- Overview of the ear
- Anatomy of the ear
- Physiology of the middle ear
- Physiology of the inner ear
- Equilibrium – Coordination and Balance
- Dysfunctions of the ear
- Treatments for hearing loss
The ear is the sense organ that enables us to hear. Hearing can be defined as the perception of sound energy via the brain and central nervous system. Hearing consists of two components: identification of sounds (what the sound is) and localisation of those sounds (where the sounds are coming from). The ear is divided into three main parts – the outer ear, the middle ear, and the inner ear. The inner ear is filled with fluid. The inner ear also contains the receptors for sound which convert fluid motion into electrical signals known as action potentials that are sent to the brain to enable sound perception. The airborne sound waves must therefore be channelled toward and transferred into the inner ear for hearing to occur. The role of the outer and middle ear is to transmit sound to the inner ear. They also help compensate for the loss in sound energy that naturally occurs when the sound waves pass from air into water by amplifying the sound energy during the process of sound transmission. In addition to converting sound waves into nerve action potentials, the inner ear is also responsible for the sense of equilibrium, which relates to our general abilities for balance and coordination.
The outer ear acts as a funnel to conduct air vibrations through to the eardrum. It also has the function of sound localisation. Sound localisation for sounds approaching from the left or the right is determined in two ways. Firstly, the sound wave reaches the ear closer to the sound slightly earlier than it reaches the other ear. Secondly, the sound is less intense when it reaches the second ear, because the head acts as a sound barrier, partially disrupting the spreading of the sound waves. All these cues are integrated by the brain to determine the location of the source of the sound. It is therefore difficult to localise sound with only one ear. The outer ear consists of the pinna and the ear canal.
The pinna is a prominent skin-covered flap located on the side of the head, and is the visible part of the ear externally. It is shaped and supported by cartilage except for the earlobe. It collects sound waves and channels them down the external ear canal through patterns formed on the pinna known as whorls and recesses. Its shape also partially shields sound waves that approach the ear from the rear, therefore enabling a person to tell whether a sound is coming directly from the front or the back.
The ear canal is roughly 3cm long in adults and slightly S-shaped. It is supported by cartilage at its opening, and by bone for the rest of its length. Skin lines the canal, and contains glands that produce secretions that mix with dead skin cells to produce cerumen (earwax). Cerumen, along with the fine hairs that guard the entrance to the ear canal, helps prevent airborne particles from reaching the inner portions of the ear canal, where they could accumulate or injure the eardrum and interfere with hearing. Cerumen usually dries up and falls out of the canal. However, it can sometimes become impact and disrupt hearing.
The middle ear is located between the external and inner ear. It is separated from the ear canal of the outer ear by the tympanic membrane (the eardrum). The middle ear functions to transfer the vibrations of the eardrum to the inner ear fluid. This transfer of sound vibrations is possible through a chain of movable small bones, called ossicles, which extend across the middle ear, and their corresponding small muscles.
The tympanic membrane is commonly known as the eardrum, and separates the ear canal from the middle ear. It is about 1cm in diameter and slightly concave (curving inward) on its outer surface. It vibrates freely in response to sound. The membrane is highly innervated, making it highly sensitive to pain. For the membrane to move freely when air strikes it, the resting air pressure on both sides of the tympanic membrane must be equal. The outside of the membrane is exposed to atmospheric pressure (pressure of the environment in which we find ourselves) through the auditory tube, so that the cavity in which it is located, called the tympanic cavity, is continuous with the cells in the jaw and thorat area. Normally, the auditory tube is flattered and closed, but swallowing, yawning and chewing pull the tube open, allowing air to enter or leave the tympanic cavity. This opening of the auditory tube allows air pressure in the middle ear to equilibrate with atmospheric pressure, so that the pressures on both sides of the tympanic membrane become equal to each other. Excessive pressure on either side of the tympanic membrane dampens the sense of the hearing because the tympanic membrane cannot vibrate freely. When external pressure changes rapidly, for example during air flight, the eardrum can bulge painfully because as the pressure outside the ear changes, the pressure in the middle ear remains unchanged. Yawning or swallowing in this instance opens up the auditory tube, allowing the pressure on both sides of the tympanic membrane to equalise, relieving the pressure distortion as the eardrum “pops” back into place. Since the auditory tube connects the jaw/throat areas to the ear, it allows throat infections to spread relatively easily to the middle ear. Middle ear infection is common in children because their auditory tubes are relatively short, compared to adults. This leads to fluid accumulation in the middle ear, which is not only painful but also disrupts the transference of sound across the middle ear. If the infection is left untreated, it can spread from the cells near the jaw, causing meningitis (inflammation of the brain lining). Middle ear infection can also cause the fusion of the ear ossicles, resulting in hearing loss.
