Cricothyroid muscle

The cricothyroid muscle is a key muscle involved in tension and lengthening of the vocal cords. It plays a crucial role in modulating pitch during speech and phonation. The cricothyroid muscle is a fan-shaped muscle and is composed of two parts: the pars recta and the pars obliqua.

Location

The cricothyroid muscle is located in the larynx, specifically on the anterior (front) aspect of the larynx. It spans between the cricoid cartilage, which forms the lower part of the larynx, and the thyroid cartilage, which is situated above the cricoid cartilage. This muscle is situated externally on the larynx, just below the Adam’s apple in the neck region.

Structure and Anatomy

The cricothyroid muscle is a critical part of the laryngeal musculature, responsible for adjusting the tension of the vocal cords. It is a paired muscle, located on both sides of the larynx, and is distinctive for its external positioning on the front of the larynx. Below is a detailed description of the anatomy of the cricothyroid muscle.

Parts of the Cricothyroid Muscle

The cricothyroid muscle is composed of two distinct parts:

• Pars Recta: The pars recta is the vertical portion of the muscle. It runs from the anterolateral surface of the cricoid cartilage to the inferior border of the thyroid cartilage. It is more vertically oriented and positioned medially.
• Pars Obliqua: The pars obliqua is the diagonal portion of the muscle. It runs from the anterolateral surface of the cricoid cartilage to the inferior horn (cornu) of the thyroid cartilage. The pars obliqua is more laterally positioned and has a slanted, oblique orientation.

These two parts work together to tilt the thyroid cartilage forward relative to the cricoid cartilage.

Attachments

The cricothyroid muscle originates and inserts into key structures within the larynx:

• Origin: The cricothyroid muscle originates from the anterolateral surface of the cricoid cartilage, which forms the base of the larynx. The cricoid cartilage is shaped like a signet ring, with the broader portion located posteriorly and the narrower arch located anteriorly where the muscle attaches.
• Insertion: The muscle inserts into two points on the thyroid cartilage:
  • The pars recta inserts into the inferior border of the thyroid cartilage.
  • The pars obliqua inserts into the inferior horn (cornu) of the thyroid cartilage.

This configuration allows the cricothyroid muscle to pull the thyroid cartilage downward and forward relative to the cricoid cartilage.

Innervation

The cricothyroid muscle has a unique innervation compared to most other laryngeal muscles.

• The muscle is innervated by the external branch of the superior laryngeal nerve, which is a branch of the vagus nerve (cranial nerve X). This is distinct because all other intrinsic laryngeal muscles are innervated by the recurrent laryngeal nerve, another branch of the vagus nerve.
• The external branch of the superior laryngeal nerve provides motor innervation to the cricothyroid muscle, allowing it to contract and adjust the tension of the vocal cords.

Blood Supply

The cricothyroid muscle receives its blood supply from arteries that also serve the laryngeal structures.

• Superior Thyroid Artery: The main blood supply to the cricothyroid muscle comes from the superior thyroid artery, a branch of the external carotid artery. The cricothyroid branch of the superior thyroid artery specifically supplies the muscle.
• Inferior Thyroid Artery: The inferior thyroid artery, a branch of the thyrocervical trunk, provides additional blood supply to the lower laryngeal structures, including portions of the cricothyroid muscle.

Relationships with Adjacent Structures

The cricothyroid muscle is closely related to several other key anatomical structures within the neck and larynx:

• Thyroid Gland: The inferior aspect of the thyroid gland is located near the cricothyroid muscle. In fact, the cricothyroid muscle is sometimes exposed during thyroid surgeries, particularly when dealing with the inferior poles of the thyroid gland.
• Cricothyroid Membrane: The cricothyroid membrane, located directly beneath the cricothyroid muscle, connects the cricoid and thyroid cartilages. This membrane is often used as an access point during emergency airway procedures, such as cricothyrotomy.
• Vocal Cords: While the cricothyroid muscle is external, its actions directly affect the vocal cords inside the larynx. By tilting the thyroid cartilage forward, the cricothyroid muscle increases tension on the vocal cords, which changes their pitch.

Fascial Covering

The cricothyroid muscle is covered by a thin layer of fascia, which separates it from surrounding structures in the neck. This fascia helps bind the muscle to the neighboring cartilages and other muscles of the larynx.

Function

The cricothyroid muscle plays an essential role in the control of voice pitch by adjusting the tension and length of the vocal cords. It is the primary muscle responsible for modulating the pitch of sound during phonation, particularly in producing higher-pitched sounds. Below is a detailed explanation of its functions.

Tension and Elongation of Vocal Cords

The cricothyroid muscle primarily functions to increase the tension on the vocal cords by altering the position of the thyroid cartilage relative to the cricoid cartilage.

