Ch. 3 Laryngeal Dissection atlas

BASIC LARYNGEAL ANATOMY

  • The larynx is composed of various anatomical landmarks crucial for the procedures in both in vivo and ex vivo settings.

  • Key anatomical structures:

    • Epiglottis (A)

    • Hyoid Bone (B)

    • Thyroid Notch (C)

    • Thyroid Cartilage (D)

    • Cricothyroid Muscle (E)

    • Cricoid Cartilage (F)

    • Trachea (G)

VOCAL FOLD PHYSIOLOGY

Contribution of Laryngeal Muscles

  • A study using fine-wire electromyography on human larynges revealed that:

    • Intrinsic Laryngeal Muscles:

    • Highly specialized for distinct actions.

    • Controlled by the timing of contraction and recruitment.

    • Exhibit a fading effect during phonation.

    • Thyroarytenoid and Lateral Cricoarytenoid Muscles:

    • Display burst-like activity at the onset of phonation.

    • Exhibit measurable fade during sustained phonation.

    • Interarytenoid Muscle:

    • Shows increased latency of contraction.

    • Maintains regular muscle tonicity during prolonged sound production.

    • Cricothyroid Muscle:

    • Exhibits the greatest measurable action with increases in pitch and volume.

    • Posterior Cricoarytenoid Muscle:

    • Activated mainly during voluntary deep inhalation and sniffing.

Process of Phonation

  • Phonation is a complex mechanism involving:

    • Brainstem reflexes.

    • Muscular actions of intrinsic laryngeal muscles.

    • Higher-level cortical control.

    • Accessory factors:

    • Lung capacity.

    • Chest wall compliance.

    • Pharyngeal, nasal, and oral anatomy.

    • Mental status.

  • Sequence of events:

    1. Inhalation occurs, leading to glottal closure.

    2. Subglottic pressure rises until it overcomes glottal closure force, allowing air to escape between vocal folds.

    3. The myoelastic-aerodynamic concept of phonation is initiated.

VOCAL FOLD MICROSTRUCTURE

Composition of Vocal Folds

  • Vocal folds consist of distinct layers essential for phonation:

    • Epithelial Layer:

    • Three types of epithelial cells cover the intra-luminal laryngeal surface:

      • Nonkeratinized stratified squamous epithelium above and below the vocal fold.

      • Transitional columnar epithelium in zones separating different epithelial types.

    • Thickness of vocal fold epithelium averages 0.05 mm and extends 4.1 mm vertically.

    • Basal cell layer connects to the superficial lamina propria via a basement membrane with type III and VII collagen fibers.

    • Superficial Lamina Propria (SLP):

    • Corresponds to Reinke’s space.

    • Thickness ranges from 1 to 2 mm and contains loosely interwoven elastic and collagenous fibers.

    • Contributes to vibratory characteristics of the true vocal fold, allowing entrained vibration between frequencies less than 100 Hz to over 1000 Hz depending on vocal fold thickness and tension.

    • Intermediate Layer:

    • Lies deep to the SLP, composed mainly of branching elastic fibers measuring 0.5 to 1.5 mm in diameter.

    • Sparse collagenous fibers present, but elastic fibers dominate this layer.

    • Deep Layer:

    • Composed of densely packed collagenous fibers twisted into bundles parallel to vocal muscle edges.

    • Vocal ligament formed by intermediate and deep layers has a mean thickness of 0.8 mm.

    • Provides support to the overlying SLP without effective vibratory characteristics.

Functional Groups of Vocal Folds

  • The vocal fold structures can be conceptualized in three functional layers:

    1. Cover:

    • Includes the epithelial layer and superficial lamina propria.

    1. Transition:

    • Comprises intermediate and deep layers of the lamina propria.

    1. Body:

    • Consists of the vocalis muscle.

Clinical Relevance

  • Anatomic understanding is crucial because different pathologies affect distinct layers:

    • Lesions near the basement membrane (e.g., nodules and polyps) require surgical approaches avoiding damage to superficial or deeper layers to minimize scar formation.

    • Preserving mucosal waves and oscillation integrity is essential for optimal phonatory function.

REFERENCES

  • Hillel AD. (2001). The study of laryngeal muscle activity in normal human subjects and in patients with laryngeal dystonia using multiple fine-wire electromyography. Laryngoscope, 111:1–47.

  • Hirano M. (1974). Morphological structure of the vocal cord as a vibrator and its variations. Folia Phoniatr (Basel), 26:89–94.