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Muscular System – Key Concepts & Muscles

General Concepts of the Skeletal Muscular System

  • The muscular system enables voluntary and involuntary movement, posture maintenance, heat production and joint stability.
  • Voluntary control is exemplified by activities like yoga (Slide 2), where conscious contraction/relaxation of multiple muscle groups allows complex body positions.
  • Basic interaction: skeletal muscle ↔ tendon ↔ bone creates a lever system converting chemical (ATP) to mechanical energy.
    • Large central portion = belly; connective extensions at each end = tendons (Slide 5).
  • Functional roles within a motion
    • Prime mover (agonist) – main force generator.
    • Antagonist – opposes the prime mover to fine-tune movement.
    • Synergist – assists prime mover, adds force or reduces unwanted movement (e.g., brachioradialis & brachialis helping biceps brachii, Slide 3).
    • Fixator – stabilises the origin of the prime mover.
  • Seven generic muscle shapes (Slide 4)
    • Fusiform, Parallel, Convergent, Pennate (uni-, bi-, multi-), Circular, Triangular, and Spiral; each shape influences force vs. range of motion trade-offs (pennate = high force/short ROM, parallel = long ROM/lower force).

Anatomical Orientation & Layering Principles

  • Superficial vs. Deep: surface muscles execute gross motion; deeper layers stabilise joints and provide fine control (Slide 6).
  • Anterior/Posterior views reveal different muscle layers; clinically important for incision planning & injection sites.

Naming Conventions (Slide 7)

  • Latin/Greek roots encode information on action, size, location, number of heads, direction of fibres, and shape.
    • Example: abductor digiti minimi
    • ab = away, duct = move, digiti = finger/toe, minimi = little → “moves little finger/toe away”.
    • Contrast: adductor digiti minimi (toward midline).
  • Practical application: decoding unfamiliar names during chart review or cadaveric dissection.

Muscles of Facial Expression (Slides 8–9)

  • Unique because they insert into dermal layers, not bone → skin displacement = expression.
  • Key prime movers & stereotypical actions
    • Occipitofrontalis – raises eyebrows (surprise) & retracts scalp.
    • Corrugator supercilii – lowers/medially draws eyebrows (frown).
    • Nasalis – flares nostrils (increased airflow).
    • Levator labii superioris / Depressor labii inferioris – elevate/depress lips (sad vs. snarl).
    • Zygomaticus major – pulls mouth corners upward (smile).
    • Orbicularis oris – puckers lips (speech, kissing).
    • Buccinator – compresses cheeks (sucking, blowing; important in infant feeding).
    • Mentalis – protrudes lower lip (pout).
    • Risorius – laterally tenses lips (grimace).
  • Clinical correlation: damage to CN VII (facial nerve) produces unilateral paralysis (Bell’s palsy), impacting these muscles.

Extrinsic Eye Muscles (Slide 10)

  • Six muscles originate on the orbit and insert on the sclera, enabling precise ocular tracking.
    • Superior, Inferior, Medial, Lateral rectus – primary cardinal movements.
    • Superior & Inferior oblique – torsional and combined vertical movements.
  • Controlled by CN III, IV, VI; dysfunction yields diplopia or strabismus.

Muscles of Mastication (Slide 11)

  • High leverage from skull processes; prime movers include masseter, temporalis, medial & lateral pterygoids (not all pictured in transcript).
  • Provide strong bite forces, critical for food processing; innervated by CN V3 (mandibular branch).

Tongue, Swallowing & Speech (Slides 12–13)

Extrinsic Tongue Muscles

  • Genioglossus – protrudes & depresses tongue; airway patency during sleep (sleep apnoea relevance).
  • Styloglossus – retracts & elevates.
  • Hyoglossus – depresses.
  • Palatoglossus – elevates posterior tongue during swallowing (only tongue muscle innervated by CN X instead of CN XII).

Suprahyoid Group (above hyoid)

  • Digastric, Stylohyoid, Mylohyoid, Geniohyoid – elevate hyoid/larynx → epiglottis closure, mouth opening.

Infrahyoid Group (below hyoid)

  • Omohyoid, Sternohyoid, Sternothyroid, Thyrohyoid – depress hyoid/larynx → vocal pitch modulation.

