6.1 Muscles of the Head, Neck, Trunk & Breathing

6.1 Muscles of the Head, Neck, Trunk & Breathing

The human muscular system contains over 600 distinct skeletal muscles, each executing coordinated mechanical actions that produce bodily movement, maintain posture, stabilize joints, and generate heat. To master human anatomy for entrance examinations and clinical practice, students must understand the foundational nomenclature governing muscle names, the collaborative functional dynamics of muscle groups, and the precise anatomical attachments and peripheral innervations of the axial skeleton.

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Principles of Skeletal Muscle Nomenclature

Rather than requiring arbitrary memorization, skeletal muscle names follow a logical anatomical naming system established by classical anatomists. The name of a muscle typically reveals one or more of its structural or functional characteristics:

1. Direction of Muscle Fascicles and Fibers: Muscle fiber orientation is described relative to an imaginary line, usually the anterior midline of the body or the longitudinal axis of a limb bone:

   • Rectus (straight/parallel): Fascicles run strictly parallel to the midline. Examples include the rectus abdominis (vertical abdominal strap) and the rectus femoris (straight muscle of the anterior thigh).
   • Transversus (perpendicular/at right angles): Fascicles run horizontally across the body axis. The prime example is the transversus abdominis of the deep lateral abdominal wall.
   • Oblique (slanted/diagonal): Fascicles run diagonally at an angle to the midline, such as the external oblique and internal oblique.

2. Relative Muscle Size:

   • Maximus (largest): Gluteus maximus (the largest muscle of the buttock).
   • Minimus (smallest): Gluteus minimus (the smallest and deepest of the primary gluteal muscles).
   • Longus (long): Fibularis (peroneus) longus or adductor longus.
   • Brevis (short): Adductor brevis or fibularis brevis.
   • Vastus (huge/great): Vastus lateralis and vastus medialis of the quadriceps femoris group.

3. Geometric Shape:

   • Deltoid: Triangular in outline, resembling the inverted Greek letter delta ($\Delta$).
   • Trapezius: Forms an expansive four-sided trapezoid across the upper back and posterior neck.
   • Rhomboid: Diamond-shaped or rhomboid (e.g., rhomboid major and rhomboid minor connecting the scapula to the vertebral column).
   • Serratus: Saw-toothed or serrated appearance along its costal origins (e.g., serratus anterior along the anterolateral ribs).
   • Orbicularis: Circular sphincter muscle encircling a body orifice (e.g., orbicularis oris encircling the mouth; orbicularis oculi encircling the eye orbit).

4. Primary Action:

   • Action verbs frequently form the initial word of a muscle's name, including flexor (decreases joint angle), extensor (increases joint angle), abductor (moves limb away from midline), adductor (draws limb toward midline), levator (elevates a structure), depressor (lowers a structure), supinator (rotates palm anteriorly/upward), and pronator (rotates palm posteriorly/downward). Examples include the flexor carpi radialis, adductor magnus, and levator scapulae.

5. Number of Origin Heads:

   • The Latin prefix specifies the number of separate tendons or heads of origin:
     • Biceps (two heads): Biceps brachii (anterior arm) and biceps femoris (posterior thigh).
     • Triceps (three heads): Triceps brachii (posterior arm).
     • Quadriceps (four heads): Quadriceps femoris (anterior thigh).

6. Anatomical Location:

   • Muscles named for the specific underlying bone or body region they overlie. Examples include the temporalis (overlying the temporal bone of the cranium), the frontalis (overlying the frontal bone), the tibialis anterior (overlying the anterior shaft of the tibia), and the intercostal muscles (residing between the ribs; Latin costa = rib).

7. Points of Origin and Insertion:

   • When named for their skeletal attachments, the site of origin (the fixed, immovable attachment) is universally listed first, followed by the site of insertion (the movable attachment point pulled during contraction). A prime example is the sternocleidomastoid: sterno refers to its origin on the manubrium of the sternum, cleido refers to its origin on the medial clavicle, and mastoid denotes its insertion into the mastoid process of the temporal bone.

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Functional Classifications: Agonists, Antagonists, Synergists & Fixators

Skeletal muscles never operate in complete isolation. Every bodily movement requires synchronized neuromuscular coordination among opposing and cooperative muscle groups, classified into four distinct functional categories:

1. Agonist (Prime Mover)

The agonist, or prime mover, is the muscle bearing primary mechanical responsibility for producing a specific movement. It generates the predominant contractile force required to mobilize the joint. For instance, the biceps brachii acts as a prime mover of forearm flexion at the elbow, and the pectoralis major serves as a prime mover of arm flexion at the glenohumeral joint.

