Ch. 15 Special Senses: Hearing Slides mostly © Marieb & Hoehn 9 th ed. Other slides by WCR

Ch. 15Special Senses: Hearing

Slides mostly © Marieb & Hoehn 9th ed.Other slides by WCR

© 2013 Pearson Education, Inc.

The Ear: Hearing and Balance

• Three major areas of ear1. External (outer) ear – hearing only

2. Middle ear (tympanic cavity) – hearing only

3. Internal (inner) ear – hearing and equilibrium• Receptors for hearing and balance respond to

separate stimuli• Are activated independently


Figure 15.24a Structure of the ear.

Externalear

Middleear

Internal ear(labyrinth)

Auricle(pinna)

Helix

Lobule

Externalacousticmeatus

Tympanicmembrane

Pharyngotympanic(auditory) tube

The three regions of the ear


External Ear

• Auricle (pinna) & external acoustic meatus (auditory canal)– Funnel sound waves to eardrum

• Tympanic membrane (eardrum)– Boundary between external and middle ears– Connective tissue membrane that vibrates in

response to sound– Transfers sound energy to bones of middle ear


Middle Ear

• Air-filled (usually), mucosa-lined cavity in temporal bone– Flanked laterally by eardrum– Remaining borders are formed by by temporal bone– Oval window, round windows: covered connections to

the inner ear

• Contains 3 bones, 2 muscles• Pharyngotympanic (auditory) tube

– Connects middle ear to nasopharynx– Equalizes pressure in middle ear cavity with external

air pressure


Ear Ossicles

• Three small bones in tympanic cavity: the malleus, incus, and stapes– Suspended by ligaments and joined by

synovial joints– Transmit vibratory motion of eardrum to oval

window– Tensor tympani and stapedius muscles

contract reflexively in response to loud sounds to prevent damage to hearing receptors


Figure 15.24b Structure of the ear.

Oval window(deep to stapes)

Semicircularcanals

Vestibule

Vestibularnerve

Cochlearnerve

Cochlea

Pharyngotympanic(auditory) tube

Entrance to mastoid antrum in the epitympanic recess

Auditoryossicles

Tympanic membrane

Round window

Stapes(stirrup)

Incus(anvil)

Malleus(hammer)

Middle and internal ear


Otitis Media

• Middle ear inflammation– Especially in children

• Shorter, more horizontal pharyngotympanic tubes• Most frequent cause of hearing loss in children

– Most treated with antibiotics– Myringotomy to relieve pressure if severe


Figure 15.25 The three auditory ossicles and associated skeletal muscles.

View

Superior

Anterior

Lateral

IncusMalleusEpitympanic

recess

Pharyngotym-panic tube

Tensortympanimuscle

Tympanicmembrane(medial view)

Stapes Stapediusmuscle


Two Major Divisions of Internal Ear

• Bony labyrinth– Tortuous channels in temporal bone– Three regions: vestibule, semicircular

canals, and cochlea – Filled with perilymph – similar to CSF

• Membranous labyrinth– Series of membranous sacs and ducts– Filled with potassium-rich endolymph


Figure 15.26 Membranous labyrinth of the internal ear.

Temporalbone

Facial nerve

Vestibular nerve

Superior vestibularganglionInferior vestibularganglionCochlear nerveMaculaeSpiral organ

Cochlear ductin cochlea

Round windowStapes inoval window

Saccule investibule

Utricle investibule

Cristae ampullaresin the membranousampullae

LateralPosteriorAnterior

Semicircular ductsin semicircularcanals


The Cochlea

• Spiral, conical, bony chamber– Size of split pea, goes from base to apex– Contains cochlear duct, which houses organ of Corti

(spiral organ)

• Cavity of cochlea divided into three chambers– Scala vestibuli—abuts oval window & stapes,

contains perilymph– Scala media (cochlear duct)—contains endolymph– Scala tympani—terminates at round window; contains

perilymph

• Scala tympani, scala vestibuli connect with each other at helicotrema (apex)


The Cochlea

• Cochlear duct (scala media) is sandwiched between scala vestibuli & scala tympani

• "Floor" of cochlear duct formed by basilar membrane, which supports organ of Corti (spiral organ)

• Cochlear branch of nerve VIII runs from cochlea to brain


Figure 15.27a Anatomy of the cochlea.

