Physiology of Laryngeal Function: Respiration and Phonation

TL;DR

The larynx is a complex organ primarily responsible for breathing and voice production. During respiration, the vocal folds open to allow air to pass, and for phonation, they close and vibrate rapidly. Understanding how air pressure, vocal fold tension, and airflow interact is key to grasping both functions.

1. The Mental Model

Think of your larynx as a finely tuned valve at the top of your windpipe. It opens wide for effortless breathing and then precisely closes and vibrates to create all the sounds of your voice.

2. The Core Material

Your larynx plays two seemingly contradictory, but beautifully integrated, roles: respiration (breathing) and phonation (voice production). It's all about how your vocal folds move and interact with airflow.

Respiration

When you breathe, your vocal folds, which are located inside your larynx, move apart, or abduct. This creates an open pathway, or glottis, for air to flow in and out of your lungs. This abductory movement is controlled by specific laryngeal muscles, primarily the posterior cricoarytenoid muscles. When you inhale, these muscles contract, pulling the vocal folds open. When you exhale, they relax, and the vocal folds passively return to a more neutral position, but still open enough for air to pass freely.

Essentially, for breathing, your larynx acts like an open gate, minimizing resistance to airflow.

Phonation

To produce sound, your larynx acts more like a vibrating reed instrument. This is a much more active process.

1. Adduction: First, your vocal folds move together, or adduct, closing off the glottis. This is controlled by other laryngeal muscles like the lateral cricoarytenoid and interarytenoid muscles. They bring the vocal folds closer, increasing resistance to airflow.
2. Subglottal Pressure Build-up: As you exhale, air from your lungs pushes up against these closed vocal folds, building up subglottal pressure (pressure below the glottis).
3. Vocal Fold Vibration (Aerodynamic-Myoelastic Theory): When the subglottal pressure becomes strong enough, it pushes the vocal folds apart, releasing a puff of air. As this air rushes past, it creates a drop in pressure (due to the Bernoulli effect – faster-moving fluid has lower pressure), which, combined with the natural elasticity (myoelasticity) of the vocal folds, pulls them back together. This cycle of opening and closing happens incredibly fast, sometimes hundreds of times per second, creating sound waves that we perceive as voice.

The pitch of your voice is determined by the rate of vocal fold vibration. Tighter, thinner vocal folds vibrate faster, producing a higher pitch. Loudness is primarily related to the force of air pushing through. More air pressure means stronger puffs, leading to a louder sound.

graph TD
    A["Inspiration/Expiration Cycle"] --> B{Vocal Fold Position?};
    B -- "Open (Abducted)" --> C["Air Passes Freely (Respiration)"];
    B -- "Closed (Adducted)" --> D["Subglottal Pressure Builds"];
    D --> E{Pressure Overcomes Resistance?};
    E -- "Yes" --> F["Vocal Folds Blow Apart"];
    F --> G["Air Rushes Through"];
    G --> H["Bernoulli Effect & Elasticity Pull Vocal Folds Together"];
    H --> I{Cycle Repeats Rapidly?};
    I -- "Yes" --> F;
    I -- "No (Vocal Folds Stay Closed)" --> D;
    G --> J["Sound Produced (Phonation)"];

3. Worked Example

Let's consider how you might produce a sustained "Ah" sound.

1. You take a breath in, and your posterior cricoarytenoid muscles quickly abduct your vocal folds, opening your airway.
2. As you prepare to speak, your lateral cricoarytenoid and interarytenoid muscles contract, adducting your vocal folds, bringing them together at the midline.
3. You start to exhale from your lungs, building up subglottal pressure below your now-closed vocal folds.
4. When this pressure reaches a critical point, it forces your vocal folds open, releasing a small burst of air.
5. As the air rushes through this narrow opening, the pressure immediately above the vocal folds drops (Bernoulli effect). Combined with the natural elasticity and tension in the vocal folds, this low pressure literally sucks them back together.
6. This opening and closing cycle repeats hundreds of times per second. If your vocal folds are vibrating about 110 times per second, you'll produce a relatively low-pitched "Ah" sound. If they're vibrating 200 times per second, it'll be higher pitched.

4. Key Takeaways

• The larynx is crucial for both breathing (respiration) and voice production (phonation).
• For respiration, vocal folds abduct (open) to allow air passage.
• For phonation, vocal folds adduct (close) and then vibrate rapidly.
• Vocal fold vibration relies on a combination of air pressure and the elasticity of the folds (aerodynamic-myoelastic theory).
• Subglottal pressure is the force that initially pushes vocal folds apart during phonation.
• The Bernoulli effect helps pull the vocal folds back together after they've been pushed open.
• Pitch is determined by the rate of vocal fold vibration; loudness by the intensity of airflow.

Common Mistakes to Avoid

• Don't confuse abduction (opening) with adduction (closing).
• Don't think of vocal folds as actively opening and closing for each vibration cycle during phonation; it's a passive, aerodynamic-elastic process after the initial adduction.
• Don't neglect the role of elasticity in vocal fold vibration; it's not just air pressure.
• Don't overlook the importance of subglottal pressure as the driving force for phonation.

5. Now Try It

Sit comfortably and hum a sustained "Mmm" sound. While humming, try to consciously feel the vibration in your throat and the gentle outward flow of air. Now, stop humming and take a deep, silent breath. Notice the difference in sensation – the absence of vibration and the wider, less restricted airflow. Can you feel your vocal folds open and close slightly between these two actions? Pay attention to the muscles in your neck and throat as you transition between silent breathing and humming. What does success look like? You should be able to clearly distinguish the sensation of an open airway for breathing versus the vibrating vocal folds during humming, and perhaps even feel the subtle muscle adjustments required for each.