The Anatomy of the Ear

Organs of human hearing are located on either side of the head

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Essential organs of human hearing and balance, the ears are located on either side of the head, at the level of the nose. Separated into an inner, middle, and outer ear, each ear is an intricate and complicated mixture of bones, nerves, and muscles. Naturally, these structures are at the heart of hearing loss problems as well as those affecting balance. The ear can be subject to bacterial infection, deafness, hearing loss, or tinnitus (ringing in the ears) due to congenital conditions, exposure to loud sounds, or earwax build-up, as well as conditions like Meniere’s disease, a major cause of vertigo (chronic dizziness). Furthermore, audition (sense of hearing) can be impacted by other neurological conditions.   

Anatomy

Structure & Location

In the broadest terms, the ear is divided into three portions: the outer ear (which includes the visible outer portion, as well as the ear canal), the middle ear, and the inner ear, representing the portion deepest in the skull. Each of these sections has a number of components. The outer ear contains the ear canal, as well as several other major parts:

  • Auricle: The outwardly visible part of the ear, this blend of skin and cartilage attaches to the skull. It has an outer (lateral) aspect as well as an inner (medial) one. Whereas the latter of these serves as an attachment, the former is more instrumental in hearing and has characteristic ridges and grooves. Notable among these is the outer rim or helix, which runs from the skull and bends around to terminate at the ear lobe. Parallel to this is another curved structure called the antihelix, which has a triangular upper fossa (or space) bound by the borders of the helix and antihelix. The auricle also has, at its center, a space next to the opening of the external acoustic meatus (ear canal) called a concha, which is partially covered by a triangular flap of cartilage known as the tragus.
  • External acoustic meatus: This is the bone and cartilage lined canal that leads from the outside to the inside of the ear. Its outer portion is surrounded by cartilage, whereas the inner part is surrounded by bones of the skull. The course of this portion is not exactly a straight line, curving first slightly up and to the back initially, before bending forward and down. The inner portion—representing about two-thirds of its course—is surrounded by temporal bone and terminates at the tympanic membrane (see below).
  • Tympanic membrane (eardrum): This portion, known commonly as the eardrum, represents the border between the external and middle ear. It is composed of a membrane attached by fibrous cartilage to the surrounding bone. It has a more flaccid part (pars flaccida), and a more taut part (pars tensa). The inner, medial surface is convexed towards the middle ear and connects with the malleus bone of the middle ear.

In turn, the middle ear (also known as the tympanum or tympanic cavity) is a complicated network of tunnels, openings, and canals mostly inside openings within the temporal bone on each side of the skull. This space, shaped like a narrow tube with concave walls, is separated from the external ear by the tympanic membrane and the inner ear by its labyrinthine (medial) wall. Roughly speaking, it has three major compartments—the mesotympanum (directly to the side of the membrane), the epitympanum or attic (located to the top of the cavity), and six major walls—the tegmental wall (roof), jugular wall (floor), membranous (lateral) wall, labyrinthine (medial) wall, mastoid (posterior) wall, as well as the carotid (anterior) wall.

It is in the middle ear that anatomists find the three auditory ossicles, which are the tiny bones (in fact the three smallest bones within the human body) that transmit sound to the labyrinth of the inner ear. These are:

  • Malleus (hammer): Attached to the tympanic membrane on its outer side, and the incuse via a joint called the incudomalleolar joint. It has a head connected to the tegmental wall of the middle ear, and a neck, which has two portions: the anterior and lateral processes. The former of these is linked to the carotid wall, and the latter is attached to the middle surface of the tympanic membrane.
  • Incus (anvil): This connects the malleus and stapes and consists of three portions: the body, as well as the long and short limbs. The first of these is connected to the malleus by way of the incudomalleolar joint and sits in a space called the epitympanic recess. The long limb runs parallel to the handle of the malleus and terminates as it accesses the tenticular process. Via the incudostapedial joint, it links to the stapes. Lastly, the short limb runs towards the back of the body, attaching to the posterior wall of the tympanic cavity.
  • Stapes (stirrup): The last of these bones connects with the incus on the side via the incudostapedial joint, while, in its middle, it accesses the oval window as part of the mechanism that carries sound to the inner ear. This bone also has a head, which connects with the lenticular process, as well as two limbs that attach to the oval base, which connect with the oval window.    

In addition, the eustachian tube (also known as the auditory or pharyngotympanic tube) connects the middle ear with nasopharynx, which is the upper throat and nasal cavity. Playing a crucial role in regulating pressure in this part of the ear, its bony part arises in the carotid wall before moving downward and forward about 30 to 35 degrees, narrowing as it progresses through an area called the pharyngeal space.