2. Antagonist

An antagonist is a muscle that opposes, reverses, or decelerates the action executed by a prime mover. When an agonist undergoes active contraction, its corresponding antagonist is typically stretched, maintaining controlled passive tension to ensure smooth, metered movement and prevent joint dislocation or hyperextension. When the movement is reversed, the functional roles invert: the antagonist becomes the agonist. In elbow flexion, the posterior triceps brachii acts as the antagonist to the biceps brachii; during active elbow extension, the triceps brachii becomes the prime mover while the biceps brachii serves as the antagonist.

3. Synergist

A synergist is a muscle that assists the prime mover in executing its action. Synergists contribute in two distinct manners:

• They add additional contractile force to amplify the mechanical output of the prime mover (e.g., the brachialis and brachioradialis muscles assist the biceps brachii in flexing the elbow).
• They neutralize or prevent unwanted collateral motions at intervening or adjacent joints. For example, when making a forceful fist, wrist extensor muscles contract synergistically to prevent the deep finger flexors from inadvertently flexing the wrist at the same time, maximizing grip strength.

4. Fixator

A fixator is a specialized synergist that immobilizes a bone or anchors a prime mover's skeletal origin. By creating an immovable, stable osseous foundation, fixators enable the prime mover to pull with maximal mechanical efficiency. Classic anatomical fixators include the postural muscles that anchor the scapula against the thoracic cage—such as the trapezius, rhomboids, and serratus anterior—permitting arm muscles like the deltoid to mobilize the humerus without displacing the shoulder girdle.

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Muscles of Facial Expression & Cranial Nerve VII

The muscles of facial expression lie within the subcutaneous tissue of the scalp, face, and anterior neck. Unlike most skeletal muscles that articulate across synovial joints to move bones, facial expression muscles originate on the bones of the skull and insert directly into the superficial fascia and dermis of the skin. Consequently, their contractions pull directly on cutaneous tissue, creating facial wrinkles, folding the skin, and conveying human emotion.

All muscles of facial expression receive motor innervation from Cranial Nerve VII (the Facial nerve). Damage to CN VII produces unilateral facial muscle paralysis, known clinically as Bell's palsy.

Major Muscles of Facial Expression

• Frontalis (Epicranius Frontalis Belly): Overlies the frontal bone of the forehead; continuous posteriorly with the galea aponeurotica (epicranial aponeurosis). Its contraction elevates the eyebrows, produces horizontal forehead wrinkles, and registers expressions of surprise or inquisitiveness.
• Orbicularis Oculi: A circular sphincter muscle enclosing the orbit of the eye. It forms the muscular substrate of the eyelids. Contraction draws the eyelids together, executing blinking, gentle eye closure, squinting against bright sunlight, and forceful winking. Paralysis impairs corneal protection, leading to exposure keratitis.
• Orbicularis Oris: A complex circular sphincter muscle encircling the mouth within the lips. Termed the "kissing muscle," it closes the oral fissure, compresses the lips against the teeth, puckers the lips forward, and facilitates articulate speech.
• Zygomaticus Major and Minor: Paired diagonal muscle straps extending from the zygomatic bone (cheekbone) inferomedially to insert into the skin and angle of the mouth. Termed the "smiling muscle," the zygomaticus major pulls the corners of the mouth superiorly and laterally during laughter and smiling.
• Buccinator: The principal horizontal muscle of the cheek, originating from the alveolar processes of the maxilla and mandible and blending anteriorly into the orbicularis oris. It compresses the cheek inward against the molar teeth. During mastication (chewing), the buccinator keeps the food bolus positioned between the grinding surfaces of the teeth, preventing food from pocketing within the oral vestibule. It also provides the positive expiratory pressure required for whistling, sucking through a straw, and playing wind instruments ("trumpeter's muscle").
• Platysma: A thin, broad sheet-like superficial muscle spanning the anterolateral neck, originating from the fascia overlying the deltoid and pectoralis major and inserting into the lower border of the mandible and cutaneous tissue of the lower mouth. It tenses the skin of the neck during exertion, depresses the mandible, and draws the lower lip inferolaterally in expressions of fright, disgust, or melancholy.

Clinical Practice Pearl: Bell's Palsy vs. Central Stroke

A hallmark nursing assessment differentiates a peripheral Cranial Nerve VII lesion (Bell's palsy) from a central upper motor neuron deficit (cerebrovascular accident / stroke):

• In Bell's palsy, the entire unilateral half of the face is paralyzed, including the forehead. The patient cannot raise the ipsilateral eyebrow or wrinkle the forehead because the final peripheral nerve trunk is disrupted.
• In a central hemispheric stroke, the forehead is clinically spared (the patient can still wrinkle both sides of the forehead symmetrically) because the motor neurons supplying the upper face receive bilateral cortical hemispheric innervation, whereas the lower face receives only contralateral cortical input.

