Hearing

Hearing

The physical sense or faculty by which humans and animals perceive sound

Ears

The two organs on the sides of the head used for hearing and balance

Otorhinolaryngology

The science that deals with the ears, nose, and throat

Ear: Regions

The ear is divided into three main regions: the external, middle, and internal ears

External Ear

The visible outer part of the body that collects sound waves and channels them inward toward the eardrum, which also consists of the auricle, external acoustic meatus, and eardrum

External Ear: Auricle

The visible, outer projecting structure of the ear made of elastic cartilage and skin

External Ear: Auricle: Other Structures

The auricle has a helix that forms its outer rim and a lobule that forms its inferior portion, with ligaments and muscles attaching it to the head.

External Ear: External Acoustic Meatus

The curved ear canal that lies in the temporal bone and leads to the eardrum

External Ear: Tympanic Membrane

A thin, cone-shaped simple cuboidal epithelium that separates the external acoustic meatus from the middle ear

External Ear: Tympanic Membrane: Perforated Eardrum

A tear in the tympanic membrane, typically caused by the pressure from a cotton swab, trauma, or a middle ear infection

External Ear: Tympanic Membrane: Otoscope

An instrument that directly illuminates and magnifies the external acoustic meatus and tympanic membrane for examination

External Ear: External Acoustic Meatus: Accessory Structures

The external acoustic meatus contains hairs and ceruminous glands that produce cerumen (earwax), which helps block dust, foreign objects, and water and protects the delicate skin of the ear canal.

External Ear: Accessory Structures: Impacted Cerumen

Some people produce large amounts of cerumen that may impact and can muffle incoming sounds, which can be treated using warm water flushing (irrigation) or specialized manual instruments

Middle Ear

The visible outer part of the body that collects sound waves and channels them inward toward the eardrum, consisting of the auricle, external acoustic meatus, and eardrum.

Middle Ear: Openings

Because the tympanic membrane separates the external and middle ears, middle ear contains two small openings: the vestibular window and cochlear window.

Middle Ear: Auditory Ossicles

The three tiny bones in the middle ear are called the auditory ossicles (malleus, incus, and stapes), which are all connected by synovial joints

Middle Ear: Auditory Ossicles: Malleus

A hammer-like bone is attached directly to the eardrum and transmits sound vibrations to the inner ear.

Middle Ear: Auditory Ossicles: Incus

An anvil-like bone between the malleus and stapes that receives sound vibrations and passes them to the stapes.

Middle Ear: Auditory Ossicles: Stapes

A stirrup-shaped bone whose base fits into the vestibular (oval) window; the cochlear (round) window is below it and is covered by the secondary tympanic membrane.

Middle Ear: Tensor Tympani Muscle

One of the two skeletal muscles attached to the ossicles, controlled by the mandibular branch of the trigeminal (V) nerve, limits ossicle movement and increases tension on the eardrum to protect the inner ear from loud sounds.

Middle Ear: Stapedius

One of the two skeletal muscles attached to the ossicles, controlled by the facial (VII) nerve, dampens large stapes vibrations, protecting the vestibular window but also decreasing hearing sensitivity.

Middle Ear: Hyperacusia

An auditory disorder where everyday environmental sounds are perceived as uncomfortably, painfully, or intolerably loud, due to the paralysis of the stapedius muscle.

Middle Ear: Skeletal Muscles: Drawback

Although tensor tympani and stapedius muscles protect the inner ear from prolonged loud noises, it does not protect the ear from brief noises.

Middle Ear: Auditory Tube

A narrow channel (made of bone and elastic cartilage) connects the middle ear to the nasopharynx, which opens during swallowing or yawning to equalize air pressure on both sides of the tympanic membrane.

Most of us have experienced our ears popping as the pressures equalize. When the pressures are balanced, the tympanic membrane vibrates freely as sound waves strike it. If the pressure is not equalized, intense pain, hearing impairment, ringing in the ears, and vertigo could develop. The auditory tube also is a route for pathogens to travel from the nose and throat to the tympanic cavity, causing the most common type of ear infection (see otitis media in Disorders: Homeostatic Imbalances at the end of this chapter).

Middle Ear: Auditory Tube: Ear Popping and Ringing

When middle-ear pressure is balanced, the tympanic membrane vibrates efficiently and transmits sound properly; when pressure is unbalanced, it can cause pain, ringing in the ears (tinnitus), and vertigo.

