Larynx (Voice Box) Explained

Overview

The larynx or upper windpipe is the body’s organ for phonation or voice production. The larynx is a well-developed organ, it accounts for all verbal communications. The capabilities of the larynx tend to increase as we age. In addition, the larynx is a vital part of the respiratory system, which allows the two-way passage for gaseous flow. As a result, it allows us to speak or produce sounds during the expiratory phase of breathing.

Structure

The larynx is located in front of the neck. It extends from the fourth to the sixth vertebrae.  In females and children, the location of the larynx is slightly higher. In males, the larynx lies exactly in front of the 3rd – 6th cervical vertebrae.

The approximate length or size of the larynx is 36 mm among females, 3mm in children, and 44 mm among males. In males, the larynx grows and develops rapidly to make a structure called Adam’s apple for low pitched and louder voice at puberty. The internal diameter of the larynx in an adult is 12mm.

Laryngeal cartilages

A skeletal framework of different cartilages, known as laryngeal skeleton, makes up the larynx. In total, there are nine laryngeal cartilages in which three are paired, and the rest of three are unpaired as:

• Paired cartilages include:
  • Cuneiform
  • Arytenoid
  • Corniculate

• Unpaired cartilages:
  • Epiglottis
  • Thyroid
  • Cricoid

Among these different cartilages, we shall look at the thyroid cartilage in more detail.

Thyroid cartilage

The thyroid cartilage is V-shaped cartilage consisting of left and right laminae. The thyroid cartilage sits as the uppermost and largest cartilage in the laryngeal cartilages. The laryngeal prominence (Adam’s apple) is the median projection of the left and right laminae of the anterior borders.

Cricothyroid joint forms as a result of articulation of cricoid cartilage and the thyroid. A ligament arises from the cricoid cartilage to attach to the thyroid. The cricothyroid ligament prevents the uncontrolled movement of each cartilage and in the case of upper airway obstruction, a surgery to excise this ligament may be required (emergency cricothyrotomy).

Laryngeal joints

Two joints, the cricothyroid joint and cricoarytenoid joint, work in the larynx

• Cricothyroid joint – a joint between the cricoid cartilage and thyroid cartilage. It permits the rotatory movements leading to relaxation and contraction of vocal cords. A few gliding movements are also permissible in this joint.

• Cricoarytenoid joint – it is a joint between the cricoid cartilage and arytenoid cartilage. It permits rotatory movements of the vocal cords and multi-directional gliding movements.

Muscles of the larynx

Intrinsic laryngeal muscles include:

• Cricothyroid
• Posterior cricoarytenoid triangular
• Lateral cricoarytenoid
• Transverse arytenoid
• Oblique arytenoid and aryepiglottic
• Thyroepiglottic and thyroarytenoid
• Vocalis

The extrinsic muscles of the larynx are the ones that attach to the hyoid bone, thus enabling movement of the thyroid cartilage. 

These are a few muscles that act on the larynx to produce different movements such as;

• Vocal cords adduction:
  • Thyroarethenoids
  • Transverse arytenoid
  • Lateral cricoarytenoid
  • Cricothyroids

• Vocal cords contraction:
  • Cricothyroids

• Vocal cord relaxation:
  • Vocalis
  • Thyroarytenoids

Laryngeal folds

The laryngeal folds are divided into vocal folds and vestibular folds. The folds are divided by a space between them, this is called the Rima glottides.

Vocal fold:

These folds are called true vocal folds because they form the inner walls of the larynx.

Vestibular fold:

Also known as false vocal cords because they sit on the vocal cords to protect the larynx. These folds have no contribution to the production of sound.

Both vocal fold and vestibular folds of the larynx divide the larynx into three parts:

• Supraglottis area
• Ventricle or sinus
• Infraglottis area

Functions of the Larynx

Production of speech

There are four basic processes for speech production:

1. Air expiration – air forces out of the larynx and lungs to produce voice. The force of expiration determines the intensity or loudness of voice.
2. Vibrators – vocal cord vibration is produced as a result of expired air. The rate of vocal cord vibration determines the pitch of the voice. Vibrators produce vowels.
3. Resonation – column of air between lips and nose and vocal cords are the resonators for voice production. Resonators determine the quality of sound.
4. Articulators – articulators constitute lips, teeth, tongue, and palate. Articulators tend to stop or narrow the exhaled air. In addition, intrinsic muscles of the tongue tend to produce consonant sounds.

Breathing and protection of the lower respiratory tract

All the cartilages and membranes of the larynx help in protecting the lower respiratory tract. During swallowing, vestibular folds and epiglottis help in sealing the larynx to prevent the entry of food into the trachea.

Neurovascular Supply

Nerve Supply

Two nerves supply the muscles of the larynx:

• Recurrent laryngeal nerve (RLN) – also known as inferior laryngeal nerve, RLN is the principal motor nerve of the intrinsic laryngeal muscles. It is a branch of the vagus nerve, and also provides sensory innervation to the larynx. This nerve supplies all intrinsic muscles other than cricothyroid muscle.
• Superior laryngeal nerve – It also originates from the vagus nerve. The internal laryngeal nerve provides innervation to the mucous membrane of the glottis, including the laryngeal cavity and vocal folds. The external laryngeal nerve innervates the cricothyroid muscle.

Venous Drainage and Arterial Supply

The blood supply for all the structures up to the vocal folds is by the superior laryngeal artery. The venous drainage at this point is via the superior laryngeal vein.

All the structures below vocal folds have their blood supply and venous drainage by the inferior laryngeal artery, and the inferior laryngeal vein respectively.

Associated Diseases

Laryngitis

Laryngitis is the infection and inflammation of the larynx on account of the trapping and lodging of foreign bodies. Laryngitis is characterized by hoarseness of voice and a dry cough. Acute laryngitis usually goes away on its own. With chronic laryngitis, treatment is targeted at the underlying condition, such as smoking or heartburn.

Singer’s nodules

Singer’s nodules or teacher’s nodules occur commonly occur in teachers, singers, or pastors. They result from repetitive vocal cords misuse or overuse. They cause callous-like growths that develop in the midpoint of the vocal folds. This leads to hoarseness of voice and a breathy sound when speaking or singing.

Vocal cord paralysis

Vocal cord paralysis occurs when the nerve impulses to your voice box (larynx) are affected. This causes paralysis of the vocal cord muscles. Vocal cord paralysis can affect your ability to speak and even breathe because your vocal cords do more than just producing sound.

Vocal cord paralysis leads to inappropriate opening and closing of vocal cords and this causes various complications in swallowing, speaking, and breathing.

Spasmodic dysphonia

This is a lifelong condition where the muscles that generate a person’s voice go into periods of spasms affecting voice and speech. In some cases, the disorder is temporary or can be improved through treatment.

Laryngeal nerve injury

This usually follows neck surgery, a neck infection, or a tumor. External laryngeal nerve damage causes weakness in producing sound. In such an injury, the cricothyroid muscle’s tightening or contracting effect on vocal cords is lost.

In recurrent laryngeal nerve injury, phonation tends to completely vanish completely.

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