (actin) slide past thick filaments (myosin), shortening the sarcomere.

• Sarcomere: Functional unit of muscle fiber; defined by Z discs.

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

Steps of Muscle Contraction

1. Events at Neuromuscular Junction: Motor neuron releases acetylcholine (ACh), triggering action potential in muscle fiber.

2. Muscle Fiber Excitation: Action potential spreads along sarcolemma and T tubules.

3. Excitation-Contraction Coupling: Action potential triggers Ca2+ release from SR; Ca2+ binds to troponin, exposing binding sites on actin.

4. Cross Bridge Cycle:

   1. Cross bridge formation (myosin binds to actin)

   2. Power stroke (myosin head pivots, pulling actin)

   3. Cross bridge detachment (ATP binds to myosin, releasing actin)

   4. Cocking of myosin head (ATP hydrolysis resets myosin)

Action Potential (AP)

• Electrical signal involving depolarization and repolarization of the muscle cell membrane.

• Essential for initiating muscle contraction.

Homeostatic Imbalances at the Neuromuscular Junction (NMJ)

• Myasthenia Gravis: Autoimmune disorder; antibodies block ACh receptors, causing muscle weakness.

• Botox: Botulinum toxin inhibits ACh release, causing paralysis.

• Rigor Mortis: After death, ATP production ceases; cross bridges cannot detach, causing stiffness.

ATP Regeneration in Muscle Contraction

• Direct Phosphorylation: Creatine phosphate donates phosphate to ADP.

• Anaerobic Pathway: Glycolysis and lactic acid formation (no oxygen required).

• Aerobic Pathway: Cellular respiration (oxygen required).

Muscle Soreness

• Often due to microscopic damage and lactic acid accumulation after intense exercise.

Additional info: For detailed molecular mechanisms, refer to Table 9.3 (not provided here), which compares skeletal, cardiac, and smooth muscle characteristics.