Middle Ear: Auditory Tube: Ear Infection

The auditory tube also is a route for pathogens to travel from the nose and throat to the tympanic cavity, causing the most common type of ear infection

Inner Ear

A labyrinth of many canals that are divided into two main division: an outer bony labyrinth that encloses an inner membranous labyrinth

Inner Ear: Bony Labyrinth

The hard, protective outer shell of the inner ear that is carved into the petrous portion of the temporal bone.

Inner Ear: Bony Labyrinth: Division

The bony labyrinth are divided into three areas: the semicircular canals, the vestibule, and the cochlea.

Inner Ear: Bony Labyrinth: Lining

The bony labyrinth is lined with periosteum and filled with perilymph, which surrounds the membranous labyrinth containing the receptors for hearing and equilibrium.

Inner Ear: Bony Labyrinth: Membranous Labyrinth

A series of fluid-filled epithelial sacs and tubes inside the inner ear that controls hearing and balance

Inner Ear: Bony Labyrinth: Endolymph

The epithelial membranous labyrinth contains endolymph, a specialized potassium-rich fluid, which helps control hearing and balance

Inner Ear: Bony Labyrinth: Vestibule

A small, oval chamber located in the oval central portion of the bony labyrinth, consisting two sacs called utricle and saccule, which are connected by a small duct

Inner Ear: Bony Labyrinth: Semicircular Canals

The three interconnected, fluid-filled tubes located in the inner ear that detect rotational head movements and help maintain balance, with a swollen enlargment (ampulla) at one end of each canal

Inner Ear: Bony Labyrinth: Semicircular Ducts

The parts of the membranous labyrinth located inside the bony semicircular canals and connect to the utricle of the vestibule.

Inner Ear: Bony Labyrinth: Nerves

The vestibular branch of the vestibulocochlear (VIII) nerve consists of ampullary, utricular, and saccular nerves, which contain first-order sensory neurons that carry information from equilibrium receptors and efferent neurons that send feedback to adjust receptor sensitivity

Inner Ear: Bony Labyrinth: Vestibular Ganglia

Cell bodies of the sensory neurons are located in the vestibular ganglia

Inner Ear: Bony Labyrinth: Cochlea

A small, snail-shell-shaped cavity, which makes three turns around a central bony core called the modiolus, converts sound vibrations into electrical signals for the brain

Inner Ear: Bony Labyrinth: Cochlea: Division

Sections through the cochlea reveal that it is divided into three channels: cochlear duct, scala vestibuli, and scala tympan

Inner Ear: Bony Labyrinth: Cochlea: Cochlear Duct

A continuation of the membranous labyrinth into the cochlea which is filled with endolymph

Inner Ear: Bony Labyrinth: Cochlea: Scala Vestibuli

A perilymph-filled channel above the cochlear duct ends at the vestibular window

Inner Ear: Bony Labyrinth: Cochlea: Scala Tympani

A perilymph-filled channel below the cochlear duct ends at the cochlear window

Inner Ear: Bony Labyrinth: Cochlea: Helicotrema

A small opening at the apex of the cochlea that connects the scala vestibuli and scala tympani, allowing fluid to pass between them

Inner Ear: Bony Labyrinth: Cochlea: Vestibular Membrane

The vestibular membrane separates the cochlear duct from the scala vestibuli.

Inner Ear: Bony Labyrinth: Cochlea: Basilar Membrane

The basilar membrane separates the cochlear duct from the scala tympani.

Inner Ear: Bony Labyrinth: Cochlea: Spiral Organ

The spiral organ is a coiled sheet of epithelial cells, resting on the basilar membrane, containing supporting cells and about 16,000 hair cells.

Inner Ear: Bony Labyrinth: Cochlea: Hair Cells

The spiral organ contains inner hair cells in one row and outer hair cells in three rows, and these hair cells are the sensory receptors for hearing.

Inner Ear: Bony Labyrinth: Cochlea: Stereocilia

Stereocilia are long, hairlike microvilli on the apical tips of hair cells that extend into the endolymph of the cochlear duct.

Inner Ear: Bony Labyrinth: Cochlea: Spiral Organ: Synapses

The bases of both inner and outer hair cells form synapses with first-order sensory neurons and motor neurons from the cochlear branch of the vestibulocochlear (VIII) nerve.

