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.