8.1 Muscles of the Head, Face, Neck & Trunk

Muscles of the Head, Face, Neck & Trunk

Core Concept: The muscular architecture of the axial skeleton coordinates vital vegetative functions—including mastication, ventilation, and intra-abdominal pressure regulation—alongside social communication via facial expression and the mechanical stabilization of the craniocervical and vertebral columns. Mastery of these muscle groups requires detailed knowledge of their skeletal attachments, cranial and spinal nerve supplies, kinetic vectors, and their clinical presentation during injury or manual therapy palpation.

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1. Muscles of Facial Expression

Unlike standard skeletal muscles that span between bony levers, the muscles of facial expression originate from facial bones or superficial fascia and insert directly into the dermis of the skin or blend with neighboring muscle fibers. When they contract, they move the overlying integument, creating facial gestures and guarding cranial orifices (eyes, nose, mouth).

Cranial Innervation: The Facial Nerve (CN VII)

All muscles of facial expression develop embryologically from the second pharyngeal arch and are innervated by Cranial Nerve VII (Facial Nerve). After exiting the stylomastoid foramen, CN VII enters the parotid gland and divides into five terminal motor branches:

1. Temporal
2. Zygomatic
3. Buccal
4. Marginal mandibular
5. Cervical (Mnemonic: "To Zanzibar By Motor Car")

Detailed Facial Muscles

• Epicranius (Occipitofrontalis): A bipartite digastric muscle connected across the cranial vault by a broad fibrous sheet, the epicranial aponeurosis (galea aponeurotica).
  • Frontalis (frontal belly): Originates on the epicranial aponeurosis; inserts into the skin and subcutaneous tissue of the eyebrows and root of the nose. Action: Elevates the eyebrows, wrinkles the forehead horizontally (expressions of surprise, attention, or inquiry).
  • Occipitalis (occipital belly): Originates on the superior nuchal line of the occipital bone; inserts into the epicranial aponeurosis. Action: Retracts and tenses the scalp posteriorly.
• Orbicularis Oculi: A circular sphincter muscle encircling the orbit:
  • Palpebral part: Lies within the eyelids; executes gentle, involuntary closing (blinking, sleep).
  • Orbital part: Surrounds the orbital rim; executes forceful closure (squinting against bright sunlight or protecting the globe from mechanical impact).
• Corrugator Supercilii: Originates from the medial end of the superciliary arch of the frontal bone; inserts into the skin of the eyebrow. Action: Draws the eyebrow medially and downward, producing vertical vertical furrows above the glabella (frowning, concentration, or distress).
• Procerus: Originates on the nasal bone; inserts into the skin of the lower forehead between the eyebrows. Action: Depresses the medial eyebrows and creates transverse horizontal wrinkles over the bridge of the nose.
• Nasalis: Consists of a transverse part (compresses the nasal aperture) and an alar part (dilates the nostrils during deep inspiration or exertion).
• Orbicularis Oris: The complex circular sphincter of the oral cavity composed of interlacing fibers from multiple facial muscles. Action: Compresses, closes, and protrudes the lips ("kissing muscle"), essential for speech articulation and retaining food during chewing.
• Zygomaticus Major & Minor: Originate on the zygomatic bone and insert into the angle of the mouth (major) and upper lip (minor). Action: Draws the angle of the mouth upward and laterally (true smiling, laughing).
• Risorius: Originates on the parotid fascia; inserts into the skin at the oral modiolus. Action: Retracts the angle of the mouth laterally (insincere smile, grimace).
• Buccinator: Deep horizontal cheek muscle originating from the alveolar processes of the maxilla and mandible and the pterygomandibular raphe; inserts into the orbicularis oris at the angle of the mouth. Pierced by the parotid (Stensen's) duct. Action: Compresses the cheek against the molar teeth, preventing food accumulation in the oral vestibule during mastication; expels air forcefully ("trumpeter's muscle").
• Mentalis: Originates from the incisive fossa of the mandible; inserts into the skin of the chin. Action: Elevates and protrudes the lower lip, wrinkling the skin of the chin (pouting, doubt).
• Platysma: A broad, thin subcutaneous muscular sheet spanning the anterolateral neck. Originates from the fascia covering the upper pectoralis major and deltoid; ascends to insert into the inferior border of the mandible and lower facial skin. Innervated by the cervical branch of CN VII. Action: Tenses the skin of the anterior neck, depresses the mandible and lower lip (expressions of horror, terror, or acute exertion).

Clinical Pearl — Bell's Palsy vs. Central Stroke: Bell's palsy is an acute lower motor neuron lesion of CN VII, causing complete unilateral flaccid paralysis of all ipsilateral facial muscles, including the inability to close the eye (lagophthalmos) or wrinkle the forehead (paralysis of frontalis). In contrast, an upper motor neuron lesion (such as an ischemic stroke) preserves frontalis function on the affected lower side because the forehead motor nucleus receives bilateral cortical input.