Inner Ear: Bony Labyrinth: Cochlea: Spiral Ganglion

The spiral ganglion contains the cell bodies of the first-order sensory neurons involved in hearing.

Inner Ear: Bony Labyrinth: Cochlea: Sensory Neurons

The inner hair cells form a single row and synapse with about 90–95% of the first-order sensory neurons; the numerous outer hair cells form far fewer synapses with these neurons.

Inner Ear: Bony Labyrinth: Cochlea: Motor Neurons

About 90% of the motor neurons in the cochlear nerve synapse with outer hair cells.

Inner Ear: Bony Labyrinth: Cochlea: Tectorial Membrane

The tectorial membrane is a flexible, gelatinous membrane that covers the hair cells of the spiral organ and has the ends of stereocilia embedded in it

Inner Ear: Bony Labyrinth: Cochlea: Basilar Membrane: Hair Cells

The bodies of the hair cells in the spiral organ rest on the basilar membrane.

Inner Ear: Bony Labyrinth: Cochlea: Inner Hair Cells: Function

Inner hair cells are the primary receptors for hearing and convert mechanical vibrations of sound into electrical signals.

Inner Ear: Bony Labyrinth: Cochlea: Outer Hair Cells: Function

Outer hair cells increase the sensitivity of inner hair cells and do not primarily serve as hearing receptors.

Sound Waves

An alternating high- and low-pressure region traveling in the same direction through some medium (such as air), originating from a vibrating object.

Sound Waves: Pitch

The frequency of a sound vibration; the higher the frequency, the higher the pitch.

Sound Waves: Pitch: Human Range

A human ear can hear pitches ranging from 500 Hz to 5000Hz (20 kHz)

Sound Waves: Pitch: Audible Range

The entire audible range extends from 20 to 20,000 Hz

Decibels

A measurement that measures how intense (size or amplitude) of the vibration.

Decibels Range

The hearing threshold—the point at which an average young adult can just distinguish sound from silence—is defined as 0 dB at 1000 Hz.

Auditory Pathway: Step 1

The auricle directs sound waves into the external acoustic meatus, which then strike against the tympanic membrane into alternating waves of high and low pressure in the air.

Auditory Pathway: Step 2

The tympanic membrane vibrates slowly for low-frequency (low-pitched) sounds and rapidly for high-frequency (high-pitched) sounds in response to the sound waves.

Auditory Pathway: Step 3

The tympanic membrane transfers its vibrations through the ossicles in order: malleus → incus → stapes.

Auditory Pathway: Step 4

As the stapes moves back and forth, its footplate vibrates in the vestibular window, producing vibrations 20 times stronger than those of the tympanic membrane because the ossicles concentrate weak vibrations from the large tympanic membrane into the much smaller vestibular window.

Auditory Pathway: Step 5

The movement of stapes pushes on the vestibular window, creating pressure waves in the perilymph of the cochlea, as the window bulges inward and pushes the fluid in the scala vestibuli.

Auditory Pathway: Step 6

The stapes pushes on the vestibular window, creating pressure waves in the perilymph of the cochlea as the vestibular window bulges inward and pushes the fluid in the scala vestibuli.

Auditory Pathway: Step 7

The pressure waves travel from the scala vestibuli → scala tympani → cochlear (round) window, causing the cochlear window to bulge outward into the middle ear.

Auditory Pathway: Step 8

The pressure waves in the scala vestibuli and scala tympani push the vestibular membrane back and forth, creating pressure waves in the endolymph of the cochlear duct.

Auditory Pathway: Step 9

Pressure waves in the endolymph vibrate the basilar membrane, causing the hair cells against the tectorial membrane to bend their stereocilia and generating nerve impulses in first-order cochlear nerve neurons.

Basilar Membrane: Frequency Tuning

Different sound frequencies cause different regions of the basilar membrane to vibrate most intensely because each region is tuned to a particular pitch.

Basilar Membrane: High-Frequency Sounds

High-frequency (high-pitched) sounds produce their greatest vibrations near the base of the cochlea, because the membrane is narrower and stiffer there

Basilar Membrane: Low-Frequency Sounds

Low-frequency (low-pitched) sounds produce their greatest vibrations near the apex of the cochlea, because the membrane is wider and flexible there

Basilar Membrane: Loudness

The loudness of a sound depends on its intensity: higher-intensity sounds cause larger basilar membrane vibrations, leading to a higher frequency of nerve impulses and often stimulating more hair cells.

