Joints (Articulations)

Definition and Function of Articulations

Articulations, also known as joints, are the sites where two or more bones meet. They allow for movement of the body, hold bones together, and sometimes provide protection.

• Immovable joints (e.g., skull sutures) offer protection.

• Movable joints enable a wide range of body movements.

Classification of Joints by Structure

Joints are classified structurally based on the material binding the bones and whether a joint cavity is present.

• Fibrous Joints: Bones connected by collagen fibers; little to no movement.

  • Sutures: Short fibers, synarthrosis (no movement), found in the skull; closed suture is called synostoses.

  • Syndesmosis: Longer fibers, amphiarthrosis (slight movement), found in ligaments (e.g., tibia/fibula, radius/ulna).

  • Gomphosis: Periodontal ligament, synarthrosis, found in tooth roots.

• Cartilaginous Joints: Bones joined by cartilage; lack a joint cavity.

  • Synchondrosis: Hyaline cartilage, found in epiphyseal plate and costal cartilage of 1st rib; synarthrosis.

  • Symphysis: Fibrocartilage (sometimes hyaline), found in intervertebral discs and pubic symphysis; amphiarthrosis.

• Synovial Joints: Bones connected by a fluid-filled cavity, articular cartilage, and an articular capsule; all are diarthrotic (freely movable).

  • Types: Plane, Condylar, Hinge, Saddle, Pivot, Ball-and-socket.

Classification of Joints by Function

• Synarthroses: Immobile joints

• Amphiarthroses: Slightly mobile joints

• Diarthroses: Freely movable joints

General Structure of Cartilaginous Joints

Cartilaginous joints are sandwiched between bones (e.g., pubic symphysis, intervertebral discs) or at the end of a long bone and the sternum. They lack a joint cavity.

• Symphysis: Example - pubic symphysis, intervertebral disc

• Synchondrosis: Example - costal cartilage in sternum, epiphyseal plate

Structural Characteristics of Synovial Joints

Synovial joints are characterized by six general features:

• Articular cartilage: Prevents crushing of bone ends

• Articular capsule: Dense irregular connective tissue; fibrous layer strengthens joint, synovial membrane produces synovial fluid

• Joint cavity: Unique to synovial joints; space filled with synovial fluid

• Synovial fluid: Lubricates joint surfaces, removes debris and microbes

• Reinforcing ligaments: Strengthen joints

• Nerves and blood vessels: Monitor pain, joint position, and provide nutrients

Bursae and Tendon Sheaths

• Bursae: Synovial membrane and fluid; reduce friction between ligaments, muscles, skin, tendons, and bones.

• Tendon sheaths: Similar to bursae but wrap around tendons (e.g., wrist) to reduce friction.

Factors Stabilizing Synovial Joints

• Shape of articular surfaces

• Muscle tone

• Number and positioning of ligaments

Common Body Movements

• Gliding: Flat surfaces slide over one another

• Angular movements:

  • Flexion: Decreasing angle between bones

  • Extension: Straightening a limb

  • Abduction: Movement away from midline

  • Adduction: Movement toward midline

  • Circumduction: Distal end moves in a circle

• Rotation:

  • Medial rotation: Toward median plane

  • Lateral rotation: Toward lateral plane

Types of Synovial Joints and Examples

• Plane joint: Intercarpal joints, intervertebral surfaces (gliding)

• Hinge joint: Elbow, interphalangeal joints (flexion/extension)

• Pivot joint: Proximal radioulnar, atlantoaxial joint (rotation)

• Condylar joint: Knuckle, wrist (flexion/extension, adduction/abduction)

• Saddle joint: Carpometacarpal of thumb (adduction/abduction, flexion/extension)

• Ball-and-socket joint: Shoulder, hip (flexion/extension, adduction/abduction, rotation)

Major Joints: Structure, Movements, Stability, and Injury

• Shoulder: Head of humerus and glenoid cavity; ball-and-socket; diarthrotic; stabilized by rotator cuffs; prone to dislocation.

• Elbow: Humerus, radius, ulna; hinge; diarthrotic; stabilized by collateral ligaments; hyperextension injury.

• Hip: Head of femur and acetabulum; ball-and-socket; diarthrotic; stabilized by acetabular labrum; dislocation possible.

• TMJ: Mandible and temporal bone; gliding/hinge; diarthrotic; stabilized by articular disc and ligaments; easily dislocated.

• Knee: Femur, tibia, patella; hinge/gliding; diarthrotic; stabilized by menisci; injuries to collateral/cruciate ligaments and cartilage.

Common Joint Injuries

• Cartilage tears: Joint lock/bind, poor self-repair, may require surgery; can lead to osteoarthritis.

• Sprains: Ligaments stretched/torn; poor repair; may require grafts or healing time.

• Dislocations: Bones forced out of alignment; accompanied by sprains, inflammation, difficulty moving; treated by reduction.

• 3 C's of Knee Injury: Collateral ligaments, cruciate ligaments, cartilages (menisci); "Unhappy triad" = ruptured ACL, ruptured tibial collateral ligament, torn meniscus.