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2. Muscles of Mastication

The four primary muscles of mastication execute movements of the temporomandibular joint (TMJ) to grind and crush food. All four develop from the first pharyngeal arch and are innervated by the mandibular division of the Trigeminal Nerve (CN V3).

Exam Trap: The lateral pterygoid is the only muscle of mastication that depresses (opens) the mandible. The masseter, temporalis, and medial pterygoid all elevate (close) the jaw. The lateral pterygoid also pulls the intra-articular fibrocartilaginous disc of the TMJ forward during opening; discoordination between its heads is a leading cause of painful TMJ clicking and subluxation.

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3. Muscles of the Neck

Cervical musculature stabilizes the heavy cranium on the cervical spine while permitting extensive multiplanar mobility.

Sternocleidomastoid (SCM)

• Anatomy: Prominent two-headed oblique muscle spanning the anterolateral neck.
  • Sternal Head: Rounded, tendinous origin on the anterior surface of the manubrium sterni.
  • Clavicular Head: Broad, fleshy origin on the superior surface of the medial third of the clavicle.
  • Insertion: Lateral surface of the mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occipital bone.
• Innervation: Motor supply from the Accessory Nerve (Cranial Nerve XI); sensory proprioceptive fibers from ventral rami of C2 and C3.
• Kinesiological Actions:
  • Bilateral Contraction: Flexes the cervical vertebral column (bringing chin toward sternum); when the cervical spine is extended, hyper-extends the upper head; elevates the sternum as an accessory muscle during forced inspiration.
  • Unilateral Contraction: Laterally flexes the head to the ipsilateral side (ear to shoulder) and rotates the head to the contralateral side (turns face toward the opposite shoulder).
• Clinical Relevance: Spasmodic contracture of the SCM produces torticollis (wry neck), fixing the head in lateral flexion and opposite rotation. The SCM physically separates the neck into the anterior cervical triangle and the posterior cervical triangle.

The Scalene Muscle Group

Composed of three paired muscles positioned deep in the lateral neck:

1. Anterior Scalene: Originates from transverse processes of C3–C6; inserts onto the scalene tubercle of the 1st rib.
2. Middle Scalene: Originates from transverse processes of C2–C7; inserts onto the superior surface of the 1st rib (posterior to subclavian artery groove).
3. Posterior Scalene: Originates from transverse processes of C5–C7; inserts onto the external border of the 2nd rib.

• Actions: Bilateral contraction flexes the neck; unilateral contraction produces ipsilateral lateral flexion; elevates the 1st and 2nd ribs as accessory muscles of inspiration.
• Clinical Trap — Interscalene Triangle & Thoracic Outlet Syndrome (TOS): The space between the anterior scalene, middle scalene, and the superior surface of the 1st rib forms the interscalene triangle. Passing through this narrow aperture are the trunks of the brachial plexus and the subclavian artery (the subclavian vein runs anterior to the anterior scalene). Spasm or hypertrophy of the scalenes compresses these neurovascular structures, producing numbness, paresthesia, and circulatory deficits down the upper limb.

Posterior & Deep Neck Musculature

• Splenius Capitis & Cervicis: Arise from the lower nuchal ligament and spinous processes of C7–T6; splenius capitis inserts on the mastoid process and superior nuchal line, while splenius cervicis inserts on C1–C3 transverse processes. Innervated by posterior rami of middle cervical nerves. Action: Bilateral cervical extension; unilateral ipsilateral lateral flexion and rotation.
• Suboccipital Muscle Group: Four paired deep muscles (Rectus capitis posterior major, Rectus capitis posterior minor, Obliquus capitis superior, Obliquus capitis inferior) bounding the suboccipital triangle. Innervated by the suboccipital nerve (posterior ramus of C1). They provide high-density proprioceptive feedback for head orientation and execute micro-adjustments at the atlanto-occipital and atlantoaxial joints. Hypertonicity here is a primary generator of cervicogenic headaches radiating in a "ram's horn" pattern over the calvarium.

