Glottis

The glottis is a crucial part of the larynx (voice box) involved in the production of sound. It comprises the vocal cords (or vocal folds) and the space between them called the rima glottidis. The vocal cords are flexible tissues that vibrate when air passes through them, playing a central role in speech and sound production. The position and tension of the vocal cords determine the pitch and tone of the voice.

Location

The glottis is located within the larynx, which is situated at the top of the trachea in the neck. It lies between the pharynx (throat) above and the trachea below, specifically at the level of the C4 to C6 vertebrae.

Structure and Anatomy

The glottis is a central part of the larynx that plays a vital role in vocalization and airway control. Its structure involves several components, including the vocal folds and the opening between them, and it is supported by muscles and cartilages that allow for fine control over airflow and sound production. Below is a detailed description of the glottis anatomy.

Vocal Folds (Vocal Cords)

The vocal folds, also known as vocal cords, are the key structures of the glottis. These are two bands of elastic tissue that vibrate when air passes through them, creating sound.

• True Vocal Folds: The true vocal folds are composed of a mucosal layer, vocal ligament, and vocalis muscle. The surface of the folds is covered by a thin layer of mucosa, and underneath lies the vocal ligament, which is supported by the vocalis muscle. This muscle allows the folds to adjust in tension, thickness, and length to produce different sounds.
• Vocal Ligament: The vocal ligament provides structural support to the vocal folds. It runs between the thyroid cartilage at the front and the arytenoid cartilages at the back. This ligament is composed of elastic fibers, which give the vocal folds their flexibility and strength.

Rima Glottidis

The rima glottidis refers to the opening or space between the vocal folds. The shape and size of this space can change depending on the position of the vocal folds.

Shape Variation: The rima glottidis is a slit-like opening that varies in shape during breathing and phonation (sound production). During quiet breathing, it is a narrow triangle, while during forced respiration, it becomes a wider, diamond-shaped gap as the vocal folds separate. When the vocal folds are brought together for phonation, the rima glottidis narrows to allow the vocal folds to vibrate.

Cartilages Involved in the Glottis

The glottis is supported by the laryngeal cartilages, which provide the framework for movement and tension adjustment of the vocal folds.

• Thyroid Cartilage: The thyroid cartilage is the largest of the laryngeal cartilages and forms the front wall of the larynx. The anterior attachment of the vocal folds is located at the inner surface of this cartilage, known as the anterior commissure.
• Arytenoid Cartilages: The arytenoid cartilages are two pyramid-shaped cartilages located at the back of the larynx. They serve as the posterior attachment point for the vocal folds. These cartilages are highly mobile and play a crucial role in opening and closing the glottis by moving the vocal folds apart or together.
• Cricoid Cartilage: The cricoid cartilage forms the base of the larynx and connects to the trachea below. The arytenoid cartilages sit on top of the cricoid, and its ring-like structure provides structural support to the larynx.

Muscles Controlling the Glottis

Several muscles are involved in adjusting the tension, position, and shape of the vocal folds, thus regulating the glottis.

• Cricothyroid Muscle: This muscle lengthens and tenses the vocal folds by tilting the thyroid cartilage forward. It plays an essential role in adjusting pitch by tightening the vocal cords.
• Thyroarytenoid Muscle: The thyroarytenoid muscle, which includes the vocalis muscle, relaxes the vocal cords, reducing their tension. It shortens and thickens the vocal folds, which deepens the pitch of the voice.
• Lateral Cricoarytenoid Muscle: This muscle is responsible for adducting (closing) the vocal folds, which narrows the rima glottidis. It works with other muscles to bring the vocal folds together during speech and swallowing.
• Posterior Cricoarytenoid Muscle: The posterior cricoarytenoid is the only muscle that abducts (opens) the vocal folds, widening the rima glottidis. This muscle is critical for allowing airflow during breathing.
• Interarytenoid Muscles: These muscles are located between the two arytenoid cartilages and work to close the vocal folds by bringing the arytenoids closer together.

Mucosal Lining

The glottis is covered by a mucosal layer that protects the underlying structures and aids in sound production.

• Epithelium: The vocal folds are lined by stratified squamous epithelium, which is thicker than the surrounding laryngeal tissue. This provides the vocal folds with durability to withstand constant vibration during phonation.
• Mucous Glands: Small mucous glands located in the surrounding tissue help keep the vocal folds lubricated. Proper lubrication is essential for smooth vocal fold vibration and reducing the risk of injury or irritation during speech.

Blood Supply

The glottis receives its blood supply from the superior and inferior laryngeal arteries, branches of the superior and inferior thyroid arteries, respectively. This rich blood supply nourishes the vocal folds and surrounding structures.

• Superior Laryngeal Artery: This artery primarily supplies the upper part of the larynx, including the vocal folds.
• Inferior Laryngeal Artery: This artery supplies the lower part of the larynx and the surrounding structures.

Nervous Innervation

The glottis is innervated by branches of the vagus nerve (cranial nerve X), which controls the movement of the vocal folds and the surrounding laryngeal structures.

• Recurrent Laryngeal Nerve: This nerve is a branch of the vagus nerve and provides motor innervation to most of the intrinsic muscles of the larynx, including those that control the vocal folds. It is crucial for both phonation and breathing.
• Superior Laryngeal Nerve: The external branch of the superior laryngeal nerve innervates the cricothyroid muscle, which adjusts tension on the vocal cords, while the internal branch provides sensory input to the mucosa above the vocal cords.

