Laryngeal Structure and Function Study Notes

Laryngeal Cartilages and Joint Movements

• Thyroid Cartilage: The "Adam’s apple" is the laryngeal prominence of the thyroid cartilage.
• Movements:
  • The thyroid and cricoid cartilages can rock and glide relative to one another, primarily influencing vocal fold length and tension.
  • The arytenoid cartilages rock and glide to influence the abduction (opening) and adduction (closing) of the vocal folds.

Anatomy and Composition of the Vocal Folds

• Five-Layer Structure:
  • Epithelium: A thin, smooth outer layer determining shape.
  • Superficial Lamina Propria: A soft, gelatin-like layer with elastic fibers for stretch and recoil.
  • Intermediate Lamina Propria: Elastic fibers likened to soft rubber bands.
  • Deep Lamina Propria: Collagen fibers that provide stiff support and prohibit overstretching.
  • Muscle (Thyroarytenoid): The bulk of the vocal fold, acting like stiff rubber bands.
• Structural Divisions:
  • Body: Formed by the deep lamina propria and the thyroarytenoid muscle.
  • Cover: Formed by the epithelium, superficial lamina propria, and intermediate lamina propria.
  • Compliance: The epithelium and superficial lamina propria are the most compliant layers.
• Glottis: The space between the vocal folds.
  • The membranous part comprises the anterior $60\%$ of the vocal folds.
  • The 密cartilaginous part comprises the posterior $40\%$ of the vocal folds.

Laryngeal Musculature and Functions

• Intrinsic Muscles and Actions:
  • Cricothyroid: Lengthens and tenses vocal folds.
  • Thyroarytenoid: Shortens and tenses vocal folds; contraction decreases the distance between the thyroid and arytenoid cartilages.
  • Posterior Cricoarytenoid: The primary abductor (rocks arytenoids away from the midline).
  • Lateral Cricoarytenoid: An adductor (rocks arytenoids toward the midline); facilitates medial compression (squeezing force between vocal processes).
  • Arytenoid (Interarytenoid): Pulls arytenoids toward each other.
• Extrinsic and Supplementary Muscles: These muscles generally have one attachment outside the larynx or do not insert directly into it, influencing laryngeal position and stability.
• Aryepiglottic Muscle: Part of the oblique arytenoid; pulls the epiglottis backward and downward to cover the laryngeal opening.

Laryngeal Control Variables and Airway Resistance

• Airway Resistance:
  • Resistance increases if the airway narrows or if airflow increases.
  • Resistance decreases if the airway widens or if airflow decreases.
• Opposing Pressure Ranking (Highest to Lowest):
  1. Coughing vigorously.
  2. Speaking loudly.
  3. Singing softly.
  4. Whispering.
• Glottal Size by Activity:
  • Large Glottis: Inspiration, producing voiceless consonants (e.g., $/s/$).
  • Small/Medium Glottis: Whispering, breathy voice.
  • No Glottis: Bearing down/lifting.
• Stiffness: Stretching the vocal folds increases stiffness.
  • Thyromuscularis contraction increases lateral stiffness.
  • Thyrovocalis contraction increases medial stiffness.

Neural Innervation

• Cranial Nerve $X$ (Vagus Nerve): Provides primary motor innervation.
  • Recurrent Laryngeal Branch: Innervates all intrinsic muscles except the cricothyroid.
  • Superior Laryngeal Branch: Specifically innervates the cricothyroid muscle.

Principles of Vocal Fold Vibration

• Vibratory Cycle:
  • The acoustic voice source occurs when vocal folds move apart and the airway opens.
  • Vertical Phase Difference: Vocal folds typically come together at the bottom before the top.
  • Intraglottal Pressure: Average pressure is greater in a converging glottis (opening) than a diverging glottis (closing).
• Closing Phase Drivers: Movement toward the midline is caused by tissue recoil force and a decrease in intraglottal air pressure.

Fundamental Frequency ($F_0$) and Sound Pressure Level (SPL)

• Fundamental Frequency ($F_0$): The rate of vocal fold vibration; perceived as pitch.
  • Increases with increased vocal fold stiffness (driven by cricothyroid and thyroarytenoid muscles).
  • Decreases as effective vibrating mass increases (influenced by lateral cricoarytenoid action).
  • Vocal fold stiffness is the dominant mechanism for $F_0$ control.
• Sound Pressure Level (SPL): Perceived as loudness.
  • Influenced primarily by tracheal pressure.
• Interaction: The greatest range of $F_0$ is available at midrange SPL, and the greatest SPL range is available at midrange $F_0$.

Voice Registers

• Pulse: The lowest register (glottal fry), often occurring at the end of breath groups.
• Modal: The middle speaking register.
• Loft: The highest register (falsetto); characterized by little to no vertical phase difference.

Development, Age, and Sex

• Puberty: Divergence in $F_0$ occurs after age $10$ as male larynges grow larger and heavier.
• Gender Differences: Young women often exhibit a posterior glottal chink (gap) during phonation.
• Aging Effects:
  • Menopause typically decreases $F_0$ in females.
  • Advanced age usually sees an increase in $F_0$ for males (due to muscle atrophy) and a decrease for females.
• Misconceptions: Speaking after inhaling helium changes the resonant characteristics of the vocal tract, but does not cause the vocal folds to vibrate faster.