Hearing: Ear Anatomy, Sound, Transduction & Auditory Pathway

Outer ear

Consists of structures that collect sound and direct sound waves toward the tympanic membrane.

Auricle (pinna)

Shell-shaped structure made of elastic cartilage covered with skin, except for the fleshy lobule; funnels sound waves into the auditory canal.

Lobule

Fleshy inferior portion of the auricle.

External auditory canal (acoustic meatus)

Slightly curved tunnel through the temporal bone that carries sound waves toward the tympanic membrane.

Ceruminous glands

Glands lining the external auditory canal that secrete cerumen.

Cerumen

Earwax that lubricates and waterproofs the auditory canal and traps debris.

Tympanic membrane (eardrum)

Thin sheet of epithelium and connective tissue separating the outer and middle ear; vibrates when struck by sound waves.

Middle ear

Hollow, air-filled chamber in the temporal bone lined with a mucous membrane.

Pharyngotympanic (auditory) tube

Tube connecting the middle ear with the nasopharynx that equalizes pressure on both sides of the tympanic membrane.

When the pharyngotympanic tube opens

Opens when we yawn or swallow.

Why ears feel pressure during elevation changes

Unequal pressure causes the tympanic membrane to stretch and bulge inward or outward, reducing its ability to vibrate.

Auditory ossicles

Three small bones of the middle ear; the smallest bones in the body.

Three auditory ossicles

Malleus, incus, and stapes.

Malleus

Hammer-shaped auditory ossicle connected to the tympanic membrane.

Incus

Anvil-shaped middle auditory ossicle.

Stapes

Stirrup-shaped auditory ossicle attached to the oval window.

Oval window

Membrane-covered opening connecting the middle ear to the inner ear; receives vibrations from the stapes.

Tensor tympani muscle

Pulls the tympanic membrane medially and tenses it.

Stapedius muscle

Reduces movement of the tympanic membrane and ossicles to help prevent damage from loud sounds; also contracts during eating and speaking.

Otitis media

Inflammation of the middle ear cavity.

Why otitis media is common in children

Children have shorter and more horizontal pharyngotympanic tubes, making it easier for pathogens to reach the middle ear.

Acute otitis media

Infection in which pathogens travel from the nasopharynx through the pharyngotympanic tube, causing fluid or pus buildup in the middle ear.

Myringotomy

Procedure in which a small hole is made in the tympanic membrane to drain the middle ear; a tube may be inserted to help prevent recurrent infection.

Inner ear

Cavity within the temporal bone containing the bony and membranous labyrinths.

Bony labyrinth

Mazelike series of tunnels in the inner ear.

Membranous labyrinth

Structure lining the inner walls of the bony labyrinth and filled with endolymph.

Endolymph

Fluid within the membranous labyrinth containing a higher concentration of K+ than Na+.

Perilymph

Fluid between the bony and membranous labyrinths containing a higher concentration of Na+ than K+.

Vestibule

Inner-ear region containing the utricle and saccule and involved in equilibrium.

Semicircular canals

Inner-ear structures containing semicircular ducts that detect rotational movement of the head.

Cochlea

Spiral-shaped inner-ear structure involved in hearing.

Round window

Membrane-covered opening of the cochlea that expands to allow movement of perilymph.

Cochlear duct (scala media)

Endolymph-filled chamber of the cochlea that connects with the saccule.

Modiolus

Screw-shaped core of bone around which the cochlea spirals.

Scala vestibuli

Perilymph-filled cochlear chamber continuous with the vestibule.

Scala tympani

Perilymph-filled cochlear chamber that ends near the round window.

Vestibular epithelium

Forms the boundary between the cochlear duct and scala vestibuli.

Basilar membrane

Membrane separating the cochlear duct from the scala tympani; supports the spiral organ and becomes more flexible toward the tip of the cochlea.

Spiral organ (organ of Corti)

Sensory organ for hearing located within the cochlear duct and supported by the basilar membrane.

Cochlear portion of CN VIII

Part of the vestibulocochlear nerve that carries auditory information from the cochlea to the CNS.

Sound waves

Displacement of air molecules caused by a moving object; adjacent molecules collide and transfer energy outward from the vibrating object.

Pitch (frequency)

Determined by how many times an object vibrates during a given period.

Hertz (Hz)

Unit of frequency equal to one cycle per second.

Human hearing frequency range

Approximately 20–20,000 Hz.

Loudness (amplitude)

Determined by the difference between regions of high and low air pressure in a sound wave.

Decibel (dB)

Unit used to measure sound loudness.

First step of sound transmission

The auricle funnels sound waves into the auditory canal toward the tympanic membrane.

Effect of sound waves on the tympanic membrane

Sound waves cause the tympanic membrane to vibrate.

Ossicle movement

Tympanic membrane vibration moves the auditory ossicles, causing the oval window to vibrate at the same frequency.

Function of the ossicles in sound transmission

Act as a lever system that concentrates and amplifies vibrations on the oval window.

Effect of oval window vibration

Creates pressure waves in the perilymph of the scala vestibuli and endolymph of the cochlear duct.