Finally, the inner ear—known also as the labyrinth—is quite intricate and easily the most complicated portion of the ear. Positioned as it is in the petrous part of the temporal bone on the side of the skull, it has several important organs and portions. Doctors consider it to be divided into a bony labyrinth, which is filled with a fluid called perilymph, in which is suspended the membranous labyrinth, which contains a fluid called endolymph. The major structures of the inner ear include:

  • Vestibule: A cavity considered a part of the membranous labyrinth, this structure contains two sacs: the utricle and the saccule. Via a structure on its outside wall called the oval window, it (along with another structure called the round window) is able to communicate with the middle ear, and it accesses the cochlea on the other side, with the semicircular canals behind and above it. As such, this is the central structure of the inner ear.
  • Cochlea: This spiral-shaped organ—its shape resembles a snail shell—consists of three compartments: the scala vestibuli, scala media (often called the cochlear duct), and scala tympani. Notably, this feature is split into a base and its spiral canal, which wraps two and a half times around a central bony column, known as the modiolus. Each of these structures serves an important role in audition; the scala vestibuli and media contain perilymph, and surround the third, which is filled with endolymph.    
  • Semicircular canals: These three semi-circular canals are arranged at different angles and loop around, with each tipped roughly 90 degrees from the other. The anterior semicircular canal emerges from the sagittal plane (the line that divides the body into left and right). The posterior one, in turn, emerges along the frontal plane (dividing the front and back of the body), and the lateral semicircular canal runs horizontally to the ground. One side of the anterior and posterior canals is merged.

Anatomical Variations

Ear anatomy can vary a great deal, and, alongside normal and relatively minor differences, there are a number of more significant and impactful variants. For instance, on the auricle, attachment—or lack thereof—of the earlobe to the face is a frequently seen genetic variation, with attached earlobes seen in anywhere from 19% to 54% of the population. There’s also a great deal of variation in the size and shape of other structures there, such as the helix, antihelix, tragus, and others, as well as differences in overall size.

There are a number of other specific malformations of the external ear recognized by doctors, including:

  • Prominent ear: This relative common variant involves ears that protrude out from the head more than 2 centimeters (cm).
  • Constricted ear: In this case, the helical rim folds over, is wrinkled, or abnormally tight.
  • Cryptotia: Due to malformation of ear cartilage, this variant gives off the appearance that the upper portion of the ear is buried inside the head.
  • Microtia: This is an underdeveloped ear.
  • Anotia: In some cases, there is a complete absence of the ear.
  • Stahl’s ear: This is when additional cartilage in the crus of the ear lends to a pointy, elf-like appearance.
  • Cauliflower ear: This condition occurs when there is excessive and abnormal cartilage formation on the top of the normal ear cartilage, resulting in misshapen, often bulkier ears. 

In addition, other variations have been observed in the middle and inner parts of the ear. Mostly involving the tympanic membrane, these include:

  • Anagenesis of the pyramidal eminence and stapedial tendon: This condition is characterized by a failure to develop of the stapedial tendon that connects the stapes to the surrounding structure.
  • Absence of the ponticulus: In rare cases the ponticulus, a small bony structure of the posterior of the middle ear is under-formed, shaped irregularly, or completely absent.
  • Absence of the subiculus: As above, doctors have observed partial or complete absence of the subiculus, a small bony structure near the oval window of the middle ear.
  • Facial dehiscence: Doctors have also observed the presence of an additional psuedomembrane covering the round window of the tympanic membrane.   

Function

Primarily, the ear serves two functions—hearing and regulation of balance. In terms of the former, the outer ear is shaped to direct sound waves from the external environment to the ear canal. These are then directed towards the tympanic membrane (eardrum), causing it to vibrate. This vibration then causes the malleus, incus, and stapes to vibrate, which leads the perilymph within the cochlea to vibrate, stimulating a small portion called the organ of Corti. As the fluid moves, tiny hairs on the surfaces of the organ of Corti are stimulated and this is translated into electrical signals that are delivered to the auditory nerve of the brain for processing.

Sense of balance and position is regulated by structures in the inner ear, most notably the semicircular canals and the utricle and saccule in the vestibule. The three semicircular canals correspond to the three dimensions (x, y, and z), and connect to the utricle at an ampulla—a widening of the canal. Within the ampulla are special sensory cells called epithelia and hair cells underneath a substance called gelatinous copula. Each semicircular canal is filled with endolymph as well, and, as the head rotates, the latter is displaced exciting the cells and generating a sense of balance. 

Balance related to the forward and back as well as upward and downward motion of the head and body is regulated by the utricle and saccule. These structures contain cells called macula, which are the primary sensory apparatus for this type of balance, and, like epithelia, they contain hair cells. Macula in the utricle are associated with forward and back mobility, whereas those in the saccule are involved in detecting vertical or downward movement. As with the semicircular canals, the motion of the head displaces these hairs and provides signaling for the sensation of motion.