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Muscles of Mastication & Cranial Nerve V

The muscles of mastication consist of four paired muscles responsible for mobilizing the mandible at the temporomandibular joint (TMJ), generating the tremendous crushing forces required to masticate food. Unlike the muscles of facial expression, all muscles of mastication receive motor innervation from the mandibular division ($V_3$) of Cranial Nerve V (the Trigeminal nerve).

Major Muscles of Mastication

• Masseter: A thick, powerful rectangular muscle situated over the lateral surface of the ramus of the mandible. It originates from the zygomatic arch and inserts into the lateral surface of the angle and ramus of the mandible. The masseter is the prime mover of jaw closure, elevating the mandible forcefully during chewing.
• Temporalis: A broad, fan-shaped muscle filling the temporal fossa on the lateral aspect of the skull. It originates from the temporal lines and temporal fossa, passes deep to the zygomatic arch, and inserts into the coronoid process of the mandible. Its anterior vertical fibers powerfully elevate the mandible, while its posterior horizontal fibers retract (pull back) the protracted mandible.
• Medial Pterygoid: Situated on the deep medial surface of the mandibular ramus. Working synchronously with the superficial masseter, it forms a mechanical muscular sling around the mandibular angle to elevate the jaw and generate lateral excursion.
• Lateral Pterygoid: Positioned horizontally deep within the infratemporal fossa. Uniquely among masticatory muscles, the lateral pterygoid protracts (pushes forward) the mandible and depresses the jaw to open the mouth. Alternating unilateral contractions of the medial and lateral pterygoids produce the side-to-side grinding movements of the lower molars.

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Muscles of the Neck & Cervical Spine

The neck contains prominent musculature responsible for supporting the heavy cranium, executing multidirectional head motions, and providing landmark boundaries for vital vascular structures.

Sternocleidomastoid (SCM)

The sternocleidomastoid is a massive, two-headed diagonal strap muscle ascending across the anterolateral neck. It serves as the primary anatomical dividing line separating the anterior cervical triangle from the posterior cervical triangle.

• Anatomical Attachments: Originates via two distinct heads—a rounded tendon from the manubrium of the sternum (sternal head) and a fleshy band from the medial third of the clavicle (clavicular head). Both heads converge into a single thick belly that inserts onto the mastoid process of the temporal bone and the superior nuchal line of the occipital bone.
• Innervation: Motor supply is delivered by Cranial Nerve XI (the Accessory nerve), while sensory proprioception is carried by cervical spinal nerves C2 and C3.
• Actions:
  • Bilateral Contraction: Flexes the cervical vertebral column forward, bowing the head toward the chest, and prevents hyperextension of the neck during sudden acceleration.
  • Unilateral Contraction: Produces two distinct simultaneous actions: it laterally flexes (tilts) the head toward the ipsilateral (same) shoulder while rotating the cervical spine so that the chin and face turn toward the contralateral (opposite) shoulder.
• Clinical Relevance: In clinical practice, the SCM is a vital landmark. Palpation of the carotid artery pulse occurs immediately medial to the anterior border of the SCM. Insertion of a central venous catheter into the internal jugular vein utilizes the triangular apex formed between the sternal and clavicular heads of the SCM. Spasmodic or congenital shortening of the SCM produces torticollis (wryneck), locking the patient's head in a tilted and rotated posture.

Trapezius

The trapezius is a massive, superficial triangular muscle spanning the posterior neck and upper thoracic thorax. Right and left trapezius muscles form a broad diamond-shaped kite across the upper back.

• Attachments: Originates along the external occipital protuberance, ligamentum nuchae, and spinous processes of C7 through T12. It inserts onto the lateral third of the clavicle, the acromion process, and the spine of the scapula.
• Innervation: Motor supply is delivered by Cranial Nerve XI (the Accessory nerve), with sensory proprioception from C3-C4.
• Actions: The trapezius stabilizes, elevates (shrugging shoulders), depresses, adducts (retracts scapulae toward midline), and superiorly rotates the scapula during overhead arm abduction. When the scapulae are fixed by other muscles, bilateral contraction of the upper trapezius extends and hyperextends the head and neck.