• Tilting the Thyroid Cartilage: The cricothyroid muscle contracts and pulls the thyroid cartilage downward and slightly forward, toward the cricoid cartilage. This movement increases the distance between the thyroid cartilage and the arytenoid cartilages, which are attached to the vocal cords at their posterior end.
• Lengthening the Vocal Cords: As the thyroid cartilage moves forward, it stretches and lengthens the vocal cords. Lengthening the vocal cords increases their tension, similar to how tightening the strings of a musical instrument raises its pitch.

Modulation of Voice Pitch

The cricothyroid muscle is responsible for controlling the pitch of the voice by adjusting the tension of the vocal cords. The tension and length of the vocal cords directly influence the frequency of their vibrations, which in turn affects the pitch of the sound produced.

• High-Pitched Sounds: When the cricothyroid muscle contracts, it lengthens and tightens the vocal cords, causing them to vibrate at a higher frequency. This produces higher-pitched sounds. The greater the tension applied to the vocal cords, the higher the pitch will be.
• Lower-Pitched Sounds: When the cricothyroid muscle relaxes, the vocal cords shorten and loosen, reducing their tension. This results in lower-frequency vibrations and, therefore, a lower-pitched sound. Other muscles, such as the thyroarytenoid muscle, work in coordination with the cricothyroid muscle to modulate the pitch of lower sounds.

Fine Control of Phonation

The cricothyroid muscle works in concert with other intrinsic laryngeal muscles to provide fine control over phonation, allowing for nuanced vocal expressions and a range of vocal sounds.

• Coordination with Other Muscles: The vocalis muscle, a part of the thyroarytenoid muscle, primarily controls vocal fold tension in shorter, lower-pitched sounds. However, for higher-pitched sounds, the cricothyroid muscle dominates by elongating the vocal cords. These muscles work together to adjust pitch and maintain vocal control across a wide frequency range.
• Smooth Transitions in Pitch: The cricothyroid muscle allows singers and speakers to smoothly transition between different pitches, enabling more expressive and controlled vocalization. Its gradual and controlled contraction and relaxation are key to achieving fine vocal control, particularly in performing dynamic vocal tasks such as modulating tone and intensity.

Role in Speech and Singing

The cricothyroid muscle is particularly important in voice modulation during both normal speech and singing. By finely adjusting vocal cord tension, the cricothyroid muscle allows for varied vocal performance.

• Singing High Notes: Singers rely heavily on the cricothyroid muscle to achieve and sustain higher notes. Its ability to stretch and tense the vocal cords is what allows for precise control over pitch during high-pitched singing.
• Speech Modulation: In regular speech, the cricothyroid muscle helps in the natural modulation of tone and pitch, allowing speakers to convey different emotions, emphases, and vocal inflections. It plays a central role in intonations and stress, which are crucial for conveying meaning in spoken language.

Breathing and Airway Regulation

Although the cricothyroid muscle’s primary role is in voice production, it indirectly contributes to maintaining an open airway during breathing.

• Vocal Cord Positioning: By adjusting the tension of the vocal cords, the cricothyroid muscle helps control the size of the rima glottidis (the opening between the vocal cords). When the vocal cords are tensed, the rima glottidis narrows, affecting airflow during respiration. In relaxed states, the rima glottidis widens to allow more air to pass during normal breathing.
• Prevention of Airway Obstruction: The cricothyroid muscle’s function in maintaining appropriate tension and length of the vocal cords is essential for preventing excessive laxity, which could potentially interfere with normal breathing by narrowing the airway.

Pressure Control and Glottal Function

The cricothyroid muscle, in conjunction with other laryngeal muscles, plays a role in controlling the glottis (the space between the vocal cords), which is important for regulating airflow and pressure during certain physiological activities.

• Valsalva Maneuver: The cricothyroid muscle helps modulate the position of the vocal cords during the Valsalva maneuver, which involves forcefully exhaling against a closed glottis. This maneuver increases thoracic and abdominal pressure and is often used during heavy lifting, childbirth, or defecation.
• Control of Subglottic Pressure: By controlling the tension of the vocal cords, the cricothyroid muscle regulates subglottic pressure, which is the air pressure beneath the vocal cords. Proper regulation of this pressure is crucial for the efficient production of sound and controlled breathing.

Clinical Significance

The cricothyroid muscle plays a key role in vocal cord tension and pitch regulation, making it clinically significant in voice disorders. Injury to the external branch of the superior laryngeal nerve, which innervates the cricothyroid muscle, can result in the inability to produce high-pitched sounds, a condition often seen in professional voice users like singers. This nerve is vulnerable during thyroid surgery, potentially leading to vocal fatigue or voice weakness if damaged.

Dysfunction or paralysis of the cricothyroid muscle can also lead to dysphonia (voice disorders), where the individual may have difficulty adjusting pitch or controlling vocal intensity. Its role in controlling vocal cord tension also makes it a focal point in conditions requiring vocal cord rehabilitation or in cases of laryngeal trauma.