Cervical Flexors/Rotators

  • Sternocleidomastoid, Scalene, Semispinalis/Splenius capitis – nodding, rotation; hypertonicity → tension headaches.

Neck & Back Musculature (Slides 14–16)

  • Posterior chain (trapezius, erector spinae, multifidus) maintains erect posture; weakness leads to kyphosis.
  • Deep neck stabilisers critical for cervical spine alignment; injury common in whiplash.

Abdominal Wall (Slide 17)

  • Rectus abdominis (listed as “rectus femoris” in slide – likely erratum) → trunk flexion, abdominal press.
  • Lateral layers (superficial → deep): External oblique → Internal oblique → Transversus abdominis.
    • Fibre orientation follows the mnemonic “Hands-in-pockets” (external oblique) vs. “Hands-on-chest” (internal).
  • Linea alba – avascular midline raphe; surgeon’s entry to minimise bleeding.
  • Posterior abdominal/lumbar muscles (quadratus lumborum, psoas) bridge spine & lower limb, aiding in hip flexion and spine stability.

Major Respiratory Muscles

  • Diaphragm (Slide 18)
    • Dome-shaped; separates thoracic and abdominal cavities.
    • Openings: caval opening (T8), esophageal hiatus (T10), aortic hiatus (T12).
    • Contraction → thoracic volume ↑, inspiration.
  • Intercostals (Slide 19)
    • External – elevate ribs (inspiration).
    • Internal – depress ribs (forced expiration).
    • Innermost – synergistic with internal; protect neurovascular bundle.

Pelvic Floor & Perineum (Slides 20–22)

  • Functions: organ support, continence, childbirth, sexual function.
  • Levator ani (pubococcygeus & iliococcygeus) – core elevator resisting intra-abdominal pressure.
  • Coccygeus (not emphasised in transcript) – posterior support.
  • Superficial perineal muscles: superficial transverse perineal, bulbospongiosus, ischiocavernosus – erection maintenance, ejaculation, vaginal compression.
  • Deep perineal muscles: external urethral & anal sphincters – voluntary continence.
  • Clinical: childbirth or prostate surgery may injure these muscles → incontinence.

Pectoral Girdle Stabilisation (Slide 23)

  • Serratus anterior, Rhomboids, Levator scapulae, Pectoralis minor, Trapezius anchor scapula/clavicle, forming a stable base for arm movement.
    • Weak serratus anterior → winged scapula (long thoracic nerve palsy).

Muscles Moving the Humerus (Slides 24–25)

  • Pectoralis major – flexes/adducts/medially rotates humerus (upper-cut motion).
  • Latissimus dorsi – extends/adducts/medially rotates (“swimmer’s pull”).
  • Deltoid – primary abductor; anterior fibers flex/medially rotate, posterior extend/laterally rotate.
  • Rotator cuff (SITS) – Supraspinatus, Infraspinatus, Teres minor, Subscapularis — stabilise glenohumeral joint; injuries common in overhead athletes.
  • Teres major & Coracobrachialis – synergistic adduction/flexion roles.

Elbow & Forearm Movers (Slides 26–27)

Flexors

  • Biceps brachii – flexion & supination (turn palm up while curling).
  • Brachialis – pure flexor (regardless of pronation/supination).
  • Brachioradialis – “beer-raising” muscle, stabilises elbow during rapid flexion.

Extensors

  • Triceps brachii – primary elbow extension (punching).
  • Anconeus – assists & abducts ulna during pronation.

Pronators/Supinators

  • Pronator teres & Pronator quadratus – palm-down rotation.
  • Supinator – palm-up rotation; partner with biceps.

Wrist & Hand (Slides 27–29)

Anterior (flexor) Compartment

  • Flexor carpi radialis / ulnaris – wrist flexion with radial or ulnar deviation.
  • Palmaris longus – weak flexor; absent in
    \approx 15\% of population; tendon used for grafts.
  • Flexor digitorum superficialis/profundus, Flexor pollicis longus – finger/thumb flexion; profundus produces “clinched fist”.