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Muscles of Breathing: Quiet & Forced Respiration

Respiration requires continuous rhythmic alterations in thoracic cavity dimensions. Because the lungs contain no contractile muscle tissue of their own, changes in intrapulmonary volume and pressure are driven entirely by the skeletal muscles of the thoracic wall and floor, adhering strictly to Boyle's law ($P_1V_1 = P_2V_2$): an increase in volume decreases internal gas pressure, drawing air into the lungs; a decrease in volume increases gas pressure, expelling air outward.

+--------------------------------------------------------------------------+
|                       RESPIRATORY CYCLE MECHANICS                        |
|                                                                          |
|  QUIET INSPIRATION (Active):                                             |
|  - Diaphragm contracts (flattens downward)     -> Increases vertical     |
|                                                   thoracic dimension     |
|  - External intercostals contract (elevate     -> Increases lateral/AP   |
|    rib cage like bucket handles)                  thoracic dimension     |
|  - Thoracic volume increases -> Intrapulmonary pressure drops below      |
|    atmospheric pressure -> Air flows INWARD                              |
|                                                                          |
|  QUIET EXPIRATION (Passive):                                             |
|  - Diaphragm & external intercostals relax                               |
|  - Elastic recoil of lungs & thoracic wall     -> Thoracic volume        |
|                                                   decreases              |
|  - Intrapulmonary pressure rises above atmospheric -> Air flows OUTWARD  |
|                                                                          |
|  FORCED EXPIRATION (Active):                                             |
|  - Internal intercostals contract (depress ribs)                         |
|  - Abdominal wall muscles contract (push viscera superiorly into dome)   |
+--------------------------------------------------------------------------+

1. The Diaphragm: Primary Muscle of Inspiration

The diaphragm is a broad, dome-shaped musculotendinous sheet that separates the thoracic cavity from the abdominopelvic cavity. It accounts for approximately 75% of resting tidal volume expansion during quiet breathing.

• Attachments: Originates circumferentially from the xiphoid process, the inner surfaces of the lower six costal cartilages and ribs, and the lumbar vertebrae via muscular crura. Its muscle fibers converge centrally to insert into a strong, aponeurotic central tendon.
• Innervation: The diaphragm is innervated exclusively by the paired phrenic nerves, which arise from cervical spinal cord segments C3, C4, and C5. This critical anatomical relationship is recalled via the clinical aphorism: "C3, 4, 5 keep the diaphragm alive." Traumatic spinal cord transections superior to C3 result in immediate respiratory arrest requiring mechanical ventilation.
• Mechanical Action: In the relaxed state, the diaphragm bulges convexly superiorly into the thoracic cage. Upon neural stimulation, its radial muscle fibers contract, pulling the central tendon downward and flattening the dome. This downward descent increases the vertical height of the thoracic cavity, lowering intrathoracic pressure and drawing atmospheric air into the expanding alveoli. In quiet resting breathing, expiration is completely passive: the diaphragm relaxes, and the natural elastic recoil of pulmonary elastance drives the dome upward, expelling air.

2. External Intercostal Muscles (Inspiration)

The external intercostals comprise 11 pairs of superficial muscles residing in the intercostal spaces between adjacent ribs. Their fibers run obliquely downward and forward (inferomedially, mirroring the orientation of "hands in front coat pockets").

• Action: When they contract during inspiration, they pull the ribs upward and outward, pivoting them like bucket handles. This expands the thoracic cavity in both the anteroposterior and transverse (lateral) dimensions, assisting the diaphragm in lowering thoracic pressure.

3. Internal Intercostal Muscles (Forced Expiration)

The internal intercostals comprise 11 pairs of deep muscles lying immediately beneath the external intercostals. Their fibers run downward and backward (inferoposteriorly), oriented at exact right angles to the external intercostals.

• Action: While quiet resting expiration is entirely passive, forced active expiration (such as during vigorous exercise, coughing, sneezing, or blowing up a balloon) requires muscular contraction. The internal intercostals contract forcefully to pull the ribs downward and inward, actively depressing the rib cage and rapidly shrinking thoracic volume.

4. Accessory Muscles of Respiration

During respiratory distress (e.g., severe asthma, COPD exacerbation, acute respiratory failure), the body recruits accessory muscles to generate deeper pressure swings:

• Accessory Inspiratory Muscles: Sternocleidomastoid (elevates the sternum), scalenes (anterior, middle, and posterior scalenes elevate ribs 1 and 2), and pectoralis minor (elevates ribs 3-5).
• Accessory Expiratory Muscles: The abdominal wall muscles (rectus abdominis, external and internal obliques, transversus abdominis). Their violent contraction compresses the abdominal viscera upward against the inferior surface of the relaxed diaphragm, forcing the dome higher into the thorax to accelerate exhalation.

