Joints

Joint Classification

Joints, also known as articulations, are the locations where two or more bones meet. They are classified based on their structure and function.

• Structural classification: Based on the material binding the bones and the presence or absence of a joint cavity. The main categories are:

  • fibrous

  • cartilaginous

  • synovial joints

• Functional classification: Based on the amount of movement allowed. The categories are:

  • synarthroses (immovable)

  • amphiarthroses (slightly movable)

  • diarthroses (freely movable)

Example: The knee is a synovial joint, which allows for a wide range of movement.

Synovial Joints

Synovial joints are the most common and movable type of joint in the body. They have a synovial cavity filled with synovial fluid.

• Defining characteristics:

  • Articular cartilage

  • joint cavity

  • synovial fluid

  • articular capsule

  • reinforcing ligaments

  • nerves

  • blood vessels

  • Articular Discs (Menisci): Pads of fibrocartilage that improve fit between bones and absorb shock (e.g., knee)

  • Bursae: Flattened sacs filled with synovial fluid that reduce friction between tendons and bones

  • Tendon Sheaths: Elongated bursae that wrap around tendons (especially in hands and feet)

• Types: Plane, hinge, pivot, condyloid, saddle, and ball-and-socket joints.

• Functions: Allow for various types of movement, including flexion, extension, rotation, and more.

  • Special Movements:

    • Elevation / Depression: Upward or downward movement (e.g., shrugging shoulders).

    • Protraction / Retraction: Anterior or posterior movement in the transverse plane (e.g., jaw).

    • Inversion / Eversion: Movement of the sole inward or outward.

    • Dorsiflexion / Plantar Flexion: Bending the foot upward or downward.

    • Supination / Pronation: Turning the palm upward or downward.

    • Opposition: Movement of the thumb to touch fingertips.

Example: The shoulder joint is a ball-and-socket synovial joint, allowing for rotation and movement in multiple planes.

Fibrous and Cartilaginous Joints

Fibrous joints are connected by dense connective tissue and allow little to no movement. Cartilaginous joints are connected by cartilage and allow more movement than fibrous joints but less than synovial joints.

• Fibrous joints: Sutures (skull), syndesmoses (between long bones), gomphoses (teeth).

• Cartilaginous joints: Synchondroses (hyaline cartilage, e.g., epiphyseal plate), symphyses (fibrocartilage, e.g., pubic symphysis).

Joint Movements

Joints allow for various types of movement, depending on their structure.

• Flexion and extension: Decreasing or increasing the angle between bones.

• Abduction and adduction: Moving limbs away from or toward the midline.

• Rotation: Movement around a longitudinal axis.

• Special movements: Elevation, depression, protraction, retraction, circumduction.

Joint Injuries and Disorders

Joints are susceptible to various injuries and disorders.

• Knee injuries: Common due to its complexity and weight-bearing function. Includes ACL tears, meniscus injuries.

• TMJ (temporomandibular joint) dysfunction: Symptoms include pain, clicking, and limited movement.

• Arthritis: Inflammation of joints.

  • Osteoarthritis (degenerative)

  • Rheumatoid arthritis (autoimmune)

  • Gouty arthritis

  • Gout: A metabolic disorder caused by uric acid crystal buildup in joints (especially big toe)

  • Bursitis: Inflammation of bursae, often from repetitive motion

Joint Replacement

Joint replacement is a surgical procedure to replace a damaged joint with an artificial one, commonly performed for severe arthritis.

Muscles Overview

Types of Muscular Tissue

Muscle tissue is classified into three types, each with distinct characteristics and functions.

• Skeletal muscle: Voluntary, striated, attached to bones, responsible for movement.

• Cardiac muscle: Involuntary, striated, found only in the heart, responsible for pumping blood.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs, responsible for movements like peristalsis.

Muscle Structure and Components

Muscles are composed of muscle fibers (cells), connective tissue, blood vessels, and nerves.

• Connective tissue sheaths: Endomysium (surrounds each fiber), perimysium (surrounds fascicles), epimysium (surrounds entire muscle).

• Fascicle orientation: Determines the shape and function of the muscle (e.g., parallel, pennate, convergent, circular).

• Muscle fibers: Contain myofibrils, which are made up of sarcomeres (contractile units).

Muscle Contraction

Muscle contraction occurs through the sliding filament mechanism, involving actin and myosin filaments.

