Vocal cords

What is Vocal cords?

Vocal cords, also known as vocal folds, are pairs of flexible, muscle-covered tissue bands located within the larynx.^[8] They are primarily composed of muscle fibers, connective tissue, and a mucous membrane that allows them to vibrate and produce sound. The vocal cords are classified into true vocal cords and false vocal cords (vestibular folds). The true vocal cords are directly involved in sound production, while the false vocal cords provide protection and support but do not contribute to phonation. These structures are highly elastic and can stretch or relax to produce different pitches of sound. They are covered by a thin layer of epithelium and supported by the vocal ligament.^[7] The vibration of the vocal cords occurs when air passes through them, producing sound waves that are modified by the throat, mouth, and nose.^[2] Their ability to open and close is vital not only for speech but also for breathing and swallowing.

Location

The vocal cords are located inside the larynx (voice box) at the level of the thyroid cartilage, near the Adam’s apple in the neck.^[5] They are situated horizontally across the glottis, which is the space between them. The larynx itself is positioned between the pharynx (above) and the trachea (below).

Anatomy

Structure and Composition

The vocal cords, also called vocal folds, consist of two pairs of folds within the larynx. The true vocal cords are involved in sound production, while the false vocal cords (vestibular folds) primarily provide protection.^[4] The true vocal cords are composed of multiple layers, allowing them to vibrate and produce sound.

Layers of the True Vocal Cords

The true vocal cords have five distinct layers:

1. Epithelium – The outermost layer is covered by stratified squamous epithelium, providing protection against mechanical stress and maintaining hydration.^[6]
2. Superficial Lamina Propria (Reinke’s Space) – This layer is loose and gelatinous, enabling the cords to vibrate freely during phonation.
3. Intermediate Lamina Propria – Composed of elastic fibers, it provides flexibility and elasticity.
4. Deep Lamina Propria – Made up of collagen fibers, it offers structural support and tensile strength.
5. Vocalis Muscle (Thyroarytenoid Muscle) – The innermost layer consists of muscle fibers responsible for tension and length adjustments of the vocal cords.^[3]

Together, the intermediate and deep layers form the vocal ligament, which supports the outer layers during vibration.

False Vocal Cords

Located above the true vocal cords, the false vocal cords consist of mucosal folds with underlying fibrous and adipose tissue. They are covered by pseudostratified ciliated columnar epithelium, which produces mucus to keep the larynx lubricated. These folds are not directly involved in phonation but assist in protecting the airway.

Attachments

The vocal cords are attached at two key points:

• Anterior Attachment – Fixed at the thyroid cartilage, specifically at the angle of the thyroid lamina.
• Posterior Attachment – Connected to the vocal process of the arytenoid cartilages, which allow them to open and close via muscle movement.

Blood Supply

The vocal cords receive blood from the superior laryngeal artery and inferior laryngeal artery, branches of the superior thyroid artery and inferior thyroid artery, respectively.^[1] Venous drainage occurs through the laryngeal veins, which empty into the internal jugular vein.

Nerve Supply

The vocal cords are innervated by branches of the vagus nerve (Cranial Nerve X):

• Recurrent Laryngeal Nerve – Provides motor control to all intrinsic muscles of the larynx, except the cricothyroid muscle.
• External Branch of Superior Laryngeal Nerve – Controls the cricothyroid muscle, regulating tension and pitch.
• Internal Branch of Superior Laryngeal Nerve – Supplies sensory innervation above the vocal cords, including the mucosa of the larynx.

Lymphatic Drainage

The lymphatic drainage of the vocal cords is unique due to the poor lymphatic network within the vocal folds, making them less prone to metastasis in early cancers. Lymphatic vessels primarily drain into the deep cervical lymph nodes located along the internal jugular vein.

Microscopic Anatomy

Under a microscope, the vocal cords show a stratified squamous epithelium on the vibratory surface to withstand mechanical stress. Beneath this, the lamina propria layers provide elasticity, while the thyroarytenoid muscle ensures movement and control. These structural adaptations allow the vocal cords to endure constant vibration, adjust to tension, and recover quickly from mechanical stresses.

Function

Sound Production (Phonation)

The primary function of the vocal cords is phonation, which involves the generation of sound. During speech or vocalization, the vocal cords come together (adduct) and vibrate as air passes through them from the lungs. This vibration creates sound waves, which are then modified by the pharynx, mouth, and nasal cavities to form speech.

