Biology of the Ears, Nose, & Throat

Introduction

The ears, nose, and throat have two things in common: they are located near each other and have separate but related functions. The ears and nose are sensory organs—necessary for the senses of hearing, balance, and smell. The throat mainly functions as a pathway through which food and fluids travel to the esophagus and air passes to the lungs. Primary care doctors often diagnose and treat disorders involving these organs, but doctors called otolaryngologists specialize in them.

The ear, which is the organ of hearing and balance, consists of the outer, middle, and inner ear. The outer, middle, and inner ear function together to convert sound waves into nerve impulses that travel to the brain, where they are perceived as sound. The inner ear also helps to maintain balance.

 

Outer Ear

The outer ear consists of the external part of the ear (pinna or auricle) and the ear canal (external auditory meatus). The pinna consists of cartilage covered by skin and is shaped to capture sound waves and funnel them through the ear canal to the eardrum (tympanic membrane), a thin membrane that separates the outer ear from the middle ear.

 

Middle Ear

The middle ear consists of the eardrum and a small air-filled chamber containing a chain of three tiny bones (ossicles) that connect the eardrum to the inner ear. The ossicles are named for their shapes. The hammer (malleus) is attached to the eardrum. The anvil (incus) is the middle bone between the hammer and the stirrup (stapes), which is attached to the oval window, a thin membrane at the entrance to the inner ear. Vibrations of the eardrum are amplified mechanically by the ossicles and transmitted to the oval window.

 

The middle ear also contains two tiny muscles. The tensor tympani muscle is attached to the hammer; it helps to tune and protect the ear. The stapedius muscle is attached to the stirrup and oval window; it contracts in response to a loud noise, making the chain of ossicles more rigid so that less sound is transmitted. This response, called the acoustic reflex, helps protect the delicate inner ear from sound damage.

 

The eustachian tube, a small tube that connects the middle ear with the back of the nose, allows outside air to enter the middle ear. This tube, which opens when a person swallows, helps maintain equal air pressure on both sides of the eardrum and prevents fluid from accumulating in the middle ear. If air pressure is not equal, the eardrum may bulge or retract, which can be uncomfortable and distort hearing. Swallowing or voluntary "popping" of the ears can relieve pressure on the eardrum caused by sudden changes in air pressure, as often occurs when flying in an airplane. The eustachian tube's connection with the middle ear explains why upper respiratory infections (such as the common cold), which inflame and block the eustachian tube, can lead to middle ear infections or changes in middle ear pressure, resulting in pain.

 

Inner Ear

The inner ear (labyrinth) is a complex structure consisting of two major parts: the cochlea, the organ of hearing; and the vestibular system, the organ of balance. The vestibular system consists of the saccule and the utricle, which determine position sense, and the semicircular canals, which help maintain balance.

 

The cochlea, a hollow tube coiled in the shape of a snail's shell, is filled with fluid. Within the cochlea is the organ of Corti, which consists, in part, of about 20,000 specialized cells, called hair cells. These cells have small hairlike projections (cilia) that extend into the fluid. Sound vibrations transmitted from the ossicles in the middle ear to the oval window in the inner ear cause the fluid and cilia to vibrate. Hair cells in different parts of the cochlea vibrate in response to different sound frequencies and convert the vibrations into nerve impulses. The nerve impulses are transmitted along fibers of the cochlear nerve to the brain. The round window is a small, membrane-covered opening between the fluid-filled cochlea and the middle ear; it helps dampen the pressure caused by sound waves in the cochlea.

 

Despite the protective effect of the acoustic reflex, loud noise can damage and destroy hair cells. Once a hair cell is destroyed, it does not appear to regrow. Continued exposure to loud noise causes progressive damage, eventually resulting in hearing loss and sometimes noise or ringing in the ears (tinnitus).

 

The semicircular canals are three fluid-filled tubes at right angles to one another. Movement of the head causes the fluid in the canals to move. Depending on the direction the head moves, the fluid movement will be greater in one of the canals than in the others. The canals contain hair cells that respond to this movement of fluid. The hair cells initiate nerve impulses that tell the brain which way the head is moving, so that appropriate action can be taken to maintain balance.

 

If the semicircular canals malfunction, as may occur in an upper respiratory infection and other conditions both temporary and permanent, a person's sense of balance may be lost or a whirling sensation (vertigo) may develop.

Aging greatly affects the function of the ears, nose, and throat. The effects of aging result from many factors such as wear and tear, noise, and the cumulative effect of infections, as well as the effect of substances such as drugs, alcohol, and tobacco.

 

A progressive loss of hearing, especially for higher-pitched sounds, is common (presbycusis). This change can alter a person's ability to understand speech. Vestibular imbalance and ringing in the ears (tinnitus) are also more common in older people but are not normal. Changes occur because some structures in the ear that help with hearing or balance deteriorate slightly. Hearing aids can help people with hearing loss hear better.