The tympanic cavity contains the body’s three smallest bones and two smallest muscles. The bones are also referred to as auditory ossicles, and connect the eardrum to the inner ear. From the outermost to innermost, the bones are called the malleus, incus and stapes.
The malleus is attached to the eardrum. It has a handle that attaches to the inner surface of the eardrum, and a head that is suspended from the wall of the tympanic cavity.
The incus is connected to the malleus on the side closer to the eardrum, and to the stapes on the side closer to the inner ear.
The stapes has an arch and a footplate. This footplate is held by a ringlike piece of tissue in an opening called the oval window, which is the entrance into the inner ear.
- Stapedius and Tensor tympani
As the eardrum vibrates in response to air waves, the chain of inner ear bones are set into motion at the same frequency. The frequency of movement is transmitted across from the eardrum to the oval window (another structure in the ear), resulting in a pressure being exerted on the oval window with each vibration. This produces wavelike movements of the inner ear fluid at the same frequency as the original sound wave. However, in order to set the fluid into motion, greater pressure is required, so that the pressure must be amplified. This amplification of the pressure of the airborne sound wave to set up fluid vibrations in the cochlea is related to two mechanisms. Firstly, the surface area of the tympanic membrane is much large than that of the oval window. In addition, the lever action of the ossicles greatly increases the force exerted on the oval window. The extra pressure generated through these mechanisms is sufficient to set the cochlea fluid in motion.
The inner ear is the deepest part of the whole ear, and is located in a place known as the bony labyrinth, which is a maze of bone passageways lined by a network of fleshy tubes known as the membranous labyrinth. A cushion of fluid, called perilymph, lies between the bony and membranous labyrinth, while a fluid called endolymph is found within the membranous labyrinth itself. Within the inner ear is a chamber called the vestibule, which plays a major role in the sense of balance. Balance is further discussed later in this article. (Equilibrium – Coordination and Balance)
Arising from the vestibule is the cochlea, which is sometimes referred to as the organ of hearing, as it is the part of the whole ear that actually converts sound vibrations to the perception of hearing. The cochlea is in the form of a snail-like spiral, so that a longer cochlea is able to fit inside an enclosed space. It is about 9mm wide at the base and 5mm high, and winds around a section of spongy bone called the modiolus. The modiolus is shaped like a screw whose threads form a spiral platform that support the cochlea, which is fleshy and unable to support itself.
The cochlea contains three fluid-filled chambers separated by membranes. The upper chamber, scala vestibule, and the bottom chamber, scala tympani, are filled with perilymph. The scala tympani is covered by a secondary tympanic membrane. The middle chamber is the scala media, or the cochlea duct. It is filled with endolymph, instead of perilymph.
The organ of corti is supported by a membrane called the basilar membrane. It about the size of a pea, and acts as a transducer, converting vibration into nerve impulses. It has hair cells and supporting cells. Hair cells have long stiff microvilli called stereocilia on their apical surfaces. Microvilli are fine hair-like structures on cells that help to increase cell surface area. On top of these stereocilia is a jelly-like membrane called the tectorial membrane. Four rows of hair cells spiral along the length of the organ of Corti. Of these, there are about 3500 inner hair cells (IHCs), each with a cluster of 50-60 stereocilia graded from short to tall. There are another 20 000 outer hair cells (OHCs) that are arranged in three rows opposite the IHCs. Each OHC has about 100 stereocilia with their tips embedded in the tectorial membrane above them. These outer hair cells adjust the response of the cochlea to different sound frequencies so as to enable the inner hair cells to function more accurately. The physiological mechanisms, by which hair cells within the cochlea act to produce hearing, are discussed in more detail below. (Physiology of the inner ear)
The function of the auditory ossicles in the middle ear is to concentrate the energy of the vibrating eardrum so as to create a greater force per unit area at the oval window, as previously described.
