In The Name Of GodIn The Name Of God

Hearing System Hearing System physiologyphysiology

1) Frequency: number of cycles of sound waves passing a stationary point per second (Hertz = Hz; sec-1)

- range of human hearing is ~ 20 - 20,000 Hz- range of human voice is ~ 350 - 3500 Hz

Characteristics of Sound Waves

2) Intensity: amplitude of a sound wave (decibels = dB)

- expressed in terms of sound pressure- sound intensity is proportional to square of pressure

dB = 10 logintensity of a particular sound wave

intensity threshold for human hearing

Normal conversation 60 dBAirplane 100 dBDamaging sound 120 - 140 dB

Outer ear consists of:

1) Pinna - collects sound waves

2) External Auditory Canal - conducts sound waves to middle ear

Anatomy of the Ear

Middle Ear consists of:

1) Tympanic membrane (eardrum) - vibrates in response to sound waves

2) Auditory ossicles - 3 bones which mechanically transduce tympanic membrane vibrations to the inner ear

a. Malleus (hammer) - tympanic membrane to incusb. Incus (anvil) - malleus to stapesc. Stapes (stirrup) - incus to oval window of cochlea

3) Eustacheon (auditory) tube - equalizes pressure between middle ear and the environment

4. Two muscle

• Tensor tympani muscle: anchored to bone at one end is attached to the malleus

• stapedius muscle: extend from a fixed anchor of bone and attached to stapes

• contraction reduces ossicular mobility• contraction therefore: reduce the intensity of lower frequency sound transmission by 30 to40 decibels that its called Attenuation reflex

function attenuation reflex

1. Protect the cochlea from damaging vibrations caused by extensively loud sound

2.To mask low frequency sound in loud environments

3. Decrease a person hearing sensitivity to his or her own speech

Function of the Ossicular Chain

I. Impedance Matching - effective transfer of sound energy from air to fluid

Sound pressure in the middle ear is amplified by 2 mechanisms:

1. Ossicular act like a lever system : displace oval window against cochlear fluid

Force increase = 1.3 fold2. Surface area of oval window = 1/17 surface area of tympanic membrane Force increase = 17 fold

NET RESULT: Ossicles increase sound pressure 22 fold

Inner ear consists of:

1) Semicircular canals - important for sense of balance and equilibrium

2) Cochlea - responsible for sound detection, discrimination and transduction into neural signal

From: W.F. Ganong, Review of Medical Physiology, 19th ed. Appleton & Lange, 1999

Cochlear structures:

From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993.

1) Cochlear duct - fluid-filled tube within cochlea

2) Scala media - endolymph filled space containing the sensory apparatus; delimited by Reissner’s and basilar membranes (high [K+] !)

3) Scala vestubuli - perilymph filled space above scala media

4) Scala tympani - perilymph filled space below scala media

Length: 34 mm

Dia. 2mm

BASE

apex

From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993.

5. Helicotrema - distal opening between scala vestibuli and scala tympani

6. Oval window - membranous opening of scala vestibuli, joins with stapes

7. Round window - membranous opening of scala tympani

8. Basilar membrane: Organ of Corti - sensory detection apparatus

PropertiesProperties of basilar membraneof basilar membrane

The basilar membrane of the cochlear duct is stiff and narrow close to the oval window. It becomes wider and

more flexible near its distal end.

0.04

0.5 mm

From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993.

Some important features of the Organ of Corti:

1) Auditory hair cells (AHC) - sensory receptor cells, have stereocilia projecting from their apical surface

2) Tectorial membrane - glycoprotein-rich flap, stereocilia tips imbedded in its surface

3) Afferent nerve fibers - synapse with AHCs, carried in the vestibulocochlear nerve

Auditory Hair Cells (AHC)The Specialized Auditory Receptor Cells

2 Populations:

1) Inner AHCs - ~ 3500 in number - arranged in a single row - provide basic auditory info to CNS

2) Outer AHCs - ~ 12000 in number - arranged in 3 parallel rows - fine tuning of auditory signal - have a limited motility, and shorten slightly in response to certain tones - amplifies the sound wave

Sound Wave Transduction in the Cochlea1. Sound waves enter via oval window; round window bulges in response (fluid is not very compressible)

2. Basilar membrane and Organ of Corti vibrate in response

AHC Stereocilia Structure / Function1) Rows of stereocilia are of constant diameter and taper at their base

2) Stereocilia act as rigid rods

3) Have several stretch-activated nonselective cation channels

4) Stereocilia tips are joined by a protein tip link

Upward Bending:

- stereocilia bend away from limbus, toward tallest stereocilia

- cation channels open, AHC depolarizes

- VG Ca++ channels open; AHC releases NT (glutamate)

Downward Bending:

- cation channels close, AHC hyperpolarizes

- VG Ca++ channels closed; no NT release

- stereocilia bend toward limbus, away from tallest stereocilia

Mechanism of transduction

Discrimination of Auditory SignalsThe two physical characteristics of sound that we can discriminate are frequency and intensity.