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4. Muscles of the Thorax & Respiration

Anterior Chest & Pectoral Girdle Muscles

• Pectoralis Major: A thick, fan-shaped muscle forming the anterior axillary fold.
  • Clavicular Head: Medial half of the clavicle.
  • Sternocostal Head: Sternum, upper six costal cartilages, and external oblique aponeurosis.
  • Insertion: Crest of the greater tubercle (lateral lip of the bicipital groove) of the humerus.
  • Innervation: Medial and lateral pectoral nerves (C5–T1).
  • Actions: Powerful adduction and medial (internal) rotation of the humerus; clavicular head flexes the arm; sternocostal head extends the flexed arm.
• Pectoralis Minor: Originates on ribs 3, 4, and 5 near their costal cartilages; inserts into the coracoid process of the scapula. Innervated by the medial pectoral nerve (C8–T1). Action: Depresses and protracts the scapula; elevates ribs during forced inspiration when the scapula is fixed. Hypertonicity can entrap the axillary artery and brachial plexus beneath the coracoid process.
• Serratus Anterior: A broad muscular sheet originating as fleshy digitations from the lateral surfaces of the upper 8 to 9 ribs; wraps posteriorly around the thoracic cage to insert along the entire medial (vertebral) border of the costal scapular surface.
  • Innervation: Long Thoracic Nerve (roots C5, C6, C7), which courses vulnerable and exposed on the muscle's superficial lateral surface.
  • Actions: Prime mover of scapular protraction (pushing or punching motions); upwardly rotates the scapula to elevate the glenoid fossa for overhead arm elevation; anchors the scapula tightly against the thoracic wall.
  • Clinical Deviation: Damage to the long thoracic nerve (from surgical dissection, carrying heavy backpacks, or sports trauma) paralyzes the serratus anterior, resulting in winged scapula (scapula alata), where the medial border and inferior angle project prominently backward like an unseated wing.

Primary & Accessory Respiratory Musculature

Inspiration Mechanics: Diaphragm contracts/flattens + External Intercostals elevate ribs
Expiration Mechanics (Quiet): Passive elastic recoil of lungs and chest wall
Expiration Mechanics (Forced): Internal Intercostals depress ribs + Abdominal wall compresses viscera

• The Diaphragm: The primary muscle of inspiration, forming a dome-shaped musculotendinous septum separating the thoracic and abdominal cavities.
  • Attachments: Originates peripherally from the xiphoid process, lower six costal cartilages/ribs, and lumbar vertebrae via muscular crura; fibers converge into a central aponeurotic plate, the central tendon.
  • Apertures: T8 (Vena Cava aperture), T10 (Esophageal hiatus), T12 (Aortic hiatus) — (Mnemonic: "Voice (8) Every (10) Afternoon (12)").
  • Innervation: Phrenic Nerve (ventral rami of C3, C4, C5) — "C3, 4, 5 keep the diaphragm alive".
  • Mechanical Action: During contraction, the diaphragm flattens downward, expanding vertical thoracic volume and generating negative intrapleural pressure, which draws atmospheric air into the lungs.
• Intercostal Muscles:
  • External Intercostals (11 pairs): Fibers run obliquely down and forward ("hands in pockets") from the lower border of one rib to the upper border of the rib below. Action: Elevate the ribs during quiet and forced inspiration (bucket-handle and pump-handle expansion).
  • Internal Intercostals (11 pairs): Deep to externals; fibers run obliquely down and backward, perpendicular to external fibers. Action: Depress the ribs during active, forced expiration.

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5. Posterior Trunk & Spine Musculature

Superficial & Intermediate Back Muscles

• Trapezius: A large triangular muscle spanning the upper back and posterior neck, subdivided into three functional zones:
  • Origin: External occipital protuberance, superior nuchal line, nuchal ligament, and spinous processes of C7–T12.
  • Insertion: Lateral third of clavicle, acromion, and crest of the spine of the scapula.
  • Innervation: Accessory Nerve (CN XI) motor; C3–C4 sensory proprioception.
  • Actions: Upper fibers elevate and upwardly rotate the scapula; middle fibers retract (adduct) the scapula; lower fibers depress and assist in upward rotation of the scapula.
• Latissimus Dorsi: Broadest muscle of the back. Originates from spinous processes of T7–T12, thoracolumbar fascia, iliac crest, and lower 3–4 ribs; converges into a flat tendon that wraps around teres major to insert into the floor of the intertubercular (bicipital) groove of the humerus. Innervated by the thoracodorsal nerve (C6–C8). Action: Powerful humerus extension, adduction, and medial rotation ("swimmer's muscle").
• Rhomboids (Major & Minor): Originate from the nuchal ligament and spinous processes of C7–T5; insert into the medial border of the scapula from the spine to the inferior angle. Innervated by the dorsal scapular nerve (C5). Action: Retract (adduct) and downwardly rotate the scapula, bracing it firmly against the posterior thoracic cage.
• Levator Scapulae: Originates from transverse processes of C1–C4; inserts into the superior angle and upper medial border of the scapula. Innervated by C3–C4 cervical nerves and dorsal scapular nerve (C5). Action: Elevates the scapula and assists in downward rotation; unilaterally flexes the neck laterally.