Helicotremaat apex

Modiolus

Cochlear nerve,division of thevestibulocochlearnerve (VIII)

Spiral ganglion

Osseous spiral lamina

Vestibular membrane

Cochlear duct(scala media)


Figure 15.27b Anatomy of the cochlea.

Vestibular membrane

Tectorial membrane

Cochlear duct(scala media;containsendolymph)

Striavascularis

Spiral organ

Basilarmembrane

Scala vestibuli(containsperilymph)

Scala tympani(containsperilymph)

Osseous spiral lamina

Spiralganglion


Tectorial membrane

Hairs (stereocilia)

Outer hair cells

Supporting cells

Inner hair cell

Afferent nervefibers

Fibers ofcochlearnerve

Basilarmembrane

Figure 15.27c Anatomy of the cochlea.


Figure 15.27d Anatomy of the cochlea.

Innerhaircell

Outerhaircell


Properties of Sound

• Sound is – Pressure disturbance (alternating areas of

high and low pressure) produced by vibrating object

• Sound wave– Moves outward in all directions– Illustrated as an S-shaped curve or sine wave


Figure 15.28 Sound: Source and propagation.

Area ofhigh pressure(compressedmolecules)

Area oflow pressure(rarefaction)Wavelength

AmplitudeDistance

Air

pre

ssure

A struck tuning fork alternately compressesand rarefies the air molecules around it, creatingalternate zones of high and low pressure.

Sound waves radiateoutward in alldirections.


Properties of Sound Waves

• Frequency– Number of waves that pass given point in given time– Wavelength

• Distance between two consecutive crests• Shorter wavelength = higher frequency of sound

– Frequency range of normal (healthy) hearing: 20 – 20,000 Hertz (Hz)

• Pitch– Perception of frequency: higher frequency = higher

pitch

• Most sounds are mixtures of many different frequencies simultaneously


Properties of Sound

• Amplitude– Height of crests

• Loudness = perception of amplitude– Subjective interpretation of sound intensity– Normal range is 0–120 decibels (dB)– Severe hearing loss with prolonged exposure above

90 dB– Loud music is 120 dB or more


Figure 15.29 Frequency and amplitude of sound waves.

High frequency (short wavelength) = high pitch

Low frequency (long wavelength) = low pitch

Pre

ssure

0.01 0.02 0.03Time (s)

Frequency is perceived as pitch.

High amplitude = loud

0.01 0.02 0.03Time (s)

Low amplitude = soft

Amplitude (size or intensity) is perceived as loudness.

Pre

ssure


Transmission of Sound to the Internal Ear

• Sound waves vibrate tympanic membrane• Ossicles vibrate and concentrate the energy

(amplify the pressure) at stapes footplate in oval window

• Cochlear fluid set into wave motion• Pressure waves move through perilymph of

scala vestibuli• Basilar membrane is “mechanically tuned”:

different parts vibrate most (i.e. resonate) in response to different frequencies


Basilar Membrane Tuning (Resonance)

• Fibers near oval window short and stiff– Resonate with high-frequency pressure waves

• Fibers near cochlear apex longer, more floppy– Resonate with lower-frequency pressure waves

• Thus basilar membrane “maps” different frequencies to different places along its length. – The “place theory” of hearing is most true for

disciminating high frequencies.


Figure 15.30a Pathway of sound waves and resonance of the basilar membrane. Slide 1

Tympanicmembrane

Roundwindow

Auditory ossicles

Ovalwindow

Cochlear nerve

Scala vestibuli

Scala tympani

Cochlear duct

Basilarmembrane

Route of sound waves through the ear

Malleus Incus Stapes

Helicotrema

3

4a

4b

Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli.

Sound waves vibrate the tympanic membrane.

Auditory ossicles vibrate. Pressure is amplified.

Sounds with frequencies below hearing travel through the helicotrema and do not excite hair cells.

Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.

4a

4b321

1

2


Figure 15.30b Pathway of sound waves and resonance of the basilar membrane.

Basilar membrane

High-frequency sounds displace thebasilar membrane near the base.

Medium-frequency sounds displace thebasilar membrane near the middle.

Low-frequency sounds displace thebasilar membrane near the apex.

Different sound frequencies cross the basilar membrane at different locations.