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Muscles of the Abdominal Wall & Posterior Trunk

The anterior and lateral abdominal walls possess no bony skeleton between the inferior margin of the rib cage and the superior rim of the pelvis. Instead, structural support, visceral protection, and movement are provided by four paired flat muscles arranged in distinct anatomical layers.

The Multilayered Abdominal Wall

Proceeding through the anterolateral abdominal wall from superficial to deep, the muscle layers are organized with alternating fiber directions, forming an exceptionally strong biological plywood architecture:

1. External Oblique (Superficial Layer): The largest and outermost lateral abdominal muscle. Its fibers run inferomedially (downward and inward). Originates from the external surfaces of the lower eight ribs (5-12); inserts into the anterior iliac crest and a broad anterior aponeurosis that fuses at the midline. Bilateral contraction flexes the vertebral column and compresses abdominal viscera; unilateral contraction rotates the trunk to the opposite side.
2. Internal Oblique (Intermediate Layer): Lies immediately deep to the external oblique. Its fibers run superomedially (upward and inward, perpendicular to external oblique fibers). Originates from the lumbar thoracolumbar fascia, iliac crest, and inguinal ligament; inserts into the costal cartilages of the lower ribs and the midline aponeurosis. Bilateral contraction flexes the spine; unilateral contraction rotates the trunk to the same (ipsilateral) side.
3. Transversus Abdominis (Deepest Layer): The innermost lateral muscle sheet. Its fibers run strictly horizontally (transversely) across the abdomen. Originates from the iliac crest, inguinal ligament, lumbar fascia, and lower costal cartilages; inserts into the midline aponeurosis. It does not produce spinal flexion or rotation; its sole, vital action is to compress the abdominal contents, stabilizing the lumbar spine and generating elevated intra-abdominal pressure.
4. Rectus Abdominis (Anterior Midline Strap): A pair of long, vertical strap muscles running along the entire length of the anterior abdominal wall, enclosed within the fibrous rectus sheath formed by the aponeuroses of the three lateral muscles. It originates from the pubic crest and pubic symphysis and inserts into the xiphoid process and costal cartilages of ribs 5-7. The muscle is divided longitudinally into three or four distinct segments by horizontal fibrous bands called tendinous intersections, which create the segmented "six-pack" contour in muscular individuals. The rectus abdominis is the prime mover of lumbar spine flexion (bringing the sternum toward the pubis, as in sit-ups) and compresses the abdomen.

The Linea Alba & Clinical Surgical Approaches

The linea alba ("white line") is a dense, fibrous collagenous band running vertically along the exact anterior midline from the xiphoid process down to the pubic symphysis. It is formed by the interlacing fibers of the aponeuroses of the external oblique, internal oblique, and transversus abdominis. Because the linea alba is composed of dense fibrous connective tissue devoid of major blood vessels or large somatic nerve trunks, it represents the classical preferred anatomical site for an exploratory midline abdominal laparotomy incision, minimizing intraoperative hemorrhage.

The Valsalva Maneuver

Simultaneous contraction of the abdominal wall musculature (particularly the transversus abdominis and rectus abdominis) against a closed glottis (preventing air from escaping the lungs) is known as the Valsalva maneuver. This action dramatically increases intra-abdominal pressure. It is physiologically required for defecation, micturition, parturition (childbirth), coughing, and stabilizing the lumbosacral spine when lifting heavy loads.

Posterior Trunk: Erector Spinae & Latissimus Dorsi

• Erector Spinae Group (Sacrospinalis): The primary intrinsic muscular column running longitudinally along the posterior vertebral column. It is organized into three parallel columns from lateral to medial (recalled by the clinical mnemonic "I Love Spaghetti"):
  1. Iliocostalis (lateral column, spanning ilium to ribs)
  2. Longissimus (intermediate column, spanning transverse processes)
  3. Spinalis (medial column, hugging the spinous processes)
  • Action: The erector spinae group serves as the prime mover of back extension. Working bilaterally, they erect and hyperextend the vertebral column, maintaining upright posture against the constant downward pull of gravity. Unilateral contraction laterally flexes the spine.
• Latissimus Dorsi: A broad, flat triangular muscle covering the lumbar and lower thoracic back. Originates from the spinous processes of T7–T12, the lumbar and sacral vertebrae (via the thoracolumbar fascia), the iliac crest, and the lower ribs, and inserts into the intertubercular sulcus of the humerus. Innervated by the thoracodorsal nerve, it is the prime mover of arm extension and a powerful arm adductor and medial rotator ("swimmer's muscle").

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Head, Neck, Trunk & Respiratory Muscles Reference Table