The vocal folds, also known as the vocal cords / chords, are flaps of tissue located in the larynx. They give us the ability to produce sound (phonation). There are two folds, one on either side of the laryngeal cavity. Each fold is comprised of a vocal ligament, a vocalis muscle and a covering mucous membrane.
Vibration of these folds in response to air traveling through the larynx allows us to speak, sing, and produce other vocal sounds. The pitch of the sound produced can be altered by changing the position and tension of the folds. These movements are controlled by the joints and muscles of the larynx.
This article will discuss the anatomy and function of the vocal cords.
|Definition||Tissue folds protruding into the cavity of the larynx|
|Structure||Vocal ligament, vocalis muscle, mucous membrane covering|
|Function||Phonation; Inspiration sphincter|
|Innervation||Vagus nerve (CN X): recurrent laryngeal nerve, superior laryngeal nerve.|
The vocal folds are located in the larynx, deep in the anterior neck. They sit approximately at the vertical midpoint of the larynx and protrude into its internal cavity.
The vocal cords are surrounded by the laryngeal cartilages; three large unpaired cartilages (thyroid, cricoid and epiglottis) and three smaller paired cartilages (arytenoid, corniculate, cuneiform). Just superior to the vocal folds are a second pair of folds termed the false vocal cords or vestibular membrane. These false folds do not typically play a role in sound production, instead their function is less well understood through they are thought to have a protective function.
The vocal folds divide the larynx into 3 parts. The section of larynx situated superior to the vocal folds is the laryngeal vestibule, sometimes called the supraglottic cavity; while the section below the folds is the infraglottic cavity. A small middle chamber of the larynx is located between the vestibular folds and vocal folds.
The vocal folds are wedged-shaped structures consisting of the vocal ligament, the vocalis muscle and a mucous membrane covering. They span the laryngeal cavity in an anterior-posterior direction and have a long free edge that protrudes into the laryngeal cavity. In a relaxed state, a small slit-like space can be found between the two folds. This opening (aperture) between the two vocal folds is called the rima glottidis.
The vocal ligament, and its covering mucosa, forms the free edge of the vocal folds. This ligament is the medial most portion of the conus elasticus (sometimes called the cricovocal or cricothyroid ligament) and is comprised of thickened elastic tissue. The vocal ligament extends across the laryngeal cavity from the thyroid cartilage (anteriorly) to the vocal processes of the arytenoid cartilages (posteriorly). The attachment to the thyroid cartilage is sometimes termed the Broyles ligament. Two small mucosal dense masses can be found within the vocal ligament. These are the anterior and posterior maculae flavae. Their function is unknown.
The vocalis muscle is located immediately lateral to the vocal ligaments. It runs from the vocal process of the arytenoid cartilage to the ipsilateral vocal ligament. It functions to “fine tune” the tension on the vocal ligament, selectively tensing or relaxing the anterior or posterior part of the vocal ligament to allow for a wider range of pitch and intensity variation.
A mucous membrane, consisting of mucosal epithelium, encases the vocal ligament and vocalis muscle. This forms a well-defined ‘fold’ of tissue located on the left and right wall of the larynx. The vocal folds meet anteriorly at the anterior commissure.
The vocal apparatus of the human body consists of the vocal folds, the rima glottidis and the interarytenoid fold, a posterior continuation of the vocal folds’ mucous membrane enclosing the posterior aspect of the rima glottidis.
Blood supply to the vocal folds and larynx is via the laryngeal arteries, namely the superior laryngeal artery, cricothyroid artery and the inferior laryngeal artery. The laryngeal arteries are accompanied by the laryngeal veins.
The vocal folds have two main functions; phonation and acting as an inspiratory sphincter. They are also a component of the upper respiratory tract and therefore play a role in respiration. These functions are achieved by movements of the vocal folds that specifically alter the shape and size of the rima glottidis, under the control of the intrinsic muscles of the larynx.
When the vocal folds are moved close to each other, but not tightly pressed together, air passing through the laryngeal cavity during expiration will cause the vocal fold edges to vibrate and produce sound.
Changes to the level of tension within the vocal folds and the expiratory effort enable us to alter the pitch and intensity of the sounds produced. To understand how this occurs, we first need to review the joints and muscles of the larynx.
Joints of the larynx
Location: joint between the cricoid cartilage (posterior aspect, external surface) and the inferior horns of the thyroid cartilage.
Action: rotation / tilting of the thyroid cartilages
Location: joint between the cricoid cartilage (posterior aspect, superior surface) and the arytenoid cartilages.
Action: allow the arytenoid cartilages to slide towards or away from each other, to tilt (anteriorly + posteriorly) and to rotate.
Location: joint between the inferior surface of the corniculate cartilage and the apex of the arytenoid cartilage (superior surface).
Action: No notable function.