Deep Back Muscles (Erector Spinae & Lumbar Stabilizers)

• Erector Spinae (Sacrospinalis): The principal extensor column of the vertebral column, organized into three longitudinal muscular columns positioned in the paravertebral gutters from lateral to medial:
  1. Iliocostalis (lateral column: lumborum, thoracis, cervicis)
  2. Longissimus (intermediate column: thoracis, cervicis, capitis)
  3. Spinalis (medial column: thoracis, cervicis, capitis) (Mnemonic: "I Love Spaghetti" = Iliocostalis, Longissimus, Spinalis)
  • Innervation: Posterior (dorsal) rami of regional spinal nerves.
  • Action: Bilateral contraction powerfully extends the vertebral column and maintains erect bipedal posture; unilateral contraction produces lateral flexion and rotation.
• Quadratus Lumborum (QL): A thick quadrilateral muscle of the posterior abdominal wall. Originates from the posterior iliac crest and iliolumbar ligament; inserts into the 12th rib and transverse processes of L1–L4. Innervated by T12 and L1–L4 ventral rami. Action: Unilaterally flexes the lumbar spine laterally and elevates the pelvis ("hip hiking"); bilaterally fixes and stabilizes the 12th rib during inspiration.

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6. The Abdominal Wall

The anterolateral abdominal wall is organized into four concentric, compressive muscular layers that enclose the peritoneal cavity:

Abdominal Wall Architecture (Superficial to Deep):
1. Rectus Abdominis (Anterior midline, divided by tendinous intersections)
2. External Oblique (Fibers run inferomedially: "hands in pockets")
3. Internal Oblique (Fibers run superomedially: perpendicular to external)
4. Transversus Abdominis (Fibers run horizontally: dynamic postural corset)

1. Rectus Abdominis: Paired longitudinal strap muscles separated at the anterior midline by the fibrous linea alba. Originates on the pubic crest and pubic symphysis; inserts into the xiphoid process and costal cartilages of ribs 5–7. Segmented by three to four transverse tendinous intersections (creating the "six-pack" appearance). Enclosed within the bilaminar rectus sheath. Action: Flexes the lumbar vertebral column, compresses abdominal contents, and tilts the pelvis posteriorly.
2. External Oblique: Most superficial lateral muscle; fibers run inferomedially ("hands in pockets"). Originates on the external surfaces of the lower eight ribs (ribs 5–12); inserts into the anterior iliac crest and a broad aponeurosis terminating at the linea alba and forming the inguinal ligament.
3. Internal Oblique: Intermediate lateral layer; fibers course superomedially, perpendicular to the external oblique. Originates from the thoracolumbar fascia, anterior two-thirds of the iliac crest, and lateral inguinal ligament; inserts into the inferior borders of ribs 10–12, linea alba, and pubic crest (via conjoint tendon).
4. Transversus Abdominis: Deepest lateral muscle; fibers run horizontally. Originates from the inner surfaces of lower costal cartilages 7–12, thoracolumbar fascia, iliac crest, and inguinal ligament; inserts into the linea alba and pubic crest. Action: Acts as a vital dynamic muscular corset, compressing abdominal viscera, elevating intra-abdominal pressure, and providing deep segmental stabilization to the lumbar spine.

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7. Manual Therapy Palpation, Draping Boundaries & Endangerment Zones

Safe clinical practice requires strict respect for anatomical boundaries and delicate neurovascular pathways:

• Anterior Cervical Triangle Endangerment Site: Bounded by the anterior border of the SCM, the inferior border of the mandible, and the anterior cervical midline. Houses the carotid sheath, containing the common carotid artery, internal jugular vein, and the vagus nerve (CN X), alongside the carotid sinus and carotid body. Clinical Directive: Direct, deep, or sustained pressure is strictly contraindicated. Bilateral palpation can trigger the carotid sinus reflex, inducing severe bradycardia, cerebral hypoperfusion, and syncope.
• Posterior Cervical Triangle: Bounded by the posterior border of the SCM, anterior border of trapezius, and middle third of the clavicle. The Accessory Nerve (CN XI) crosses obliquely within the investing fascia and is vulnerable to blunt compressive trauma. The trunks of the brachial plexus and subclavian artery emerge through the base.
• Draping Boundaries for Trunk Work:
  • Pectoral Region: Modesty draping must cover the breast tissue completely; for female clients, firm drape boundaries are maintained along the superior border of the clavicle and axillary margin, exposing only the subclavicular muscular fibers of pectoralis major.
  • Lumbosacral Region: The gluteal cleft and pelvic basin remain securely draped; the drape is tucked firmly along the anterior/superior crest of the ilium, exposing only the paraspinal and quadratus lumborum muscles down to the sacral base.

Additional Named Muscles in the Official List

The unit wording also uses triangularis for depressor anguli oris and may use older or variant names. Learn the modern name, the published synonym, position, and action.