• Pitch Control – The tension and length of the vocal cords are adjusted by the cricothyroid muscle to produce different pitches. Tightened cords create higher pitches, while relaxed cords produce lower pitches.
• Volume Control – The force of air passing through the vocal cords determines the loudness of the sound. Stronger airflow results in louder sounds.
• Tone and Resonance – The quality of sound is influenced by the size and shape of the vocal cords, as well as the resonance provided by surrounding anatomical structures.

Airway Protection

The vocal cords serve as a protective barrier for the lower respiratory tract. They prevent food, liquids, and foreign particles from entering the trachea and lungs. This protection is achieved through:

• Adduction During Swallowing – The vocal cords close tightly during swallowing to block the airway.
• Cough Reflex Activation – If particles or irritants enter the larynx, the vocal cords trigger a cough reflex to expel them, preventing aspiration.
• Closure During Straining – The vocal cords close completely during activities like lifting heavy objects or defecation (Valsalva maneuver) to stabilize the thoracic cavity and support intra-abdominal pressure.

Breathing Regulation

During breathing, the vocal cords remain open (abducted) to allow the free flow of air into and out of the lungs. The degree of opening can adjust based on the body’s oxygen demand, such as during exercise or rest.

• Widening for Deep Breathing – The cords spread further apart during forceful inhalation to increase airflow.
• Narrowing for Controlled Breathing – The cords partially close during activities like whispering or speaking softly to control airflow and pressure.

Control of Airflow and Pressure

The vocal cords regulate air pressure within the respiratory system. By opening and closing in different patterns, they help:

• Maintain Subglottic Pressure – Essential for coughing, sneezing, and speaking.
• Pressure Modulation – Necessary for activities like singing or playing wind instruments, where airflow precision is critical.

Pitch Modulation

Pitch modulation is achieved through changes in the length and tension of the vocal cords. The cricothyroid muscle stretches the cords to produce high-pitched sounds, while the thyroarytenoid muscle relaxes them to create lower-pitched sounds. This ability allows for a wide range of vocal expression, including singing and emotional tones.

Lubrication and Humidification

The vocal cords contribute to humidifying and lubricating the air that passes through them. Their surface is lined with mucus-secreting glands that keep them moist and reduce friction during vibration. This lubrication prevents damage caused by repeated movements and exposure to air.

Sensory Feedback and Reflex Actions

The vocal cords contain sensory receptors that detect irritation, mechanical pressure, or foreign particles. These receptors are part of the vagus nerve reflex arc, enabling immediate responses such as:

• Closure to Prevent Aspiration – Immediate closure in response to irritation.
• Cough Reflex – Rapid expulsion of irritants from the airway.
• Gag Reflex – Protects against accidental swallowing of harmful substances.

Emotional Expression

The vocal cords are essential for non-verbal communication and emotional expression. Variations in pitch, tone, and volume convey emotions such as anger, sadness, or excitement, adding depth to verbal communication.

Control of Intra-abdominal Pressure

By closing tightly during the Valsalva maneuver, the vocal cords help increase intra-abdominal pressure. This is important during activities such as:

• Lifting Heavy Objects – Provides stability to the thoracic and abdominal cavities.
• Childbirth – Assists in pushing during labor.
• Defecation and Urination – Facilitates muscle contractions in the abdominal and pelvic regions.

Thermoregulation and Filtration

While not their primary role, the vocal cords contribute to filtration and humidification of inhaled air. They help trap dust, particles, and microbes, preventing them from entering the lungs. The mucus on their surface also maintains moisture and temperature regulation within the respiratory tract.

Clinical Significance

The vocal cords are vital for speech, breathing, and airway protection, making them susceptible to various disorders. Laryngitis, an inflammation caused by infections, allergies, or overuse, can lead to hoarseness or loss of voice. Vocal cord nodules, polyps, and cysts often develop due to vocal strain, especially in singers and speakers, affecting voice quality. Paralysis of the vocal cords can occur due to nerve damage, trauma, or neurological conditions, leading to breathing or swallowing difficulties. In severe cases, bilateral vocal cord paralysis can cause airway obstruction, requiring emergency intervention like a tracheostomy. Vocal cord cancer, often linked to smoking or alcohol use, may present as persistent hoarseness and requires early diagnosis for better outcomes.^[8] Spasmodic dysphonia, a neurological disorder, causes involuntary spasms of the vocal cords, disrupting speech. Diagnosis of vocal cord issues often involves laryngoscopy, and treatments may include speech therapy, medications, surgery, or laser therapy.