KA

Comprehensive Notes – Muscular System (Chapter 11)

Learning Objectives

  • Describe actions and roles of skeletal muscle agonists, antagonists, and synergists (prime movers, fixators)
  • Explain organisation of muscle fascicles, connective-tissue wrappings and their influence on force generation
  • Apply Latin/Greek naming rules to predict a muscle’s location, action, shape, or size
  • Identify major skeletal muscles of face, neck, torso, limbs and state origin, insertion and prime action
  • Connect individual muscle actions to normal activities (breathing, chewing, locomotion, gestures)

Fundamental Terminology

Points of Attachment

  • Origin – fixed or less-movable bony attachment
    • e.g. pelvis for rectus femoris
  • Insertion – movable bony attachment that is pulled toward the origin
    • e.g. femur / patella / tibia for rectus femoris via patellar tendon

Functional Roles

  • Prime Mover (Agonist)
    • Provides the mayor force for a specific movement
    • Example: Biceps brachii = elbow flexion; Triceps brachii = elbow extension
  • Antagonist
    • Performs opposite action of agonist, resists or brakes movement, stabilises joints
    • Maintains posture or limb position; controls rapid motions
  • Synergist / Fixator
    • “Syn” = together, “erg” = work
    • Assists prime mover, adds force, prevents unwanted movements
    • When it stabilises the origin it is often called a fixator

Connective-Tissue Organisation Around Fibres

  • Epimysium – dense irregular CT surrounding entire muscle
  • Perimysium – looser CT bundling fibres into fascicles
  • Endomysium – delicate CT investing each cell; houses capillaries + nerves

Fascicle Arrangements & Resultant Muscle Shapes

  • Arrangement determines line of pull, force production & ROM
    • Parallel – fibres run lengthwise (e.g. sartorius)
    • Fusiform – parallel but with swollen belly (e.g. biceps brachii)
    • Circular – sphincters, concentric rings (e.g. orbicularis oris)
    • Convergent / Triangular – broad origin → narrow insertion (e.g. pectoralis major)
    • Pennate (feather-shaped)
    • Uni- (extensor digitorum), Bi- (rectus femoris), Multi- (deltoid)

Muscle-Naming Toolbox (Mnemonic)

  • Directional prefixes: ad (toward), ab (away), sub (under), epi (on top), apo (to side)
  • Motion suffix: -ductor (something that moves)
  • Size descriptors: longissimus (longest), longus (long), brevis (short), maximus/medius/minimus (large/medium/small)
  • Fibre orientation: rectus (straight)
  • Number of heads: uni / bi / tri / quad = 1, 2, 3, 4 respectively
  • Location: externus (outside) vs internus (inside)
  • 5-step analysis strategy:
    1. Read the name
    2. Visualise the muscle – joint crossed, path, side
    3. Infer actions from that picture
    4. Memorise precise attachments
    5. Place muscle in larger functional group

Muscles of Facial Expression

  • Frontalis – raises eyebrows, wrinkles forehead
  • Orbicularis oculi – closes eye, squinting
  • Zygomaticus major/minor – elevate corners (smile)
  • Orbicularis oris – purses lips (whistle, kiss)
  • Buccinator – compresses cheeks (suck, blow)
  • Table of detailed targets / motions (slide shows occipitofrontalis, corrugator supercilii etc.)
    • Brow furrowing/unfurrowing, nostril flare (nasalis), smiling, pouting (mentalis) illustrated as prime movers + origins/insertions

Extra-ocular Muscles (Eye Movements)

  • Recti: Superior (up), Inferior (down), Medial (to midline), Lateral (away)
  • Obliques: Superior (down + out), Inferior (up + out)
  • Levator palpebrae – elevates upper eyelid

Mastication & Tongue

  • Jaw closers: Masseter (prime), Temporalis (synergist)
  • Pterygoids: medial (close + protract + grind), lateral (open + protract + grind)
  • Extrinsic tongue muscles: genioglossus, styloglossus, hyoglossus, palatoglossus (speech, chewing, swallowing)

Muscles of the Neck

  • Suprahyoids (digastric, stylohyoid, mylohyoid, geniohyoid) – elevate hyoid/larynx (swallowing)
  • Infrahyoids (sternohyoid, omohyoid, sternothyroid, thyrohyoid) – depress hyoid/larynx
  • Sternocleidomastoid – unilateral: rotate head opposite side; bilateral: neck flexion (“look down”)
  • Splenius capitis – unilateral: rotate same side; bilateral: extension (“look up”)

Torso Musculature

Anterior / Lateral Thorax & Abdomen

  • Serratus anterior – protract scapula (“boxer’s muscle”) & forced expiration accessory
  • Pectoralis major – arm flexion, adduction, medial rotation, assists forced expiration
  • Pectoralis minor – scapular protraction; with scapula fixed → elevates ribs for forced inspiration
  • Intercostals:
    • External – fibres run “hands in pockets”; elevate ribs for inspiration
    • Internal – fibres run opposite; depress ribs for expiration
  • Diaphragm – prime inspiratory muscle; contraction flattens dome creating negative intrathoracic pressure
  • Abdominal wall:
    • External oblique, Internal oblique, Transversus abdominis (compress abdomen, rotation, lateral flexion)
    • Rectus abdominis – flex vertebral column; Linea alba = midline CT seam