In addition to this, the ossicles and their adjacent muscles also serve a protective function. In response to a loud noise, the tensor tympani pulls the eardrum inward and tenses it. At the same time, the stapedius reduces movement of the stapes. These actions of the muscles are known collectively as the tympanic reflex. This reflex muffles the transfer of vibrations from the eardrum to the oval window. It is thought that the tympanic reflex is an evolutionary adaptation for protection against loud but slowly building noises such as thunder. However, because it has a time delay of about 40 ms, it is not quick enough to protect the inner ear from sudden loud noises such as gunshots. It also does not adequately protect the ears from sustained loud noises such as factory noises or loud music. These noises can irreversibly damage the stereocilia of the hair cells in the inner ear, leading to hearing loss.
The muscles of the middle ear also assist in coordinating speech with hearing, so that the sound of our own speech is not so loud as to damage our inner ear and drown out soft or high-pitched sounds from other sources. Just as we are about to speak, the brain signals the middle ear muscles to contract, dampening the sense of hearing in coordination with the sound of our own voice. This makes it possible to hear other people while we are speaking ourselves.
As previously mentioned, the cochlea is the organ that enables sound perception. The physiology of the cochlea revolves around the functioning of the inner and outer cochlea hair cells. In addition to the cells themselves, there are several other components of the cochlea that contribute to the ability to hear.
The inner hair cells transform the mechanical force of sound (cochlea fluid vibration) into the electrical impulses of hearing (action potentials sending auditory messages to the brain). They communicate with nerve fibres that make up the auditory nerve leading to the brain. When the rate of neurotransmitter (chemicals released by cells in response to stimuli) release from these hair cells is increased, the rate of firing in the nerve fibres is also increased. This occurs when the voltage of the hair cells becomes more positive. Conversely, when the voltage of the hair cells becomes more negative, the hair cells release less neurotransmitter and the firing rate in nerve fibres decreases.
Unlike the inner hair cells, the outer hair cells do not signal the brain about incoming sounds. They instead actively and rapidly elongate in response to changes in the voltages of the cell membrane. This behaviour is known as electromotility. When the outer hair cells elongate, the motion of the basilar membrane is amplified. This modification of the basilar membrane is believed to improve and tune the stimulation of the inner hair cells. The outer hair cells therefore enhance the receptors of the inner hair cells, increasing their sensitivity to sound intensity and rendering them highly discriminatory between various pitches of sound.
The activity of the inner and outer hair cells is possible through various other components within the cochlear. They key components are listed as follows:
The vibration of the auditory ossicles, as previously described, eventually leads to the vibration of the basilar membrane on which the hair cells rest through sequence of chain reactions. During the vibration of the auditory ossicles, the stapes vibrates rapidly in and out, leading to the basilar membrane vibrating down and up, and the secondary tympanic membrane vibrating out and in. This can occur as often as 20 000 times per second.
In order for inner hair cells to function properly, the tips of their stereocilia must be bathed in endolymph, which has an exceptionally high potassium ion (K+) concentration, creating a strong electrochemical gradient (large difference in voltage) from the tip to base of a hair cell. This electrochemical gradient provides the energy that allows the hair cell to function. The interaction between stereocilia and endolymph is further discussed below. (Stereocilia)
The stereocilia of the outer hair cells have their tips embedded in the tectorial membrane, while the stereocilia of the inner hair cells come very close to the membrane. The tectorial membrane is anchored to a structure called the modiolus, which holds it relatively still as the basilar membrane and hair cells vibrate. Vibration of the basilar membrane therefore causes shearing of the hair cells against the tectorial membrane, bending the hair cell stereocilia back and forth.
A protein functions as a mechanically gated ion channel on the top of each stereocilia of the inner hair cells. In addition, there is a fine stretchy protein filament known as a tip link that extends like a spring from the ion channel of one stereocilium to the side of the streocilium next to it. On each inner hair cell, the stereocilia progressively increase in height, so that all but the tallest ones have tip links leading to taller stereocilia beside them. When a taller stereocilium bends away from a shorter one, it pulls on the tip link, so that the ion channel of the short stereocilium is opened. The endolymph bathing the stereocilia has a very high concentration of K+ ions, so that when the channel is pulled open, there is a rapid flow of K+ into each hair cell. This makes the voltage of the hair cell become positive when the channel is open. When the stereocilium is bent the other way, the channel closes and the cell voltage becomes negative. When the cell voltage is positive, the inner hair cells release a neurotransmitter that stimulates the sensory nerves at the base of the hair cell. This leads to the generation of action potentials in the cochlea nerve.