1) Place Principle of Frequency

High frequency - cause vibration of the basilar membrane at the base of the cochlea where the membrane is narrow and stiff

Low frequency - cause vibration of the basilar membrane at the apex of the cochlea where the membrane is wide and more compliant

2. Volley or frequency principle: It was used for determination low freq. sound from 20 to 200 HZ It cause volley of impulse synchronized at same at same freq.

Discrimination of Auditory Signals (continued)

3) Mechanisms for discrimination of LOUDNESS

Louder sound

firing rate of AHCs stimulation of special“high threshold” AHCs

number of stimulated AHCs

amplitude of basilarmembrane vibration

Determination of the direction Determination of the direction soundsound

1.By the time lag between the entry of sound into one ear and into the opposite area.

• it is benefit at Frq.<3000 Hz

•The medial superior olivary nucleus has important role in determination time lag between acoustic signal entering the tow ear

2.Differnce between the intensity of the sound in the two ear

• the intensity mechanism operated best at Frq.>3000 Hz

• it is concern with Lateral superior olivary nucleus

Figure 9.19 Pathways in the Figure 9.19 Pathways in the auditory systemauditory system

Subcortical Mechanisms of Subcortical Mechanisms of Sound LocalizationSound Localization

The lateral and medial superior The lateral and medial superior olivesolives react to differences in what react to differences in what is heard by the two earsis heard by the two earsMedial – arrival Medial – arrival time differencestime differencesLateral – Lateral – amplitude differencesamplitude differences

Both project to the superior colliculusBoth project to the superior colliculusThe deep layers of the The deep layers of the superior colliculus superior colliculus are laid are laid

out according to auditory space, allowing out according to auditory space, allowing location of sound sources in the world; the location of sound sources in the world; the shallow layers are laid out retinotopicallyshallow layers are laid out retinotopically

Tonotopic Tonotopic mapping/organizationmapping/organization• The organization of frequency in terms of place. The organization of frequency in terms of place.

http://tonks.disted.camosun.bc.ca/courses/psyc290/brain/tonotopic.GIF

• Info is carried through auditory system in frequency channels.

Two Streams of Auditory Two Streams of Auditory CortexCortex

Auditory signals are conducted to two Auditory signals are conducted to two areas of association cortexareas of association cortexPrefrontal cortexPrefrontal cortexPosterior parietal cortexPosterior parietal cortex

Anterior auditory pathway Anterior auditory pathway may be more may be more involved in involved in identifying sounds (whatidentifying sounds (what))

Posterior auditory pathway Posterior auditory pathway may be more may be more involved in locating sounds involved in locating sounds (where)(where)

Perception of different Perception of different characteristics of soundcharacteristics of soundFrequencyFrequency

Starts at the basilar membraneStarts at the basilar membrane and frequency sharpening and frequency sharpening occurs throughout the auditory pathway occurs throughout the auditory pathway

IntensityIntensityStarts at the hair cells Starts at the hair cells (OHC are stimulated by weaker (OHC are stimulated by weaker

stimulus)stimulus)Frequency of impulsesFrequency of impulses

DirectionDirectionInter-aural time difference Inter-aural time difference

Pattern recognition Pattern recognition Cortical function Cortical function

Interpretation of speechInterpretation of speechComplex cortical phenomenonComplex cortical phenomenon

Electrical stimulation in Wernicke’s area Electrical stimulation in Wernicke’s area of a conscious person of a conscious person occasionally causes a occasionally causes a highly complex thoughthighly complex thought..

. The types of thoughts that might be experienced include . The types of thoughts that might be experienced include complicated visual scenes that one might remember from complicated visual scenes that one might remember from childhood, childhood, auditory hallucinations auditory hallucinations such as a specific musical such as a specific musical piece, or even a statementpiece, or even a statement

made by a specific person.made by a specific person.

For this reason, it is believed that activation of Wernicke’s For this reason, it is believed that activation of Wernicke’s area can callarea can call

forth forth complicated memory patterns complicated memory patterns that involve more than that involve more than one sensory modality even though most of the individual one sensory modality even though most of the individual memories memories ..may be stored elsewheremay be stored elsewhere

Hearing abnormalityA. Conductive Defects

1. Otitis - inflammation of the external or middle ear2. Otosclerosis - calcification of the stapes3. Tympanic perforation (broken eardrum)4. Foreign body insertion

B. Sensorineuronal Defects

1. Drug-induced - aminoglycoside antibiotics2. Congenital3. Infections - syphilis, measles, mumps, meningitis, flu4. Acoustic neuroma - benign tumor of vestibulocochlear nerve