Apex(long,floppyfibers)

Fibers of basilar membrane

Base (short,stiff fibers)

2020,000Frequency (Hz)

2000 200


Excitation of Hair Cells in the Spiral Organ

• Cells of spiral organ– Supporting cells– Cochlear hair cells

• One row of inner hair cells• Three rows of outer hair cells• Have many stereocilia and one kinocilium

• Afferent fibers of cochlear nerve coil about bases of hair cells


Figure 15.27c Anatomy of the cochlea.

Tectorial membrane

Hairs (stereocilia)

Outer hair cells

Supporting cells

Inner hair cell

Afferent nervefibers

Fibers ofcochlearnerve

Basilarmembrane


Excitation of Hair Cells in the Spiral Organ

Stereocilia protrude from hair cells, some embed in tectorial membrane above•Passing pressure wave causes deflection of basilar membrane•Shearing action of basilar membrane and tectorial membrane causes cilia to bend•Opens mechanically gated ion channels via pull on tip links

– Inward current causes graded potential and release of neurotransmitter glutamate from hair cell onto sensory neuron

•Cochlear fibers transmit impulses to brain

Source: Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Siegel GJ, Agranoff BW, Albers RW, et al., editors; 1999. Downloaded 2011-12-04 from http://www.ncbi.nlm.nih.gov/books/NBK28026/

Hair cell transduction by ion channel opening


Sensory pathway for hearing

1. Hair cells in specific area of basilar membrane become stimulated

2. Sensory neuron axons (cell bodies in spiral ganglion) make up cochlear branch of vestibulocochlear nerve (VIII)

3. Sensory neuron axons synapse onto neurons in cochlear nucleus (medulla oblongata)

4. Information ascends bilaterally (often synapsing on the way) to inferior colliculus (midbrain)

5. Inferior colliculus neurons synapse at medial geniculate nucleus (thalamus)

6. Projection fibers from thalamus reach primary auditory cortex (temporal lobe)


Auditory pathwayMedial geniculatenucleus of thalamus

Primary auditorycortex in temporal lobe

Inferior colliculus

Lateral lemniscus

Superior olivarynucleus (pons-medulla junction)

Cochlear nuclei

Midbrain

Medulla

Vestibulocochlearnerve

Spiral ganglionof cochlear nerve

Bipolar cell

Spiral organ

Vibrations

Vibrations

Tonotopic organization

•Different frequency sounds excite different basilar membrane regions (apex: low frequencies; base: high frequencies)

•Cochlear nucleus (first auditory area in CNS) has a “map” of basilar membrane, i.e. frequency map: tonotopic map

•Tonotopic map seen in successive higher centers, up to & including primary auditory cortex

Tonotopic organization of primary auditory cortex

Source: Lynch, downloaded 2011-12-04 http://www.colorado.edu/intphys/Class/IPHY3730/image/figure8-16.jpg


Localizing sounds

• Most auditory information crosses over but some doesn’t, so brainstem and cortical areas get inputs from both ears

• Right versus left arrival time difference

• Right versus left intensity difference

• Both are used to localize sounds


Conduction deafness

• Sound energy is not conducted from outside world to the receptors, i.e. doesn’t make it to inner ear

• Causes include:

• Water or excess cerumen in external ear

• Scarring or perforation of tympanic membrane

• Immobility of ear ossicles (fluid, pus, tumor; otosclerosis)

Otosclerosis: abnormal bony growth around stapes footplate prevents normal stapes movement.


Sensorineural (nerve) deafness

• Most common cause of permanent deafness

• Damage to hair cell receptors

• Normal (young) range: 20–20,000 Hz; hearing loss later, high frequencies go first

• Loud noise, infection, some drugs

• Damage to nerve or to central auditory pathways


Normal organ of Corti, with tectorial membrane removed to show hair cells.

Ryan AF. Protection of auditory receptors and neurons: evidence for interactive damage. PNAS 97:6939-6940, 2000.

©2000 by National Academy of Sciences

Damaged organ of Corti.

Hair cells in healthy and damaged cochleas


Treating Sensorineural Deafness

• Cochlear implants for congenital or age/noise cochlear damage– Convert sound energy into electrical signals– Inserted into drilled recess in temporal bone– So effective that deaf children can learn to speak

Documents

Ch. 15 Special Senses: Hearing Slides mostly © Marieb & Hoehn 9 th ed. Other slides by WCR