Skeletal Muscle Anatomy and Function Notes

10.1 Muscle Function and Classification

  • Skeletal Muscles: Arranged in opposing groups across the joints, allowing one group to reverse or modify the action of another.
  • Muscle Classification:
    • Prime Movers (Agonists): Main muscles responsible for a specific movement.
    • Antagonists: Muscles that oppose the action of the prime movers.
    • Synergists: Assist the prime movers in performing an action; may also include fixators that stabilize joints during movement.

10.2 Naming Skeletal Muscles

  • Muscles are named based on various criteria including:
    • Location: Where the muscle is found in the body.
    • Shape: Distinctive forms of muscles (e.g., trapezius).
    • Relative Size: Comparison of the muscle size (e.g., maximus, minimus).
    • Fiber Direction: Orientation of muscle fibers (e.g., rectus for straight fibers).
    • Number of Origins: Points from which a muscle originates (e.g., biceps has two origins).
    • Attachment Sites: Where the muscle attaches (origin/insertion).
    • Action: What movement the muscle primarily performs.

10.3 Fascicle Arrangements

  • Common Arrangement Patterns:
    • Circular: Muscle fibers arranged in concentric rings.
    • Convergent: Fibers converge towards a single tendon.
    • Parallel: Fibers run parallel to the long axis; these muscles shorten the most.
    • Fusiform: Spindle-shaped with a central belly.
    • Pennate: Fibers attach obliquely to a tendon; they are strong but shorten less.

10.4 Lever Systems in the Body

  • Lever Functions:
    • Lever: A bar (bone) that moves around a pivot (fulcrum).
    • Load and Effort: Application of force to move a load.
  • Mechanical Advantage vs. Disadvantage:
    • Mechanical Advantage: When effort is applied further from the fulcrum than the load—slow and strong action.
    • Mechanical Disadvantage: When effort is applied closer to the fulcrum—fast movement with a large degree of movement.
  • Types of Levers:
    • First-Class Levers: Can operate at both advantages or disadvantages (effort-fulcrum-load).
    • Second-Class Levers: Always operate at a mechanical advantage (fulcrum-load-effort).
    • Third-Class Levers: Always operate at a mechanical disadvantage (fulcrum-effort-load).
    • Majority of skeletal muscles operate in third-class lever systems.

10.5 Muscle Actions Based on Origin and Insertion

  1. Facial Muscles: Small muscles attached to skin, responsible for facial expressions such as closing eyes, opening mouth, smiling, etc.
  2. Mastication Muscles: Masseter and temporalis elevate the mandible; deep muscles aid in grinding movements.
  3. Neck Muscles: Facilitate swallowing and head movements.
  4. Thoracic Muscles: Diaphragm aids in breathing; thoracic muscles stabilize and move the scapula.
  5. Abdominal Muscles: Act as a girdle, supporting and protecting abdominal contents and aiding in trunk movement.
  6. Pelvic Floor Muscles: Support pelvic viscera, resist abdominal pressure, and control urination/defecation.
  7. Shoulder Muscles: Nine muscles act on the shoulder joint with four forming a stabilizing “rotator cuff.”
  8. Arm Muscles: Anterior muscles flex the forearm; posterior muscles extend it.
  9. Forearm Muscles: Move hand and fingers; anterior are mainly flexors while the posterior are extensors.
  10. Intrinsic Hand Muscles: Aid in precise finger movements, categorized into three groups: thenar, hypothenar, midpalmar.
  11. Hip and Knee Muscles: Act on thigh and leg; anterior and medial muscles flex/adduct, while posterior muscles extend/rotate the thigh.
  12. Leg Muscles: Anterior muscles are dorsiflexors, lateral muscles are plantar flexors and everters, and posterior muscles are also plantar flexors.
  13. Intrinsic Foot Muscles: Support foot arches and aid in toe movements, similar organization to intrinsic hand muscles.