Basilar Membrane: Pitch vs. Loudness

Pitch depends mainly on the location of maximal basilar membrane vibration, while loudness depends mainly on the intensity of vibration and resulting neural activity.

Sound Transduction

The process where the ear converts physical sound waves into electrical signals that the brain understands

Sound Transduction: Inner Hair Cells

Inner hair cells convert mechanical vibrations produced by sound into electrical signals that can be transmitted to the brain.

Sound Transduction: Stereocilia Movement

As the basilar membrane vibrates, the stereocilia on hair cells bend back and forth, opening mechanically gated cation channels in the membrane

Sound Transduction: Potassium Entry

When mechanically gated cation channels open, K⁺ from the endolymph enters the hair cell cytosol, producing a depolarizing receptor potential.

Sound Transduction: Tip Links

A tip link is a protein that connects a mechanically gated cation channel to the tip of the neighboring taller stereocilium and helps convert sound vibrations into electrical signals.

Sound Transduction: Resting State

At rest, the stereocilia stand upright and partially open mechanically gated cation channels, allowing a few K+ ions to enter and produce a weak depolarizing receptor potential.

Sound Transduction: Resting Receptor Potential

The weak receptor potential opens a few voltage-gated Ca2+ channels, allowing a few Ca2+ ions to enter the hair cell, triggering exocytosis of a small number of neurotransmitter-containing vesicles.

Sound Transduction: Resting Nerve Impulse Frequency

Low neurotransmitter release produces a low frequency of nerve impulses in the first-order auditory neuron.

Sound Transduction: Active State

When stereocilia bend toward the tallest stereocilium, this fully stretches tip links and fully opens the cation channels, allowing a large influx of K+ ions and producing a stronger depolarizing receptor potential.

Sound Transduction: Hair Cells: Active Receptor Potential

Complete opening of mechanically gated cation channels allows more K⁺ to enter, producing a strong depolarizing receptor potential.

The strong receptor potential opens more voltage-gated Ca2+ channels, triggering exocytosis of a large number of neurotransmitter-containing vesicles.

Hair Cells: Strong Depolarization

Strong depolarization opens more voltage-gated Ca²⁺ channels and causes greater neurotransmitter release.

Hair Cells: Increased Nerve Impulses

More neurotransmitter release produces a higher frequency of nerve impulses in the first-order auditory neuron.

Stereocilia: Bending Away from Tallest Stereocilium

When stereocilia bend away from the tallest stereocilium, the tip links become slack and the mechanically gated cation channels close.

Hair Cells: Hyperpolarization

When cation channels close, K⁺ cannot enter the hair cell, causing the membrane to become more negative and producing a hyperpolarizing receptor potential.

Hair Cells: Hyperpolarization Effect

Hyperpolarization causes little neurotransmitter release and results in very few nerve impulses in the first-order auditory neuron.

Hair Cells: Stereocilia Direction

Bending stereocilia toward the tallest stereocilium increases depolarization and auditory signaling, while bending them away decreases signaling.

Hair Cell: Sound Transduction Sequence

Stereocilia bend → mechanically gated cation channels open or close → K⁺ movement changes membrane potential → Ca²⁺ channels respond → neurotransmitter release changes → auditory nerve impulse frequency changes.

Otoacoustic Emissions: Definition

Otoacoustic emissions are usually inaudible sounds produced by vibrations of the outer hair cells of the cochlea.

Otoacoustic Emissions: Cause

Outer hair cells can vibrate in response to sound waves and signals from efferent neurons, producing otoacoustic emissions.

Outer Hair Cells: Movement

When outer hair cells depolarize and repolarize, they rapidly shorten and lengthen.

Outer Hair Cells: Tectorial Membrane

Movement of outer hair cells appears to change the stiffness of the tectorial membrane.

Outer Hair Cells: Amplification

The vibrations of outer hair cells are thought to enhance movement of the basilar membrane, amplifying responses of the inner hair cells.

Otoacoustic Emissions: Pathway

Vibrations of outer hair cells create a traveling wave that moves back toward the stapes and exits the ear as an otoacoustic emission.

Otoacoustic Emissions: Hearing Test

Otoacoustic emissions can be detected with a sensitive microphone placed near the eardrum and provide a fast, inexpensive, noninvasive way to screen newborns for hearing problems.