 

The sense of smell may decline with age, making tastes less distinct. Changes in the voice also occur with age. The tissues in the larynx may stiffen, affecting the pitch and quality of the voice and causing hoarseness. Changes in the tissues of the throat (pharynx) may lead to the leakage of food or fluids into the trachea during swallowing (aspiration). If persistent or severe, aspiration may cause pneumonia.

The nose is the organ of smell and a main passageway for air into and out of the lungs. The nose warms, moistens, and cleans air before it enters the lungs. The bones of the face around the nose contain hollow spaces called paranasal sinuses. There are four groups of paranasal sinuses: the maxillary, ethmoid, frontal, and sphenoid sinuses. Sinuses reduce the weight of the facial bones and skull while maintaining bone strength and shape. The air-filled spaces of the nose and sinuses also add resonance to the voice.

 

The supporting structure of the upper part of the external nose consists of bone, while the lower part consists of cartilage. Inside the nose is the nasal cavity, which is divided into two passages by the nasal septum. The nasal septum is composed of both bone and cartilage and extends from the nostrils to the back of the throat. Bones called nasal conchae project into the nasal cavity, forming a series of folds (turbinates). These folds greatly increase the surface area of the nasal cavity. Polyps may develop between the folds, often in people with asthma, allergies, or cystic fibrosis and in those using aspirin for long periods.

 

Lining the nasal cavity is a mucous membrane rich with blood vessels. The increased surface area and the many blood vessels enable the nose to warm and humidify incoming air quickly. Cells in the mucous membrane produce mucus and have tiny hairlike projections (cilia). Usually, the mucus traps incoming dirt particles, which are then moved by the cilia toward the front of the nose or down the throat to be removed from the airway. This action helps clean the air before it goes to the lungs. Sneezing automatically clears the nasal passages in response to irritation, just as coughing clears the lungs.

 

Like the nasal cavity, the sinuses are lined with a mucous membrane composed of cells that produce mucus and have cilia. Incoming dirt particles are trapped by the mucus, then moved by the cilia into the nasal cavity, through small sinus openings (ostia). Because these openings are so small, the drainage can easily be blocked by conditions such as colds or allergies, which produce swelling of the mucous membranes. Blockage of normal sinus drainage leads to sinus inflammation and infection (sinusitis).

 

One of the most important functions of the nose is its role in the sense of smell. Smell receptor cells are located in the upper part of the nasal cavity. These cells are special nerve cells that have cilia. The cilia of each cell are sensitive to different chemicals and, when stimulated, create a nerve impulse that is sent to the nerve cells of the olfactory bulb, which lies inside the skull just above the nose. The olfactory nerves carry the nerve impulse from the olfactory bulb directly to the brain, where it is perceived as a smell.

 

The sense of smell, which is not fully understood, is much more sophisticated than the sense of taste. Distinct smells are far more numerous than tastes. The subjective sense of taste while eating (flavor) involves taste and smell as well as texture and temperature. This is why food seems somewhat tasteless when a person has a decreased sense of smell, as may occur when the person has a cold. Because the smell receptors are located in the upper part of the nose, normal breathing does not draw much air over them. Sniffing, however, increases the flow of air over the smell receptor cells, greatly increasing their exposure to odors.

The throat (pharynx) is located behind the mouth, below the nasal cavity, and above the esophagus and windpipe (trachea). It consists of an upper part (nasopharynx), a middle part (oropharynx), and a lower part (hypopharynx). The throat is a muscular passageway through which food is carried to the esophagus and air is carried to the lungs. Like the nose and mouth, the throat is lined with a mucous membrane composed of cells that produce mucus and have hairlike projections (cilia). Dirt particles caught in the mucus are carried by the cilia toward the esophagus and are swallowed.

 

The tonsils are located on both sides of the back of the mouth, and the adenoids are located at the back of the nasal cavity. The tonsils and adenoids consist of lymphoid tissue and help fight off infections. They are largest during childhood and gradually shrink throughout life. When the tonsils and adenoids are removed surgically, either for obstructive sleep apnea (when breathing is temporarily blocked during sleep) or repeated infections (adenotonsillitis), other lymphoid tissue such as lymph nodes in the head and neck take over their immune function. The uvula is a small flap of tissue visible in the back of the throat between the tonsils. As part of the soft palate, the uvula helps prevent food and fluids from entering the nasal cavity during swallowing and assists in the formation of certain sounds during speech. A long uvula may cause snoring and occasionally contributes to sleep apnea.

 

At the top of the trachea is the voice box (larynx), which contains the vocal cords and is primarily responsible for producing the sound of the voice. When relaxed, the vocal cords form a V-shaped opening that air can pass through freely. When contracted, they vibrate as air from the lungs passes over them, generating sounds that can be modified by the tongue, nose, and mouth to produce speech.

 

The epiglottis is a stiff flap of cartilage located above and in front of the larynx. During swallowing, the epiglottis covers the opening to the larynx to prevent food and fluids from entering the trachea. Thus, the epiglottis protects the lungs.