RK

CH 10 – The Muscular System

Module 10.1 – Functional Organization of the Muscular System

  • Scale & Significance
    • Muscular tissue constitutes ≈ 50\% of total body mass, more than any other organ system.
    • Roughly \sim700 individual skeletal muscles have been catalogued.
  • Diversity of Form & Performance
    • Muscles differ dramatically in size, shape, attachment sites, and physiological roles.
    • Mechanical performance is governed by two fundamental variables:
    • Arrangement of fibers (fascicle organization).
    • Mode of attachment to the skeleton (direct, indirect, aponeurotic, etc.).
  • Two Anatomical Divisions
    • Axial Muscles
    • Attach to and support/position the axial skeleton (skull, vertebral column, thoracic cage).
    • Critical for posture, head/neck motion, and ventilation mechanics.
    • Appendicular Muscles
    • Stabilize, move, and brace the limbs (pectoral & pelvic girdles + free limbs).
  • Tendinous Continuity
    • Tendons transmit the contractile force generated by muscle fibers to bones or other tissues, thereby producing movement.

Module 10.2 – Fascicle Organization & Leverage

  • Why Fascicle Orientation Matters
    • Dictates range of motion, speed of shortening, and tension capability.
    • Provides the engineer-like “trade-off” between force production and excursion distance.

1. Parallel Muscles

  • Architecture: fascicles run parallel to the long axis; possesses a thick central "belly".
  • Kinematics: entire muscle can shorten up to ≈ 30\% of resting length (because individual fibers shorten the same percentage).
  • Force Law: tension is ∝ total number of myofibrils in cross-section (CSA).
  • Example: biceps brachii (classic fusiform parallel muscle).

2. Convergent Muscles

  • Architecture: broad origin with fascicles converging on a single tendon/raphe.
  • Functional Flexibility: different subdivisions can be activated independently, allowing multi-vector pulls.
  • Trade-Off: sacrifices maximal pull on a single point compared with parallel muscles of the same size.
  • Example: pectoralis major.

3. Pennate Muscles ("feather-like")

  • General Features
    • Fascicles insert obliquely on a tendon; pull is at an angle relative to tendon line of action.
    • Because packing density is greater, they house more fibers & myofibrils than a parallel muscle of identical volume → higher peak tension.
    • Shorter fiber excursion; tendon moves a smaller distance per fiber shortening.
  • Sub-classes
    • Unipennate – fibers on one side only (e.g., extensor digitorum).
    • Bipennate – fibers on both sides of central tendon (e.g., rectus femoris).
    • Multipennate – tendon branches inside the muscle (e.g., deltoid).

4. Circular (Sphincter) Muscles

  • Architecture: concentric fascicle rings surrounding an orifice.
  • Action: contraction decreases lumen diameter, functioning as biological valves.
  • Example: orbicularis oris encircling the mouth.

Biomechanical Leverage (preview)

  • Each fascicle pattern interacts with the lever system of bones & joints to fine-tune speed vs. force; will be elaborated in kinesiology modules.

Module 10.3 – Muscle Naming Conventions

Functional Anatomy Vocabulary

  • Origin: relatively fixed, proximal attachment of a skeletal muscle.
  • Insertion: movable, distal attachment.
  • Action: the canonical movement produced when the muscle contracts concentrically.

Cooperative Muscle Roles

  • Agonist (Prime Mover) – main muscle producing a motion (e.g., biceps brachii for elbow flexion).
  • Synergist – assists the agonist either by adding extra force or by stabilizing the origin (e.g., brachioradialis during elbow flexion).
    • Fixator: specialized synergist that immobilizes another joint to provide stable base.
  • Antagonist – produces the opposite action (e.g., triceps brachii antagonizes elbow flexion but acts as agonist for extension).

Decoding Muscle Names

  • Names embed five data clusters:
    1. Region – e.g., abdominis, brachii, femoris.
    2. Position/Direction/Fascicle orientation – anterior, oblique, rectus, profundus, etc.
    3. Number of Origins (Heads) – biceps (2), triceps (3), quadriceps (4).
    4. Shape or Other Distinctive Form – deltoid (triangle), orbicularis (circle), serratus (saw-tooth).
    5. Action/Function – flexor, abductor, supinator, plus colorful historico-functional terms (sartorius = “tailor”, buccinator = “trumpeter”).
  • Size Descriptors: maximus, minimus, longus, brevis, magnus, major, minor, vastus help distinguish multiple muscles in one region.
  • Practical Tip: Learning these labels creates a mnemonic shorthand for anatomy, surgery, PT, & kinesiology.

Module 10.4 – Axial vs. Appendicular Muscles

  • Axial Group
    • Represents ≈ 60\% of all skeletal muscles.
    • Functions: posture, head/neck motion, vertebral alignment, thoracic volume modulation (breathing).
  • Appendicular Group
    • Remaining ≈ 40\%.
    • Functions: locomotion, manipulation, fine motor skills, load-bearing stabilization of limbs & girdles.
  • Visual Survey
    • Anterior & posterior views (Fig. 10.4) map axial (axial skeleton anchored) vs. appendicular (limb-associated) muscles; color coding in texts highlights distribution.

Integrative & Clinical Connections

  • Biomechanics: Fiber arrangement + lever class influences athletic performance (e.g., sprinters favor bipennate gastrocnemius for force, while dancers benefit from long parallel sartorius for flexibility).
  • Injury Patterns:
    • Pennate muscles are prone to tendinous micro-tears due to high force densities.
    • Circular muscles pathology (e.g., pyloric stenosis) disrupts visceral passage.
  • Rehabilitation: Understanding agonist–antagonist pairs guides PNF stretching & strength-balance protocols, reducing joint injury risk.
  • Surgical Landmarks: Terminology (rectus abdominis, linea alba) crucial for incision planning to minimize nerve/vascular damage.
  • Philosophical/Ethical Note: Mastery of muscular anatomy underpins responsible clinical practice—misidentification during injections or physical therapy can cause iatrogenic harm.

Quantitative Quick-Reference

  • Total muscles catalogued: \sim700
  • Muscle mass fraction: \approx50\% body weight.
  • Parallel muscle maximal shortening: 30\% of resting length.
  • Distribution: Axial 60\% vs. Appendicular 40\% of skeletal muscles.

Study Prompts & Mnemonics

  • “P → Parallel = Power per fiber low; Pennate = Packed for Peak power.”
  • “CIA” for cooperative roles: Contractor (Agonist), Inhibitor (Antagonist), Assistant (Synergist).
  • Shape clues: Deltoid = Greek Δ (delta); Trapezius = trapezoid; Serratus = “saw blade.”

Potential Exam Questions

  • Describe how the architecture of a bipennate muscle influences its force output relative to a parallel muscle of equal mass.
  • Given the name “flexor carpi ulnaris,” deduce region, action, and positional clues.
  • Explain why the triceps brachii is simultaneously an antagonist for elbow flexion and an agonist for elbow extension.