Effect of inner-ear pressure waves

Cause the basilar membrane to vibrate.

High-frequency sounds

Travel only a short distance into the cochlea and vibrate a narrow, stiff region of the basilar membrane.

Low-frequency sounds

Travel farther into the cochlea and vibrate a wider, more flexible region of the basilar membrane.

Helicotrema

Area at the tip of the cochlea where the scala vestibuli and scala tympani connect; very low-frequency sounds that cannot be heard may travel here.

How pitch is determined

Determined by which area of the basilar membrane vibrates.

How loudness is determined

Determined by how much the basilar membrane vibrates at a particular location.

Effect of a louder sound

Contains more energy and produces stronger movement of the tympanic membrane, ossicles, perilymph, endolymph, and basilar membrane.

Hair cells

Sensory receptor cells of the spiral organ that transduce mechanical vibrations into neural signals.

Inner hair cells

Single row of about 3,500 hair cells primarily responsible for detecting sound.

Outer hair cells

Three rows of hair cells that increase sensitivity of the inner hair cells and amplify basilar membrane vibration.

Stereocilia

Microvilli projecting from hair cells that bend during sound transduction.

Tip links

Filaments connecting stereocilia.

Tectorial membrane

Membrane that sits on the outer hair cells and above the inner hair cells.

First step of hair-cell transduction

Basilar membrane moves toward the tectorial membrane, bending stereocilia toward the tallest stereocilium.

Effect of bending stereocilia toward the tallest stereocilium

Opens K+ channels in the hair cell.

Why K+ enters auditory hair cells

Endolymph contains a very high K+ concentration, so K+ moves into the hair cells when channels open.

Effect of K+ entering a hair cell

Depolarizes the hair cell.

Effect of hair-cell depolarization

Triggers neurotransmitter release, causing action potentials in axons of the cochlear nerve.

Hair-cell transduction sequence

Basilar membrane moves → stereocilia bend toward tallest stereocilium → K+ channels open → K+ enters → hair cell depolarizes → neurotransmitter released → cochlear nerve action potentials.

What happens when the basilar membrane moves away from the tectorial membrane

Stereocilia bend in the opposite direction, K+ channels close, and the hair cell hyperpolarizes.

Percentage of auditory neurons contacting inner hair cells

About 90%.

Primary role of outer hair cells

Increase sensitivity of inner hair cells and amplify vibration of the basilar membrane.

Tinnitus

Perception of sound when no external sound is present, often described as ringing, buzzing, whistling, or hissing.

Most common cause of tinnitus

Bent or misshapen stereocilia that release neurotransmitters onto cochlear nerve endings.

Other possible causes of tinnitus

Fused auditory ossicles, inflamed tympanic membrane, certain medications, or damage to neural pathways.

Conduction hearing loss

Hearing loss caused by an outer- or middle-ear problem that prevents sound waves from reaching the inner ear.

Causes of conduction hearing loss

Excess cerumen, middle-ear infection, perforated tympanic membrane, or fused auditory ossicles.

Bone conduction

Transmission of vibrations through the temporal bone directly to inner-ear fluid; occurs with less accuracy.

Sensorineural hearing loss

Hearing loss caused by a defect in the cochlea, cochlear nerve, or CNS auditory pathways.

Sensory hearing loss

Failure to generate action potentials in the cochlea, usually because of hair-cell dysfunction.

Causes of sensory hearing loss

Loud sounds, certain medications, or aging.

Neural hearing loss

Failure of action potentials to propagate through the cochlear branch of CN VIII or CNS auditory pathways.

Causes of neural hearing loss

May include strokes or tumors such as an acoustic neuroma.

Cochlear implant

Device with surgically implanted electrodes that bypass damaged hair cells and directly stimulate cochlear nerve endings.

Auditory pathway step 1

Action potentials travel through the cochlear portion of CN VIII to the cochlear nuclei at the medulla-pons junction.

Auditory pathway step 2

Axons from the cochlear nuclei synapse in the superior olivary nucleus of the pons.

Superior olivary nucleus

Compares information from both ears to determine the location of a sound.

Auditory pathway step 3

Auditory stimuli travel to the inferior colliculus of the midbrain.

Inferior colliculus in hearing

Coordinates auditory stimuli with head and trunk movement during the startle reflex.

Startle reflex

Auditory reflex that causes the head to turn in response to a sound.

Auditory pathway step 4

Auditory information is relayed to the medial geniculate nucleus of the thalamus.

Medial geniculate nucleus

Thalamic relay that may help process combinations of sounds based on frequency and time interval.

Auditory pathway step 5

Thalamic neurons stimulate the primary auditory cortex in the superior temporal lobe.

Primary auditory cortex

Area where conscious awareness of sound begins and location, pitch, and loudness are analyzed.

Auditory association connections

Auditory information connects with brain areas involved in language, emotions, and memory.

Complete auditory pathway

Cochlear nerve (CN VIII) → cochlear nuclei → superior olivary nucleus → inferior colliculus → medial geniculate nucleus of thalamus → primary auditory cortex.