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

Chapter 8: Joints

Articulations (Joints)

Articulations, or joints, are sites where two or more bones meet. They play a crucial role in providing mobility and holding the skeleton together.

• Definition: An articulation is a junction between two bones.

• Function: Joints allow for movement and provide mechanical support.

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

• Cartilaginous Joints: Bones united by cartilage; no joint cavity. Examples: Synchondroses (epiphyseal plates), symphyses (pubic symphysis).

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

Classification of Joints by Function

Functional classification is 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).

Cartilaginous Joints

• General Structure: Bones are united by cartilage; lack a joint cavity.

• Types:

  • Synchondroses: Hyaline cartilage unites bones (e.g., epiphyseal plates, first rib and manubrium).

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

Synovial Joints

These joints are the most movable and are characterized by a fluid-filled joint cavity.

• Six General Features:

  1. Articular cartilage (hyaline): covers bone ends.

  2. Joint (synovial) cavity: small space containing synovial fluid.

  3. Articular capsule: two layers (fibrous and synovial membrane).

  4. Synovial fluid: lubricates and nourishes articular cartilage.

  5. Reinforcing ligaments: strengthen the joint.

  6. Nerves and blood vessels: supply the joint.

Bursae and Tendon Sheaths

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

• Tendon Sheaths: Elongated bursae that wrap around tendons.

• Comparison: Both reduce friction, but tendon sheaths are specifically associated with tendons.

Stabilization of Synovial Joints

• Articular surfaces: Shape determines possible movements.

• Ligaments: Prevent excessive or undesirable motion.

• Muscle tone: Keeps tendons taut and is the most important stabilizing factor.

Body Movements at Synovial Joints

• Gliding: One flat bone surface glides over another (e.g., intercarpal joints).

• Angular Movements: Flexion, extension, hyperextension, abduction, adduction, circumduction.

• Rotation: Turning of a bone around its own long axis (e.g., atlas and axis, hip, shoulder).

• 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

• Shoulder (Glenohumeral): Head of humerus and glenoid cavity of scapula; most mobile, least stable; frequent dislocation.

• Elbow: Humerus, radius, ulna; hinge joint; flexion/extension.

• 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.

• Knee: Femur, tibia, patella; largest, most complex; hinge with some rotation; prone to injury.

Common Joint Injuries

• Sprains: Ligament stretching or tearing.

• Dislocations: Bones forced out of alignment.

• Torn Cartilage: Common in knee; may require surgery.

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

Types of Arthritis

• Osteoarthritis: Degenerative; wear and tear.

• Rheumatoid Arthritis: Autoimmune; inflammation of synovial membrane.

• Gouty Arthritis: Uric acid crystal deposition.

Lyme Disease

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

• Consequences: Joint pain, arthritis, neurological symptoms.

• Geography: Less common on the West Coast due to lower tick populations and different tick species.

Chapter 9: Muscles & Muscle Tissue

Types of Muscle Tissue

• Skeletal Muscle: Striated, voluntary, attached to bones, multinucleated.

• Cardiac Muscle: Striated, involuntary, found in heart, intercalated discs.

• Smooth Muscle: Non-striated, involuntary, found in walls of hollow organs.

Muscle Terminology

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

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

Characteristics 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 of Muscle Tissue

• Movement of body parts and substances

• Maintenance of posture

• Stabilization of joints

• Heat generation

Gross Structure of Skeletal Muscle

• Composed of: Muscle fibers (cells), connective tissue, blood vessels, 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: Actin (thin) and myosin (thick) filaments.

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

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

Sliding Filament Model of Contraction

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

• Sarcomere: Functional unit of muscle contraction.

• Key Steps: Cross-bridge formation, power stroke, detachment, cocking of myosin head.

Steps of Muscle Contraction

1. Action potential arrives at neuromuscular junction.

2. Acetylcholine released, binds to receptors on sarcolemma.

3. Depolarization triggers action potential in muscle fiber.

4. Calcium released from SR.

5. Calcium binds to troponin, exposing binding sites on actin.

6. Myosin heads bind to actin, initiating cross-bridge cycle.

Neuromuscular Junction (NMJ)

• Structure: Axon terminal, synaptic cleft, motor end plate.

• Action Potential: Electrical signal that travels along the sarcolemma.

• Events: Release of acetylcholine, depolarization, initiation of muscle action potential.

Excitation-Contraction Coupling

• Sequence linking action potential to muscle contraction.

• Involves calcium release and cross-bridge formation.

Cross Bridge Cycle (4 Steps)

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 head)

Homeostatic Imbalances at the NMJ

• Myasthenia Gravis: Autoimmune disorder; antibodies block acetylcholine receptors.

• Botox: Blocks acetylcholine release, causing muscle paralysis.

• Rigor Mortis: Lack of ATP after death prevents detachment of cross bridges, causing stiffness.

ATP Regeneration in Muscle

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

• Anaerobic Pathway: Glycolysis and lactic acid formation

• Aerobic Pathway: Cellular respiration (mitochondria)

Muscle Soreness

• Often due to microscopic damage and lactic acid accumulation.

Additional info: For detailed molecular mechanisms, refer to Table 9.3 (not included here) for a comparison of muscle tissue types.