Types of Arthritis

• Bursitis: Inflammation of bursae; caused by blow/friction; treated with rest, ice, anti-inflammatory drugs.

• Tendonitis: Inflammation of tendons; caused by overuse; similar symptoms/treatment as bursitis.

• Osteoarthritis: Degenerative, irreversible; cartilage breakdown; bone spurs; stiffness, crepitus; treated with exercise, pain relievers.

• Rheumatoid arthritis: Autoimmune; joint pain, swelling, anemia, osteoporosis; inflammation of synovial membrane; treated with steroids, immune suppressants.

• Gouty arthritis: Uric acid crystals in joints; affects big toe; untreated leads to joint immobilization; treated with medication, hydration, diet changes.

Lyme Disease

• Caused by bacteria from tick bites; symptoms include rash, flu-like symptoms, joint pain.

• Treated with antibiotics.

• Less common on West Coast due to Western Fence lizard's blood killing the bacteria.

Muscles & Muscle Tissue

Types of Muscle Tissue

• Skeletal muscle: Attached to skeleton; longest fibers; striated; multinucleated; voluntary; powerful but tires easily.

• Cardiac muscle: Only in heart; striated; involuntary.

• Smooth muscle: Walls of hollow organs; no striations; elongated cells; involuntary; moves fluids.

Muscle Terminology

• Myo and mys: Muscle

• Sarco: Flesh

Characteristics of Muscle Tissue

• Excitability: Ability to respond to stimuli

• Contractibility: Ability to shorten forcibly

• Extensibility: Ability to stretch

• Elasticity: Ability to return to original shape

Functions of Muscle Tissue

• Produce movement

• Maintain posture

• Stabilize joints

• Generate heat

Gross Structure of Skeletal Muscle

• Made of nerves, arteries, veins, connective tissue sheaths, tendons, aponeuroses

• Epimysium: Covers whole muscle

• Perimysium: Covers fascicle

• Endomysium: Covers muscle fiber

Microscopic Structure of Muscle Fibers

• Nuclei

• Sarcolemma: Plasma membrane

• Sarcoplasm: Cytoplasm

• T-Tubules: Allow action potential to travel

• Sarcoplasmic reticulum: Stores/releases Ca2+

• Actin, troponin, tropomyosin: Thin filaments

• Myosin: Thick filaments

• Z disc, A band, I band, H band, M line

Functional Roles in Muscle Fibers

• Myofibrils: Contain sarcomeres, basis for contraction

• Myofilaments: Proteins for contraction

• T-tubules: Conduct action potential

• Sarcoplasmic reticulum: Releases Ca2+

Sliding Filament Model of Muscle Contraction

Thin filaments slide past thick filaments, causing overlap. The I band and Z disc shorten, H zone disappears, but filament length remains unchanged.

Sarcomere Structure

• Z disc

• I band

• A band

• M line

• H zone

• Thin and thick filaments

Molecular Composition of Myofilaments

• Thin filaments: Actin (G actin), tropomyosin, troponin

Movements of Sarcomere in Sliding Filament Model

During contraction, Z bands move closer, I band shortens, A band remains unchanged.

Steps of Muscle Contraction

1. Action potential opens voltage-gated Ca2+ channels at axon terminal.

2. Ca2+ triggers release of acetylcholine (ACh) into synaptic cleft.

3. ACh binds to ion channels, causing depolarization (end plate potential).

4. Action potential travels away from NMJ, opening more Na+ channels.

5. AP reaches T-tubules, opens Ca2+ channels in sarcoplasmic reticulum.

6. Ca2+ binds to troponin, moves tropomyosin, allows cross-bridge formation.

Neuromuscular Junction (NMJ) and Muscle Fiber Excitation

• AP travels to axon terminal, opens Ca2+ channels, releases ACh.

• Muscle fiber excitation: rapid charge change triggers contraction.

Excitation-Contraction Coupling

• AP travels down T-tubules, releases Ca2+ from SR, Ca2+ binds to troponin, cross-bridging occurs.

Cross-Bridge Cycle

1. Myosin heads attach to actin (cross-bridge formation).

2. Power stroke: myosin head pivots, pulls actin to M line (ADP and P released).

3. ATP attaches, myosin head detaches from actin.

4. ATP hydrolyzes to ADP and P, myosin head resets.

Homeostatic Imbalances at NMJ

• Myasthenia gravis: Autoimmune disease; destroys ACh receptors at NMJ.

Rigor Mortis

• Caused by myosin heads remaining attached to actin after death due to lack of ATP.

ATP Regeneration During Muscle Contraction

• Phosphorylation of ADP by creatine phosphate (CP): CP donates phosphate to ADP to form ATP.

• Anaerobic pathway: Glycolysis and lactic acid formation; fast, less ATP.

• Aerobic respiration: Glycolysis and cellular respiration; slow, more ATP; uses glucose, fatty acids.

ATP Functions in Muscle Fiber

• Move and detach cross bridges

• Pump Ca2+ back into SR

• Pump Na+ out and K+ in after excitation-contraction coupling

ATP is the only energy source for contractile activities and must be regenerated quickly.