Posterior (extensor) Compartment

  • Extensor carpi radialis longus/brevis, Extensor carpi ulnaris – wrist extension/deviation.
  • Extensor digitorum, Extensor digiti minimi, Extensor indicis – digit extension.
  • Abductor pollicis longus, Extensor pollicis brevis/longus – thumb abduction/extension (forming the anatomical ‘snuff box’).

Intrinsic Hand Muscles

  • Thenar & hypothenar groups, lumbricals, interossei enable fine motor skills (e.g., typing, surgical suturing).

Hip & Thigh (Slides 30–32)

Iliopsoas Group

  • Psoas major & Iliacus (Iliopsoas) – strongest hip flexors; posture maintenance.

Gluteal Group

  • Gluteus maximus – powerful hip extension (climbing stairs).
  • Gluteus medius/minimus & Tensor fascia lata – hip abduction, pelvic stabilisation during gait; weakness → Trendelenburg gait.

Lateral Rotators

  • Piriformis, Obturators, Gemelli, Quadratus femoris – externally rotate femur; sciatic nerve may pierce piriformis (piriformis syndrome).

Adductors & Medial Thigh

  • Adductor longus/brevis/magnus, Pectineus, Gracilis – hip adduction; groin pulls common in sports.

Quadriceps (Anterior Compartment)

  • Rectus femoris, Vastus lateralis/medialis/intermedius – knee extension; rectus also flexes hip.

Sartorius

  • Longest muscle; “tailor’s position” (flexes, abducts, laterally rotates hip, flexes knee).

Hamstrings (Posterior Compartment)

  • Biceps femoris, Semitendinosus, Semimembranosus – knee flexion & hip extension; susceptible to strain in sprinting.

Leg & Foot (Slides 33–36)

Anterior Leg

  • Tibialis anterior – dorsiflexion & inversion; overuse → shin splints.
  • Extensor hallucis longus, Extensor digitorum longus – toe extension.

Lateral Leg

  • Fibularis (peroneus) longus/brevis – eversion & plantar flexion; stabilise ankle.

Posterior Leg

  • Superficial: Gastrocnemius, Soleus, Plantaris attach via calcaneal (Achilles) tendon (strongest in body) → plantar flexion, propulsion in gait.
  • Deep: Tibialis posterior, Flexor digitorum longus, Flexor hallucis longus, Popliteus – inversion, toe flexion, knee unlocking.

Intrinsic Foot

  • Dorsal: Extensor digitorum brevis – toe extension.
  • Plantar four layers:
    1. Abductor hallucis, Flexor digitorum brevis, Abductor digiti minimi – maintain arch, initial toe flex/abduction.
    2. Quadratus plantae, Lumbricals – adjust pull of flexor tendons, toe flex/ext interplay.
    3. Flexor hallucis brevis, Adductor hallucis, Flexor digiti minimi brevis – great & little toe control.
    4. Dorsal & Plantar interossei – toe ab/adduction (DAB/PAD rule) & fine adjustments during balance.
  • Plantar musculature forms dynamic arches combating body weight, essential for shock absorption.

Clinical & Practical Implications

  • Balanced strength across agonist/antagonist pairs prevents joint instability and overuse injuries.
  • Muscular naming literacy accelerates learning across systems (cardiology: adductor canal, etc.).
  • Understanding layered anatomy assists surgeons in minimally invasive approaches and informs physiotherapists in targeted rehabilitation.
  • Ethical aspect: knowledge of musculoskeletal mechanics underpins ergonomic design, reducing occupational health issues.

Numerical & Statistical References

  • 7 canonical skeletal muscle shapes.
  • Population variance: \approx 15\% lack palmaris longus.
  • Diaphragmatic apertures correspond to thoracic vertebral levels T8,\ T10,\ T12 (caval, esophageal, aortic).

Foundational Principles Revisited

  • Sliding filament theory explains contraction universally across listed muscles, tying cell biology (actin–myosin interaction) to gross movement.
  • Lever classes (I, II, III) in biomechanics correspond to different muscle–bone arrangements (e.g., triceps = class I, biceps = class III).
  • Neuromuscular control via motor unit recruitment clarifies why fine facial expressions require many small motor units vs. powerful gastrocnemius units.