Joints: Classification and Structure

Functional and Structural Joint Classification

Joints, or articulations, are classified based on their function (degree of movement) and structure (type of tissue connecting bones).

• Functional Classifications:

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

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

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

• Structural Classifications:

  • Fibrous: Bones joined by fibrous tissue; no joint cavity (e.g., sutures, syndesmoses).

  • Cartilaginous: Bones joined by cartilage; no joint cavity (e.g., synchondroses, symphyses).

  • Synovial: Bones separated by a fluid-filled joint cavity; most joints in the body.

Example: The knee and elbow are synovial, diarthrotic, and structurally classified as hinge joints.

Synovial Joints: Features and Stability

Synovial joints are characterized by six distinguishing features and are the most common and movable type of joint.

• Six Features:

  1. Articular cartilage

  2. Joint (synovial) cavity

  3. Articular capsule

  4. Synovial fluid

  5. Reinforcing ligaments

  6. Nerves and blood vessels

• Synovial Fluid Formation: Produced by filtration from blood plasma and secreted by synovial membrane.

• Bursae and Tendon Sheath: Reduce friction between moving structures; bursae are sacs, tendon sheaths are elongated bursae around tendons.

• Stability Factors: Shape of articular surfaces, number and position of ligaments, muscle tone (most important).

Range of Motion and Types of Synovial Joints

Synovial joints allow various movements, classified by axes of motion and joint type.

• Axes of Motion:

  • Nonaxial: Gliding movements (plane joints).

  • Uniaxial: Movement in one plane (hinge, pivot).

  • Biaxial: Movement in two planes (condylar, saddle).

  • Multiaxial: Movement in three planes (ball-and-socket).

• Six Major Types:

  • Plane (intercarpal joints)

  • Hinge (elbow, knee)

  • Pivot (proximal radioulnar joint)

  • Condylar (knuckle joints)

  • Saddle (thumb carpometacarpal joint)

  • Ball-and-socket (shoulder, hip)

Specific Joints: Knee, Shoulder, Elbow, Hip, TMJ

Major synovial joints have unique structures and functions.

• Knee:

  • Three joints: femoropatellar, lateral tibiofemoral, medial tibiofemoral.

  • Menisci: fibrocartilage pads for shock absorption and stability.

  • Ligaments: patellar, collateral (medial/lateral), cruciate (anterior/posterior).

  • Common injuries: torn meniscus, ACL injury, patellar dislocation.

• Shoulder:

  • Articulation: head of humerus and glenoid cavity of scapula.

  • High mobility, less stability; frequent dislocations.

  • Rotator cuff muscles stabilize joint.

• Elbow:

  • Articulation: humerus, radius, ulna.

  • Movements: flexion, extension.

  • Ligaments: ulnar collateral, radial collateral, annular.

• Hip:

  • Articulation: head of femur and acetabulum of pelvis.

  • Four ligaments: iliofemoral, pubofemoral, ischiofemoral, ligamentum teres.

  • More stability, less mobility than shoulder.

• TMJ (Temporomandibular Joint):

  • Articulation: mandibular condyle and temporal bone.

  • Stability: relatively weak, prone to dislocation.

Ligaments and Joint Correspondence

Each major joint is stabilized by specific ligaments; knowing their correspondence is essential for understanding joint function and injury.

Arthritis and Lyme Disease

Joint diseases affect structure and function.

• Three Types of Arthritis:

  • Osteoarthritis: Degenerative, wear-and-tear; pain, stiffness.

  • Rheumatoid arthritis: Autoimmune; inflammation, joint deformity.

  • Gouty arthritis: Uric acid crystal deposition; sudden pain, swelling.

• Lyme Disease: Caused by Borrelia burgdorferi (tick-borne); symptoms include joint pain, swelling, and can be treated with antibiotics.

Muscle Tissue: Structure and Function

Types and Characteristics of Muscle Tissue

Muscle tissue is classified into three types, each with unique structure and function.

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

• Cardiac Muscle: Involuntary, striated, found in heart.

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

Characteristics: Excitability, contractility, extensibility, elasticity.

Connective Tissue Sheaths

Muscle fibers are wrapped in connective tissue layers for support and force transmission.

• Epimysium: Surrounds entire muscle.

• Perimysium: Surrounds fascicles (bundles of fibers).

• Endomysium: Surrounds individual muscle fibers.

Microscopic Structure of Muscle Fiber

Muscle fibers contain specialized structures for contraction.

• Sarcolemma: Plasma membrane of muscle cell.

• Sarcoplasm: Cytoplasm; contains myoglobin for oxygen storage.