Associated Conditions

Many diseases and health conditions can affect the functioning of the ear, in terms of both hearing and balance. There are a great many to be mindful of, but the most common of these include:

  • Tinnitus: This persistent ringing in the ear can be subjective—likely occurring due to abnormal activity in the auditory nerve of the brain—or objective, in which a muscle spasm or other process in the middle ear is the cause. Tinnitus may be the result of age-related hearing loss, overexposure to loud noises, physical injury, Meniere’s disease (see below), or neurological disorders. Treatment may include correcting the hearing loss with hearing aids, modifying lifestyle, or cognitive behavioral therapy (CBT).
  • Vertigo: Simply put, this is an improper and consistent perception of dizziness, which can be so severe as to prevent the ability to stand or walk. Like tinnitus it can be the product of Meniere’s disease, certain types of migraine headaches, infections, stroke, multiple sclerosis, or other neurological conditions. Treatment varies based on the underlying cause of the condition, from taking certain medications to making lifestyle changes, among other therapies.
  • Meniere’s disease: Also known as idiopathic endolymphatic hydrops, this disorder of the inner ear is a major cause of vertigo and can lead to tinnitus, fluctuations in hearing ability, pain, headaches, nausea, and other symptoms. Not entirely understood by doctors, this condition is thought to be related to changes in fluid levels within the inner ear. Incurable, it’s managed by treating symptoms or working preventatively. Lifestyle changes may be recommended to address high blood pressure which can contribute to Meniere’s disease. Certain medications may also be prescribed. Some combat nausea, like dexamethasone (Decadron) and Phenegran, while there are others like the sedative lorazepam (Ativan). 
  • Inflammation: Infections of the ear are quite common and can vary in terms of location and severity. Among the more common is otitis media, an infection of the middle ear. Another frequently seen type is infection of the outer ear, commonly known as swimmer’s ear. Symptoms include pain in the ear, fever, feeling of pressure in the ear, as well as inability to sleep. Since bacteria are at the root cause of these conditions, antibiotics are prescribed to take on these issues. If untreated, these conditions can leave lasting damage within the ear.
  • Deafness: Hearing loss up to and including deafness is another common pathology of the ear, and types are often divided based on which part of the ear is affected. Among these forms is high tone deafness (sensorineural hearing loss), which occurs due to damage caused by overexposure to loud sounds. This type can be managed with the use of hearing aids or cochlear implants.
  • Impacted cerumen: The excessive build-up of ear wax (cerumen), can affect hearing ability and blocks passages between outer and middle ear. This wax can be physically removed to treat the condition.
  • Auricular hematoma: Due to bleeding within parts of the ear, this condition, in which blood collects within tissues, arises. This collection of blood can then negatively impact the supply that reaches parts of the ear. This is often the result of trauma or injury, and it’s usually treated by careful draining of the problematic area. 

Tests

Ear infection diagnosis
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A range of medical tests and examinations are administered to assess physical health of the ear as well as sense of hearing. Most common of these are:

  • Otoscopy: This is the most commonly administered test and essentially involves the doctor examining the ear canal using a special tool called an otoscope. Infection of the middle and outer ear, as well as a host of other problems, can be seen visually.
  • Pure-tone testing: Administered to assess overall audition, this test involves patients wearing headphones and having to raise a hand if and when they hear certain tones. The doctor notes the quietest sounds that a person can hear at different pitches.
  • Speech testing: Hearing loss can also be tested by having patients repeat certain words or phrases played at specific volumes.
  • Tympanometry: To test the motion and health of the tympanic membrane, doctors will insert a small probe into each ear, which will push air into each one. Among other conditions, understanding the motion of this part can tell audiologists of a person has an ear infection.
  • Acoustic reflex measure: Among the tests to assess extent of hearing loss, acoustic reflex measure seeks to stimulate some of the musculature in the middle ear. The extent to which there is stimulation says a great deal about how well the person is hearing, with less activity (or complete absence of response) a sign of deafness or sensory loss.
  • Static acoustic impedance: Rupture, holes in, build-up of fluid behind, blockage, or other issues with the tympanic membrane are measured using this test. Basically, it looks at how much air there is in the ear canal.
  • Auditory brainstem response (ABR) test: A test of inner ear function (as well as neural pathways from there), this examination involves using electrodes placed on the skin to measure brain activity in response to stimuli.
  • Otoacoustic emissions (OAE) test: Another way to assess the inner ear is by looking at otoacoustic emissions (OAEs), which are the sounds emitted by the vibrations of hair cells in response to stimulus. The level of OAE is, therefore, a reliable test of hearing ability. This test is performed by inserting a small, specialized probe into the ear that both emits sounds and measures the response.  
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