The vocal cords, which are also referred to as the vocal folds, are situated within the larynx (voice box), which is placed on top of the trachea (windpipe). The muscles from the hyoid bone (U-shaped bone at the base of the tongue) support the larynx, enabling it to move up or down. The larynx consists of 3 paired and 3 unpaired cartilages. One of the unpaired cartilages is the thyroid cartilage. In some men, the front section of this cartilage is quite pronounced. This section is referred to as the 'Adam's apple'. The other two unpaired cartilages are referred to as cricoid cartilage and epiglottis. The epiglottis is a flap-like cartilaginous structure. When we swallow, the epiglottis folds down over the glottis and the larynx moves up, thereby covering the entrance to the trachea. This allows food or water to go down the esophagus (food pipe, which lies behind the larynx and trachea), and protects the vocal folds and airways by preventing food or water from going down the airways or the windpipe.
Vocal Cords Function and Anatomy
The human respiratory system consists of the nose, pharynx (throat), larynx (voice box), trachea (windpipe), bronchi (airways), and lungs. These organs work in tandem to facilitate the exchange of carbon dioxide and oxygen. The larynx is a cartilaginous structure that is located at the top of the trachea. It contains two true vocal folds.
Stretching horizontally across the voice box, these membranous folds are around 11-24 mm long, and are covered by stratified squamous epithelium.
These run from the back of the thyroid cartilage to the front of the arytenoid cartilage, and are controlled by the vagus nerve. Just above these folds lie the vestibular folds (false vocal folds) that protect the delicate true vocal cords. The vestibular folds are formed by a thick mucous membrane and vestibular ligament. They are responsible for resonance, and production of deep tones, but it is the true vocal cords that are instrumental in speech generation. Between the false and true folds lie the laryngeal ventricles that extend on either side of the middle laryngeal cavity. The movement of the vocal folds, which involves changing the shape, position, and tension of the cords, is facilitated by intrinsic muscles of the larynx that connect the cartilages of the larynx. The extrinsic muscles connect the voice box to the surrounding anatomical structures and help adjust the position of the larynx in the neck.
Intrinsic Muscles of Larynx and Vocal Cords
The intrinsic muscles connect the cartilages of the voice box and help alter the position, shape, and tension of the vocal cords by bringing them together or spreading, and stretching them. These muscles include:
Posterior cricoarytenoid muscles
Lateral cricoarytenoid muscles
Transverse interarytenoid muscles
The cricothyroid muscle extends from the cricoid to the thyroid cartilage. It raises the cricoid and lowers the thyroid, which in turn reduces the cricothyroid space. This creates tension and lengthens the vocal folds. This changes their stiffness and the period for which the vocal folds vibrate, thereby producing higher-pitched sounds.
The posterior cricoarytenoid muscles start on the back of the cricoid cartilage and run upward and to the side of the arytenoid cartilage. As they rotate the arytenoid cartilage, they cause the vocal folds to pull apart. This is called abduction.
The lateral cricoarytenoid muscles start at the muscle of the arytenoid (which lines the rear part of the larynx) and run lateral to the cricoid cartilage. When they contract, they rotate the arytenoid cartilages, causing the mucous part of vocal folds to adduct or come together.
The interarytenoid muscles run horizontally between the two arytenoids. They adduct the vocal cords. They help the lateral cricoarytenoid muscle in closing the rear section of the glottis.
All these muscles are supplied by the recurrent laryngeal branch of the vagus nerve, with the exception of the cricothyroid muscles, which are supplied by another branch of vagus nerve (external branch of the superior laryngeal nerve).
The thyroarytenoid muscles draw the arytenoid cartilages forward, which relaxes and shortens the vocal cords. They also rotate the arytenoid cartilage inward, thus bringing the vocal folds together and narrowing the rima glottidis. The only intrinsic laryngeal muscles that open the vocal folds are the posterior cricoarytenoid. They open and close rapidly, as the air expelled from the lungs is forced upwards. When the adductor muscles get activated, they provide resistance to exhaled air from the lungs. Air flows rapidly through the closed vocal cords. As the air rushes through them, the pressure between the folds lowers, and the folds come back together.
The oscillating vocal cords interrupt the airflow, which is followed by the production of a glottal tone. However, vocal cords alone are not involved in the generation of speech. The tone produced is further changed in the vocal tract. Thus, the generation of speech involves the lungs, the vocal folds within the larynx, and the articulators (tongue, palate, lips, cheeks, etc.). The lungs reduce the size of chest cavity with the help of diaphragm while expelling air upwards through the windpipe, towards the larynx, thereby providing an airstream that has sufficient pressure to cause the vibration of the vocal cords. While the muscles of the larynx adjust the tension and the length of the vocal folds to adjust the pitch and tone, the vocal folds and the articulators work together to produce different types of sounds.
Thus, vocal cords are integral to the function of phonation or speech generation. They work in tandem with the articulators in order to produce a wide range of sounds that can reflect varied moods or emotions. During normal speech, the vocal cords oscillate more than 100 times per second, whereas they open and close more than 400 times in a second when we sing.