Function

The glottis plays a crucial role in the respiratory system, especially in sound production, breathing, and protecting the lower airway from foreign particles. Below is a detailed explanation of the functions of the glottis.

Sound Production (Phonation)

The most prominent function of the glottis is in the production of sound.

• Vibration of the Vocal Folds: The vocal folds (vocal cords), which form the core part of the glottis, vibrate when air passes through them, producing sound. When the vocal folds are brought close together, air from the lungs is forced through the narrow space, causing them to vibrate. The frequency of the vibration determines the pitch of the sound produced. Faster vibrations produce higher-pitched sounds, while slower vibrations produce lower-pitched sounds.
• Modulation of Pitch and Tone: The pitch and tone of the voice are controlled by adjusting the tension, length, and thickness of the vocal folds. This is done by the muscles surrounding the glottis, such as the cricothyroid muscle, which increases tension for higher pitches, and the thyroarytenoid muscle, which relaxes the vocal folds for deeper tones.
• Resonance and Articulation: The glottis works with other parts of the vocal tract, including the tongue, lips, and pharynx, to articulate different sounds. By adjusting the shape and size of the rima glottidis (the opening between the vocal folds), the glottis helps modulate the sound’s volume and resonance, contributing to speech and singing.

Regulation of Airflow

The glottis plays a key role in controlling the flow of air into and out of the lungs during respiration.

• Opening and Closing of the Vocal Folds: During quiet breathing, the vocal folds are slightly apart, allowing air to pass through the rima glottidis into the trachea and bronchi. During inhalation, the vocal folds move apart (abduction) to widen the airway, allowing more air to pass through, especially during activities such as exercise.
• Airflow Restriction for Vocalization: During phonation, the vocal folds partially close, restricting airflow. This controlled airflow creates the vibrations necessary for sound production. The size of the rima glottidis can be adjusted to control the amount of air passing through, contributing to variations in speech volume and intensity.

Protection of the Lower Airway

The glottis is essential for protecting the lower respiratory tract, including the trachea and lungs, from foreign particles and substances.

• Cough Reflex: The glottis plays a major role in initiating the cough reflex, a protective mechanism triggered by irritation in the airway. If foreign particles such as dust, food, or fluids accidentally enter the airway, sensory receptors in the larynx send signals to the brain to trigger a cough. The vocal folds close tightly, trapping air in the lungs. When the folds suddenly open, the trapped air is forcefully expelled, clearing the airway.
• Closure During Swallowing: During swallowing, the glottis closes to prevent food, liquids, and other substances from entering the trachea and lungs. This closure, combined with the action of the epiglottis, ensures that ingested material is directed into the esophagus, not the airway, thus protecting the lungs from aspiration.

Pressure Control and Effort Closure

The glottis is involved in controlling pressure within the thoracic and abdominal cavities, especially during activities that require increased intra-abdominal pressure.

• Valsalva Maneuver: The Valsalva maneuver is an example of the glottis controlling pressure. It occurs when the vocal folds are fully closed while the person attempts to exhale. This maneuver temporarily increases pressure in the thoracic and abdominal cavities, aiding in functions such as lifting heavy objects, defecation, and childbirth. The glottis prevents air from escaping the lungs during these activities, allowing for a buildup of pressure.
• Breath-Holding: The glottis also allows individuals to voluntarily hold their breath by closing the vocal folds, trapping air in the lungs. This is useful in situations where breath control is needed, such as during underwater swimming or exerting physical force.

Regulation of Intrapulmonary Pressure

The glottis helps regulate intrapulmonary pressure, which affects the air pressure inside the lungs and contributes to breathing mechanics.

• Respiratory Effort and Glottic Modulation: When inhaling, the vocal folds remain open, allowing air to flow freely into the lungs. During forced expiration, such as during intense physical activity or coughing, the vocal folds can partially close, increasing the pressure within the lungs. This allows for more forceful expulsion of air and, in turn, aids in clearing mucus or foreign particles from the airways.
• Maintaining Airway Resistance: The glottis can modulate airway resistance by changing the size of the rima glottidis. This regulation helps maintain optimal air pressure within the lungs, ensuring efficient ventilation during breathing.

Voice Intensity and Volume Control

The glottis plays an important role in controlling the loudness or intensity of the voice.

• Subglottal Pressure: The intensity of the voice is determined by the amount of air pressure beneath the vocal folds, known as subglottal pressure. When a person wants to speak or sing loudly, the vocal folds close tightly, and more subglottal pressure builds up before the folds are opened. This leads to a louder sound when the air is released through the vocal folds.
• Control of Air Release: By finely adjusting the opening of the vocal folds, the glottis allows for varying amounts of air to pass through, thus controlling the volume of the voice. A narrow opening restricts airflow and results in a softer voice, while a wider opening allows more airflow for louder vocalization.

Clinical Significance

The glottis plays a critical role in both respiration and voice production, and any damage or dysfunction in this area can lead to significant clinical issues. Conditions such as laryngitis, vocal cord nodules, or paralysis of the vocal folds can impair the glottis, resulting in hoarseness, loss of voice (aphonia), or difficulty breathing. Glottic stenosis, a narrowing of the glottic space, can cause airway obstruction and severe breathing difficulties, potentially requiring surgical intervention.

The glottis is also crucial in procedures like intubation and tracheostomy, where airway management is essential. Damage to the glottis during such procedures can lead to long-term vocal and respiratory issues. Additionally, conditions like laryngeal cancer often involve the glottis, making it a common site for tumors, and early detection in this area is vital for effective treatment and preservation of both voice and airway function.