Muscle Contraction, Joint Anatomy, and Skeletal Movement

Posted on Oct 3, 2025 in Human Biology

Muscle Contraction and Anatomy Fundamentals

Steps in Skeletal Muscle Contraction

  1. Calcium ions are released from the sarcoplasmic reticulum.
  2. Calcium ions bind to troponin.
  3. Myosin cross-bridges bind to the actin filament.
  4. The myosin head pivots toward the center of the sarcomere (Power Stroke).
  5. The myosin head binds an ATP molecule and detaches from actin.
  6. The free myosin head splits ATP, resetting the head position.

Key Muscle Fiber Structures and Definitions

  • Triad (E): Terminal cisternae abutting a T-tubule.
  • Myoglobin (A): Oxygen-binding protein found in muscle fibers.
  • Myofibril (B): Bundles of proteins (myofilaments).
  • Sarcoplasm (C): Cytoplasm of the muscle fiber.
  • Transverse Tubule (T-Tubule) (D): Transmits action potential from the sarcolemma to the cell interior.

Sarcomere Structure Terminology

  • A Band (D): Area in the sarcomere where thick and thin filaments are found.
  • I Band (C): Area in the sarcomere where only thin filaments are found.
  • Z Line (A): Separates sarcomeres from each other.
  • H Band (E): Area containing only thick filaments.
  • M Line (B): Composed of supporting proteins holding thick filaments together at the H zone.

Joint Structure and Classification

Joints are classified structurally into four main types:

  1. Fibrous
  2. Cartilaginous
  3. Bony (Synostosis)
  4. Synovial

Functional Classification of Joints

  1. Synarthrosis (Immovable Joint):
    • Example: Gomphosis (teeth).
  2. Amphiarthrosis (Slightly Movable Joint):
    • Syndesmosis: Bones connected by a ligament.
    • Symphysis: Bones connected by fibrocartilage.
  3. Diarthrosis (Freely Movable Joint): Synovial joints.

Accessory Structures of Synovial Joints

  • Synovial Fluid: Provides shock absorption, lubrication, and nutrient distribution.
  • Fat Pads: Protect articular cartilage and serve as packing material for joints.
  • Bursae: Small pockets of synovial fluid.
  • Meniscus (Articular Disc): Subdivides synovial cavities, channels the flow of synovial fluid, and allows for variations in the shape of articular surfaces.

Movements at Synovial Joints

  • Monaxial: Movement in 1 plane (e.g., elbow).
  • Biaxial: Movement in 2 planes (e.g., wrist).
  • Triaxial: Movement in 3 planes (e.g., shoulder).

Skeletal Movement Terminology

Flexion
Decreases the angle between articulating bones.
Extension
Increases the angle between articulating bones (returning to normal position).
Hyperextension
Movement past the anatomical position.
Pronation
Palm faces down.
Supination
Palm faces up.
Inversion
Twists the sole of the foot medially.
Eversion
Twists the sole of the foot laterally.
Dorsiflexion
Lifting the toes up.
Plantar Flexion
Pointing the toes down.
Opposition
Movement of the thumb toward the palm or other fingers.
Reposition
The opposite of opposition (returning the thumb to anatomical position).
Protraction
Moving the jaw forward.
Retraction
Pulling the jaw back.
Depression
Opening the mouth.
Elevation
Closing the mouth.
Lateral Flexion
Bending the neck side to side.

Movement Application Questions

  • Injury to the ankle by turning the sole inward? Inversion
  • Fetal position (intervertebral joints)? Flexion
  • Abduction example? Spreading the fingers
  • Nodding the head? Flexion and Extension
  • Grasping upper lip with teeth (moving jaw forward)? Protraction
  • Move body upward? Elevation
  • Joint with greatest mobility? Shoulder

Muscle Fiber Composition and Attachments

Collagen fibers of the epimysium, perimysium, and endomysium come together at the ends of muscles to form:

  • A Tendon (bundle) or
  • An Aponeurosis (sheet)

These structures attach skeletal muscles to bones.

Muscle Tissue Components

  • Epimysium: Layer of collagen fibers that surrounds the entire muscle.
  • Fascicle: Muscle fibers bundled within perimysium.
  • Myosatellite Cells: Stem cells that assist in muscle repair.
  • Sarcolemma: Plasma membrane of a muscle fiber; surrounds the sarcoplasm.
  • Sarcoplasmic Reticulum: Specialized for storage and release of calcium ions.

Rotator Cuff and Muscle Fiber Types

  • Rotator Cuff Muscles: Shoulder muscles.
  • SITS Mnemonic Note: Teres minor is not one of the SITS muscles.
  • Fiber most resistant to fatigue: Slow fiber.