The conversion of sound energy into a neural signal that is interpreted by the brain as sound perception, as described above, is known as sound transduction. The following diagram summarises this process:
Loud vs Soft sounds
The organ of Corti allows us to discriminate between different sound intensities. Loud sounds produce more vigorous vibrations of the organ of Corti, thereby exciting a greater number of hair cells over a greater area of basilar membrane. This leads to a high frequency of action potentials being initiated in the cochlea nerve. Intense activity in the cochlea nerve fibres from a broad region of the organ of Corti is therefore detected by the brain and interpreted as a loud sound. The reverse applies to detect soft sounds.
High-pitched vs Low-pitched sounds
The basilar membrane enables us to differentiate between high and low pitched sounds. The membrane is spanned by short stiff fibres of various lengths. At its lower end, the basilar membrane is attached, narrow and stiff. At the top end, however, it is unattached, wider and more flexible. The vibration of one region of the basilar membrane causes a wave of vibration to travel down its length and back again. This is referred to as a standing wave, and is akin to plucking a string at one end, causing a wave vibration (like on a guitar). The peak amplitude of the standing wave is near the top end during low-frequency sounds and near the bottom end during higher-frequency sounds. When the brain receives signals mainly from inner hair cells at the top end, it interprets this sound as being low-pitched. Likewise, when the brain receives signals mainly from inner hair cells at the bottom end, the sound is interpreted as being high-pitched. In the reality of everyday life, speech, music and other everyday sounds are not pure tones. Instead, they create complex patterns of vibration in the basilar membrane that have to be decoded and interpreted by the brain.
Although we think of the ear as the sense organ for hearing, it did not evolve originally for this purpose. It was instead originally an adaptation for coordination and balance, collectively known as the sense of equilibrium. Vertebrates only evolved the cochlea, middle ear structures and consequent auditory function of the ear later on. In humans, the parts of the ear that allow for the sense of equilibrium are the vestibular apparatus (or the vestibule). These consist of the three semicircular canals, and the two chambers – the saccule and the utricle. There are two components of the sense of equilibrium. One is static equilibrium, referring to the ability to detect the direction of the head when the body is not moving. The second is dynamic equilibrium, referring to the perception of motion or acceleration. Acceleration can in turn be divided into linear acceleration, which is a change in velocity (rapidity) in a straight line, and angular acceleration, which is a change in the rate of rotation of the head. The saccule and utricle detect static equilibrium and linear acceleration, while the semicircular canals only detect angular acceleration.
Both the saccule and the utricle contain a small patch of hair cells and their supporting cells, which collectively are known as a macula. The macula lying vertically on the wall of the saccule is called the macula sacculi, while the macula lying horizontally on the floor of the utricle is called the macula utriculi. Each hair cell of a macula has about 40-70 stereocilia (structures on the hair cells that sense mechanical stimuli), as well as one true cilium (a tail-like cell projection) called a kinocilium. The tips of the stereocilia and the kinocilium are embedded in a jelly-like membrane called the otholithic membrane. This membrane is weighed down by granules which are referred to as otoliths. The otoliths add to the density and inertia of the membrane, aiding in sensing gravity and motion.
Detecting tilt of the head
Horizontal tilt of the head is detected by the macula utriculi, while vertical tilt of the head is detected by the macula sacculi. When the head is upright, the otolithic membrane weighs down directly on the hair cells, keeping stimulation to a minimum. However, when the head is tilted, the weight of the membrane bends the stereocilia, stimulating the hair cells. Any orientation of the head causes a combination of stimulation to the utricles and saccules of both ears. The overall orientation of the head is interpreted by the brain by comparing the inputs from both organs to each other, and to other input from the eyes and stretch receptors in the neck.
