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).

Structure of Synovial Joints

Synovial joints are the most common and movable type of joint in the body. They have six general features:

• Articular cartilage: Hyaline cartilage covering bone ends to reduce friction.

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

• Articular capsule: Two-layered capsule enclosing the joint cavity (fibrous layer and synovial membrane).

• Synovial fluid: Lubricates and nourishes articular cartilage.

• Reinforcing ligaments: Strengthen the joint (intrinsic, extracapsular, or intracapsular).

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

Bursae and Tendon Sheaths

• Bursae: Flattened fibrous sacs lined with synovial membrane; reduce friction where ligaments, muscles, skin, tendons, or bones rub together.

• Tendon Sheaths: Elongated bursae wrapped around tendons, especially where tendons are crowded (e.g., wrist).

Stabilization of Synovial Joints

• Articular surfaces: Shape determines possible movements but plays a minor role in stability.

• Ligaments: Prevent excessive or undesirable motion; the more ligaments, the stronger the joint.

• Muscle tone: Most important stabilizing factor; keeps tendons taut as they cross joints.

Types of Synovial Joints (by Movement)

• Plane: Gliding movements (e.g., intercarpal joints).

• Hinge: Flexion and extension (e.g., elbow, knee).

• Pivot: Rotation (e.g., proximal radioulnar joint, atlas-axis).

• Condylar: Flexion, extension, adduction, abduction (e.g., knuckle joints).

• Saddle: Similar to condylar but with greater movement (e.g., thumb carpometacarpal joint).

• Ball-and-socket: Multiaxial movement (e.g., shoulder, hip).

Common Body Movements

• Flexion/Extension/Hyperextension

• Abduction/Adduction

• Rotation

• Circumduction

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

Major Synovial Joints: Structure, Movements, and Injuries

• Shoulder (Glenohumeral): Ball-and-socket; greatest range of motion; stability sacrificed for mobility; prone to dislocation.

• Elbow: Hinge joint; stable due to strong ligaments; allows flexion and extension.

• Hip: Ball-and-socket; less mobile than shoulder but more stable; deep socket and strong ligaments.

• Temporomandibular Joint (TMJ): Modified hinge; allows opening/closing and side-to-side movement; prone to dislocation.

• Knee: Largest, most complex; hinge with some rotation; stabilized by menisci, ligaments, and muscle tone; common injuries include "3 C's": cartilage, cruciate ligaments, collateral ligaments.

Joint Injuries and Disorders

• Sprains: Ligament stretching or tearing.

• Dislocations: Bones forced out of alignment.

• Arthritis: Inflammatory or degenerative diseases of joints.

  • Osteoarthritis: Most common, wear-and-tear.

  • Rheumatoid arthritis: Autoimmune, chronic inflammation.

  • Gouty arthritis: Uric acid crystal deposition.

• Lyme Disease: Caused by Borrelia burgdorferi (tick-borne); symptoms include joint pain, skin rash, flu-like symptoms; less common on the West Coast due to lower tick prevalence.

Muscles and Muscle Tissue

Types of Muscle Tissue

There are three basic types of muscle tissue, each with unique structure and function:

Type	Location	Control	Striations	Function
Skeletal	Attached to bones	Voluntary	Yes	Movement, posture
Cardiac	Heart	Involuntary	Yes	Pumps blood
Smooth	Walls of hollow organs	Involuntary	No	Moves substances

Muscle Terminology

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

• sarco-: Prefix referring to flesh, often used for muscle cell structures (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 of body parts and substances

  • Maintaining posture and body position

  • Stabilizing joints

  • Generating heat

Gross and Microscopic Structure of Skeletal Muscle

• Gross Structure: Skeletal muscle is an organ made of muscle fibers, blood vessels, nerve fibers, and connective tissue sheaths.

  • Connective tissue sheaths:

    • Epimysium: Surrounds entire muscle.

    • Perimysium: Surrounds fascicles (bundles of fibers).

    • Endomysium: Surrounds each muscle fiber.

• Microscopic Structure: Muscle fibers contain myofibrils, sarcoplasmic reticulum, and T tubules.

  • Myofibrils: Rod-like elements containing contractile units (sarcomeres).

  • Myofilaments: Actin (thin) and myosin (thick) filaments.

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

  • T tubules: Invaginations of sarcolemma; conduct impulses deep into muscle fiber.

Sliding Filament Model of Contraction

Muscle contraction occurs as myosin heads bind to actin, pulling thin filaments toward the center of the sarcomere, shortening the muscle.

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

• During contraction: I bands shorten, H zones disappear, A bands move closer but do not change length.

Steps of Muscle Contraction

1. Events at the Neuromuscular Junction (NMJ): Motor neuron releases acetylcholine (ACh), which binds to receptors on the sarcolemma, generating an action potential.

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

3. Excitation-Contraction Coupling: Action potential triggers calcium release from SR; calcium 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, myosin releases actin)

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

Homeostatic Imbalances at the NMJ

• Myasthenia gravis: Autoimmune disease; ACh receptors destroyed, causing muscle weakness.

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

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

Regeneration of ATP in Muscle

• Direct phosphorylation: Creatine phosphate + ADP → ATP + creatine

• Anaerobic pathway: Glycolysis and lactic acid formation

• Aerobic respiration: Glucose + O2 → CO2 + H2O + ATP

Muscle soreness is often due to microtrauma and lactic acid accumulation.

Key Equations

• Direct phosphorylation: $\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

• Aerobic respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 32\text{ATP}$

Additional info: For detailed labeling and diagrams, refer to textbook figures of synovial joints and sarcomeres. Table 9.3 (not provided) likely compares muscle tissue types as summarized above.