• Striations: Caused by arrangement of myofilaments (thick myosin, thin actin).

• Sarcomere: Functional unit; Z discs, I bands, H zone change during contraction.

Example: During contraction, Z discs move closer, I bands and H zone shorten.

Regulatory Proteins and Structures

• Troponin: Binds calcium, regulates contraction.

• Tropomyosin: Blocks myosin binding sites on actin.

• Sarcoplasmic Reticulum: Stores and releases calcium.

• T-tubules: Conduct action potentials into muscle fiber.

Muscle Contraction: Phases and Mechanisms

Contraction involves electrical and mechanical events.

• Neuromuscular Junction: Acetylcholine (ACh) released, triggers action potential.

• Action Potential Phases: End plate potential, depolarization, repolarization.

• Excitation-Contraction Coupling: Action potential leads to calcium release, cross-bridge formation.

• Cross Bridge Cycle: Myosin binds actin, pulls, releases, and repeats.

Motor Units and Muscle Twitch

• Motor Unit: One motor neuron and all muscle fibers it innervates.

• Muscle Twitch: Single contraction; phases: latent, contraction, relaxation.

• Wave Summation and Tetanus: Repeated stimuli increase force; incomplete and complete tetanus.

• Muscle Tone: Constant, partial contraction for joint stability.

Types of Muscle Contractions

• Isometric: Muscle tension without movement.

• Isotonic: Muscle changes length; concentric (shortening), eccentric (lengthening).

ATP Formation and Muscle Fiber Types

Muscles use different pathways to generate ATP, affecting performance.

• ATP Formation:

  • Direct phosphorylation (creatine phosphate): short duration.

  • Anaerobic glycolysis: moderate duration, produces lactic acid.

  • Aerobic respiration: long duration, most ATP.

• Muscle Fiber Types:

  • Slow oxidative: endurance, aerobic.

  • Fast oxidative: intermediate, aerobic.

  • Fast glycolytic: rapid, anaerobic.

Smooth Muscle Structure and Function

• Smooth vs Skeletal Muscle: Smooth lacks troponin and sarcomeres; contracts via calmodulin and different calcium sources.

• Unitary vs Multi-unit Smooth Muscle: Unitary contracts as a group; multi-unit contracts independently.

Muscle Disorders

• Myasthenia Gravis: Autoimmune; weakens muscles by affecting ACh receptors.

• Muscular Dystrophy: Genetic; progressive muscle weakness and degeneration.

Muscular System: Organization and Function

Functional Groups of Muscles

Muscles work in groups to produce movement.

• Prime Movers (Agonists): Main muscle responsible for movement.

• Antagonists: Oppose or reverse movement.

• Synergists: Assist prime movers.

• Fixators: Stabilize origin of prime mover.

Muscle Naming Criteria and Fascicle Arrangement

• Criteria: Location, shape, size, direction, number of origins, location of attachments, action.

• Fascicle Arrangement: Parallel, convergent, pennate, circular.

Lever Systems in the Body

Muscles and bones act as levers to move loads.

• Terms: Lever (bone), fulcrum (joint), effort (muscle force), load (weight).

• Mechanical Advantage: Less effort needed; disadvantage: more speed, less force.

• Classes of Levers:

  • First-class: Fulcrum between effort and load (e.g., neck muscles).

  • Second-class: Load between fulcrum and effort (e.g., calf raise).

  • Third-class: Effort between fulcrum and load (e.g., biceps curl).

Major Muscle Groups and Functions

• Face and Neck: Muscles for facial expression, mastication, neck movement.

• Vertebral Column: Erector spinae (iliocostalis, longissimus, spinalis) for back extension.

• Respiration: Diaphragm and intercostals for inspiration/expiration.

• Abdominal Wall: Muscles for trunk movement and support.

• Pelvic/Urogenital Diaphragm: Support pelvic organs (see Table 10.13).

• Rotator Cuff: Stabilize shoulder joint.

• Lower Limbs:

  • Iliopsoas, Sartorius: Flex and rotate thigh.

  • Adductors: Adduct and rotate thigh.

  • Quadriceps Femoris: Extend leg.

  • Gluteus Maximus: Extends thigh; largest gluteal muscle.

  • Hamstrings: Extend thigh, flex leg.

  • Tibialis Anterior: Dorsiflexion of foot.

  • Fibularis Longus: Plantar flexion of foot.

Muscle Fiber Types Comparison Table

ATP Production Pathways Table

Equations Related to Muscle Physiology

• ATP Hydrolysis:

• Force Generation:

Additional info: Academic context was added to clarify muscle fiber types, ATP production, and joint classifications, as well as to provide examples and explanations for each major point.