• Steps: Action potential, release of calcium, binding of myosin to actin, power stroke, detachment, and re-cocking of myosin head.

• ATP: Required for muscle contraction and relaxation.

Equation:

Regulation of Muscle Contraction

Muscle contraction is regulated by neural input, calcium ions, and the availability of ATP.

• Neuromuscular junction: Site where motor neuron communicates with muscle fiber.

• Acetylcholine (ACh): Neurotransmitter that initiates muscle contraction.

Muscle Injuries and Repair

Muscle injuries can include strains, tears, and overuse injuries. Repair involves satellite cells and connective tissue.

Fascicle Orientation

Fascicle orientation affects muscle strength and range of motion.

• Parallel: Fibers run parallel to the long axis (e.g., sartorius).

• Pennate: Fibers attach obliquely to a central tendon (e.g., rectus femoris).

• Convergent: Fibers converge from a broad origin to a single tendon (e.g., pectoralis major).

• Circular: Fibers arranged in concentric rings (e.g., orbicularis oris).

Microscopic Anatomy of Muscle

Muscle fibers contain myofibrils, which are composed of repeating units called sarcomeres. Sarcomeres contain thick (myosin) and thin (actin) filaments.

• Sarcomere: The functional unit of muscle contraction.

• Z-line: Defines the boundaries of each sarcomere.

• M-line: Center of the sarcomere.

• I-band: Contains only thin filaments.

• A-band: Contains thick filaments and overlapping thin filaments.

Muscle Actions

Muscles act as levers, with bones serving as levers and joints as fulcrums.

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

• Second-class lever: Resistance between fulcrum and effort (e.g., standing on tiptoe).

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

Muscle Origins, Insertions, and Actions

Muscle origin is the fixed attachment, while insertion moves with contraction. Muscles are classified as prime movers (agonists), antagonists, synergists, and fixators.

Muscle Disorders

Common muscle disorders include muscular dystrophy, myasthenia gravis, and muscle hypertrophy/atrophy.

HTML Table: Joint Types and Characteristics

HTML Table: Muscle Tissue Types

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the lever systems and microscopic anatomy of muscle. The tables are inferred from the study guide's listed topics and standard Anatomy & Physiology content.

Muscle metabolism refers to how muscle fibers produce ATP to power contraction. Because stored ATP is limited, muscles rely on multiple energy systems.

1. Immediate Energy: Phosphagen System

• Uses creatine phosphate (CP) and ADP to form ATP.

• Enzymes: Creatine kinase and myokinase.

• Duration: ~10 seconds of maximum effort (e.g., sprinting).

2. Short-Term Energy: Anaerobic Glycolysis

• Glucose → Pyruvate → Lactic acid (no oxygen).

• Produces 2 ATP per glucose.

• Duration: ~30–40 seconds.

• Byproduct: Lactate accumulates → temporary fatigue.

3. Long-Term Energy: Aerobic Respiration

• Occurs in mitochondria; requires oxygen.

• Produces ~36 ATP per glucose.

• Sources: glucose, fatty acids, amino acids.

• Duration: hours (used in endurance activities).

ATP Uses:

• Myosin head movement (power stroke)

• Ca²⁺ pump in SR

• Na⁺/K⁺ ATPase for membrane potential restoration

Muscle Fatigue and Recovery

• Muscle Fatigue: Inability to maintain force of contraction. Causes:

  • Depletion of ATP and glycogen

  • Ion imbalance (Na⁺, K⁺, Ca²⁺)

  • Lactic acid buildup → pH drop

  • Insufficient oxygen

• EPOC (Excess Post-Exercise Oxygen Consumption):

  • Restores oxygen reserves in myoglobin

  • Replenishes ATP and creatine phosphate

  • Converts lactic acid → pyruvate in liver

  • Returns body to resting state

Muscle Fiber Types (expanded)

Types of Muscle Contractions

Muscle Tone and Tension Control

• Muscle Tone: Partial contraction of resting muscles; maintains posture and readiness.

• Motor Unit: One motor neuron + all muscle fibers it controls.

• Recruitment: Increasing active motor units increases force.

• Twitch Phases:

  1. Latent period – delay before contraction begins

  2. Contraction phase – tension rises

  3. Relaxation phase – Ca²⁺ reabsorbed, tension falls

• Tetanus: Sustained contraction due to rapid stimulation (incomplete vs. complete).

Muscle Disorders

Cardiac and Smooth Muscle (expanded comparison)