Muscles of the larynx
Opening (abduction) and closing (adduction) of the vocal folds and rima glottidis
During adduction the arytenoid cartilages rotate in a clockwise direction so that the vocal processes approximate (lateral cricoarytenoid muscles). The arytenoid cartilages also slide medially (transverse and oblique arytenoid muscles). These two actions bring the vocal ligaments close together allowing vibration of the folds as air passes through the rima glottidis. If the same action occurs but without activation of the transverse arytenoid muscles a small gap persists between the folds allowing for a lower intensity of sound to be produced, known as whispering. In abduction (posterior cricoarytenoid muscles) the opposite movements occur, and no phonation occurs.
Tensing and relaxing of the vocal folds
By tilting the thyroid cartilage and increasing the distance between the thyroid prominence and the arytenoid cartilages, the cricothyroid muscles act to tighten the vocal ligaments. This tension results in raising the pitch of the produced sound. Conversely to the tensors, muscles that relax the vocal ligaments will lower the voice pitch. Relaxation is achieved by the thyroarytenoid muscles pulling the arytenoid cartilages anteriorly, decreasing the distance between the arytenoid and thyroid cartilages. The vocalis muscle acts to finely tune the tension of the vocal ligament. This action allows us to produce highly controlled changes in the pitch of the voice, as occurs during singing.
Inspiratory sphincter action of the vocal folds
The vocal cords also function as a sphincter, to prevent the inhalation of liquid or particles through the lower air respiratory tract and into the lungs. In this case, all intrinsic muscles of the larynx reflexively contract, the vocal folds are adducted tightly together and the arytenoid cartilages are brought closer to the epiglottis. These actions occur in addition to the closure of the larynx by the epiglottis.
During regular breathing the vocal folds and rima glottidis are relaxed and open, allowing for the movement of air through the laryngeal cavity and into the lower respiratory tract. If forced inspiration is required the rima glottidis is actively widened. Forced inspiration is achieved by fully abducting the vocal folds.
Effort closure is when we actively increase intraabdominal pressure by holding inhaled air within the thoracic cavity, for example during coughing, defecation or heavy lifting. This is achieved by complete closure of the rima glottidis, along with the lower larynx.
Attachment: Vocal process of arytenoid cartilage (Lateral surface) to ipsilateral vocal ligament. Sometimes considered to be deep portion of the thyroartenoid muscle.
Action: selective tensioning or relaxing of anterior and/or posterior part of vocal ligament.
Attachment: Angle of thyroid laminae (posterior aspect, lower half) and adjacent cricothyroid ligament to anterolateral surface of arytenoid. Muscle lies lateral to the vocal fold.
Action: Shortens and relaxes vocal ligament. Internal rotation of arytenoids, thus assists in approximating the vocal folds and closing rima glottidis.
Attachment: Arch of cricocartilage (external, anterolateral aspect) to the inferior horn and margin of the thyroid cartilage.
Action: Tilts the thyroid cartilage, thereby increasing the distance between the thyroid prominence and the arytenoid cartilages. This stretches (elongate) and tenses the vocal ligaments.
|Posterior cricoarytenoid muscle||
Attachment: Posterior aspect of lamina of cricoid cartilage to muscular process of ipsilateral arytenoid cartilage.
Action: External rotation of arytenoid cartilage, abducts vocal folds and opens rima glottidis
|Lateral circoartytenoid muscle||
Attachment: Arch of cricoid cartilage (superior aspect) to muscular process of ipsilateral arytenoid cartilage.
Action: Internal rotation of arytenoid cartilage, adducts vocal folds and closes rima glottidis. Additional action of shortening and relaxing vocal ligament.
|Transverse arytenoid muscle||
Attachment: Posterior aspect of arytenoid cartilage (lateral border) to contralateral arytenoid cartilage.
Action: Adducts arytenoid cartilages and vocal folds, closing posterior aspect of rima glottidis
|Oblique arytenoid muscles||
Attachment: Posterior aspect of arytenoid cartilage to contralateral arytenoid cartilage. Superficial to the transverse arytenoid muscle
Action: Adducts arytenoid cartilages and vocal folds, full closure of rima glottidis forming laryngeal sphincter.
Attachment: Also aryepiglottic part of oblique arytenoid muscle; Lateral continuation of the oblique arytenoid muscle.
Action: Assists oblique arytenoid muscle in adduction arytenoid cartilages and vocal folds, full closure of rima glottidis forming laryngeal sphincter.
Attachment: Also thyro-epiglottic part of thyro-arytenoid muscle; continuation of aryepiglotticus muscle into the aryepiglottic fold. Angle of thyroid cartilage and adjacent cricothyroid ligament to arytenoid cartilage (anterolateral surface).
Action: Depresses the epiglottis
Sensory and motor supply to the larynx, vocal folds and their controlling musculature is provided by branches of the vagus nerve (the 10th cranial nerve).
The recurrent laryngeal nerve innervates the intrinsic muscles of the larynx, with the exception of the cricothyroid muscle which is innervated by the superior laryngeal nerve (external branch). Sensory supply is via the internal branch of the superior laryngeal nerve (upper portion of the larynx) and the recurrent laryngeal nerve (lower half of the larynx).
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