Detecting linear acceleration
When we begin to move forward after being stationary, the heavy otolithic membrane of the macula utriculi briefly lags behind the rest of the tissues. When we stops moving, the macula stops as well, but the otolithic membrane keeps moving for a moment, bending the stereocilia forward. The hair cells covert this pattern of stimulation into nerve signals which are relayed to the brain to be interpreted. This results in the brain interpreting changes in linear velocity (ie. detecting linear acceleration). If we begin to move upwards after being stationary (for example, going upwards in an elevator), the otolithic membrane of the vertical macula sacculi lags briefly behind and pulls down on the hair cells. When we stop moving, the otolithic membrane keeps moving for a moment, bending the hair cells upward. The brain therefore receives signals from the macula sacculi, enabling it to interpret vertical acceleration.
Each of the three semicircular canals houses a semicircular duct. Collectively, they detect rotational acceleration. Two ducts are positioned vertically at right angles to each other. The third duct lies at an angle of approximately 30 degrees from the horizontal plane. The different orientations of the three ducts cause different ducts to be stimulated, depending on what plane the head is rotating in. The head can be turned from side to side (eg. Gesturing “no”), up and down (eg. Gesturing “yes”), or tilted from side to side (eg. Touching to ears to each of your shoulders, one at a time). All the semicircular ducts are filled with a fluid called endolymph. Each duct opens into the utricle and has a dilated sac at one end called an ampulla. Inside the ampulla are hair cells and their supporting cells. These are referred to as the crista ampullaris. A jelly-like membrane called the cupula extends from the crista ampullaris to the roof of the ampulla. The stereocilia of the hair cells are embedded in the cupula. As the head turns, the duct rotates, but the endolymph in it lags behind. The endolymph thus pushes against the cupula, causing the stereocilia to bend, stimulating the hair cells. However, after 25-30 seconds of continual rotation, the endolymph catches up with the movement of the duct and stimulation of the hair cell ceases.
Deafness refers to a loss of hearing, which may be temporary or permanent, partial or complete.
Conductive deafness occurs when sound waves are not properly conducted through the external and middle portions of the ear to set the fluid in the inner ear in motion. Possible causes include:
- Physical blockage of the ear canal with earwax
- Eardrum rupture
- Middle ear infection with accompanying fluid accumulation
- Restriction of the movement of the ossicles, due to bony adhesions between the stapes and oval window
In sensorineural deafness, the sound waves are transmitted to the inner ear, but they are not converted into nerve signals that are interpreted by the brain as sounds. The defect can lie in the organ of Corti or the auditory nerves, or rarely, in some pathways and parts of the brain.
Neural prebycusis is one of the most common causes of partial hearing loss. It is a progressive age-related process that occurs over time as the hair cells “wear out” with use. Even exposure to ordinary modern-day sounds can eventually damage hair cells over long periods of time. An adult loses on average more than 40% of their cochlea hair cells by the age of 65. Those hair cells that process high-frequency sounds are the most vulnerable to destruction.
Vertigo refers to the sensation of rotation in the absence of equilibrium – in other words, dizziness. Vertigo can be caused by viral infections, certain drugs, and tumours such as acoustic neuroma. Vertigo can also produced normally in individuals through excessive stimulation of the semicircular ducts. In some individuals, excessive stimulation of the utricle can also produce motion sickness (carsickness, airsickness, seasickness).
Meniere’s Syndrome is a disease of the internal ear affecting both hearing and equilibrium. Patients initially experience episodes of dizziness and tinnitus (ringing noise in the ears), and later develop a low-frequency hearing loss. The causes relate to the blockage of a duct in the cochlea which drains excess endolymph away. Blockage of the duct causes an increase in endolymphatic pressure and swelling of the membranous labyrinth in which the inner ear hair cells are located.
Hearing aids can be useful in treating conductive deafness but are less beneficial for sensorineural deafness. They increase the intensity of airborne sounds and may modify the sound spectrum to suit the patient’s particular pattern of hearing loss at higher or lower frequencies. However, the receptor cell-neural pathway system must still be intact and functioning for the sound to be perceived, so hearing aids are useless in sensorineural deafness.
Recently, cochlear implants have become available. The implants are electronic devices which are surgically implanted. They convert sound signals into electrical signals that can directly stimulate the auditory nerve, so as to bypass a defective cochlea system. Cochlear implants cannot restore normal hearing but they permit recipients to recognise sounds. Success can range from an ability to hear a phone ringing, to being able to carry on a conversation over the telephone.
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