Posterior Torso / Back

  • Trapezius – scapular elevation, retraction; upper fibres produce shrug
  • Rhomboideus major & minor – retract scapula, synergise with trapezius
  • Latissimus dorsi – extension, adduction, medial rotation of humerus (rowing, swimming)
  • Erector spinae (iliocostalis, longissimus, spinalis) – spine extension, posture, forced inspiration support
  • Quadratus lumborum – lateral flexion of trunk, works with obliques

Pectoral Girdle Movers

  • Anterior rotation/protraction: pectoralis minor, serratus anterior
  • Posterior retraction/elevation: trapezius, rhomboids, levator scapulae

Muscles Moving the Humerus

  • Pectoralis major – flexion, adduction, medial rotation
  • Latissimus dorsi – extension, adduction, medial rotation; depresses scapula
  • Deltoid – abduction (middle fibres), flexion/medial rotation (anterior), extension/lateral rotation (posterior)

Rotator Cuff ("SITS")

  • Supraspinatus – initiates abduction (supraspinous fossa → greater tubercle)
  • Infraspinatus – lateral rotation (infraspinous fossa → greater tubercle)
  • Teres minor – lateral rotation (lateral scapula border → greater tubercle)
  • Subscapularis – medial rotation (subscapular fossa → lesser tubercle)

Forearm Movers

  • Biceps brachii – elbow flexion, supination; long & short heads from scapula → radial tuberosity
  • Brachialis – strong flexor, deep to biceps (humerus → coronoid ulna)
  • Brachioradialis – flexes in mid-pronation (humerus → radial styloid)
  • Triceps brachii – elbow extension (scapular & humeral origins → olecranon)
  • Pronator teres – pronates forearm (medial epicondyle + coronoid ulna → lateral radius)
  • Supinator – supinates (lateral epicondyle + ulna → proximal radius)

Hip & Thigh

Iliopsoas (Posterior Abdomen)

  • Iliacus (iliac fossa) + Psoas major (lumbar vertebrae) merge → lesser trochanter
  • Produce hip flexion & lateral rotation (bowing motion)

Gluteal Group

  • Gluteus maximus – hip extension & abduction
  • Gluteus medius & minimus – hip abduction, medial rotation; key for pelvic stability during gait

Anterior Compartment (Quadriceps)

  • Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius share common tendon → patella → tibial tuberosity
    • All extend knee; rectus femoris additionally flexes hip
  • Sartorius – flexes, abducts, laterally rotates hip; flexes knee (“tailor’s cross-leg”) (ilium → medial tibia)

Posterior Compartment (Hamstrings)

  • Biceps femoris (lateral), Semitendinosus, Semimembranosus (medial)
    • Originate ischial tuberosity → tibia/fibula; extend hip, flex knee; antagonise quadriceps

Medial Compartment

  • Gracilis – hip adduction (pubis/ischial ramus → medial tibia); aids knee flexion

Lower Leg / Foot

  • Tibialis anterior – dorsiflexion, inversion; permits heel-walking
  • Fibularis (Peroneus) longus – eversion, plantar flexion; maintains lateral arch
  • Gastrocnemius + Soleus → calcaneal tendon; plantar flexion (tip-toe, propulsion)

Breathing Muscles – Normal vs Forced

  • Normal inspiration: diaphragm \downarrow, external intercostals \uparrow ribs
  • Normal expiration: passive recoil when diaphragm relaxes
  • Forced inspiration: sternocleidomastoid, scalenes, pectoralis minor lift ribs
  • Forced expiration: abdominal wall muscles & internal intercostals push diaphragm up

Clinical & Functional Connections

  • Rotator cuff integrity essential to shoulder stability; common site of athletic injury
  • Erector spinae endurance crucial for maintaining lumbar curve, preventing low-back pain
  • Diaphragm dysfunction (e.g., phrenic nerve damage) severely impairs ventilation
  • Gluteus medius weakness → Trendelenburg gait, highlighting role in frontal-plane pelvic control
  • Balance between hamstrings & quadriceps prevents anterior cruciate ligament injuries

Study Strategy Reminders

  • Visualise joint crossed to infer movement (Step 2 of 5-step approach)
  • Memorise origins/insertions with mnemonics ("SITS" for cuff; “Boxer’s muscle” for serratus anterior)
  • Group muscles by compartment & shared action to build context

Numerical / Statistical Details (Embedded Examples)

  • Pectoralis major originates from costal cartilages 1\text{–}6 or 1\text{–}7 (variation)
  • Latissimus dorsi attaches to lower thoracic vertebrae T7\text{–}T12 and ribs 9\text{–}12
  • Quadriceps: four heads (biological example of “quad” =4)

Ethical / Practical Implications

  • Understanding muscle antagonism prevents overuse/imbalance injuries in training programmes
  • Knowledge of accessory breathing muscles guides respiratory therapy in COPD patients
  • Surgical repair of rotator cuff or ACL demands precise reattachment to restore original line of pull

Quick Reference – Prime Movers by Motion

  • Elbow flexion: biceps brachii
  • Elbow extension: triceps brachii
  • Shoulder abduction: deltoid, supraspinatus (initiation)
  • Hip extension: gluteus maximus, hamstrings
  • Knee extension: quadriceps
  • Plantar flexion: gastrocnemius & soleus
  • Dorsiflexion: tibialis anterior