The Sense Of Hearing

The Sense Of Hearing: 

The Sense Of Hearing

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SOUND SOUND WAVES

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S – shaped Curve or Sine Wave Frequency - number of waves that pass a given point in a given time (cycles/sec) Amplitude - height of the wave

Physiology of Hearing: 

Physiology of Hearing transmission of sound depends on elastic medium travels more slowly than light light --- 300,000 km/s (186,000 miles/s) sound --- 0.331 – 0.344 km/s (0.2 miles/s) -- 20 o C at sea level (↑ with temperature and altitude) speed of sound solid > liquid > air

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LOUDNESS - correlated with amplitude - measured in decibels (dB) - 1 dB = 0.01 bel intensity of sound log -------------------------- 0 dB (1000 Hz) standard sound - 120–160 dB (painful), 90–110 extremely high , 60- 80 very loud 40-50 moderate and 30 faint PITCH - correlated with frequency - frequency range audible to human ear is 20 – 20,000 cycles/sec (Hertz) - greatest sensitivity ranges from 1,000 – 4,000 cycles/sec - best pitch discrimination is 1,000 – 3,000 cycles/sec

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EXTERNAL AUDITORY CANAL - transmits sound pressure waves to the tympanic membrane - contains glands that secrete cerumen EXTERNAL EAR PINNA - funnels sound wave to the external auditory meatus - plays a role in sound localization

Physiological Anatomy of Ear: 

Physiological Anatomy of Ear The Ear has 3 parts: External, Middle and the Internal ear. The External Ear (1) Pinna: Helps to collect sound waves and to localize the source of sound. In lower animals Pinna can be moved by muscular actions in the direction of sound source to collect sound in humans these muscles have little action.

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(2) External Auditory canal- 2.5 cm long (i)helps in transporting the sound waves to the middle ear (ii) secrete wax and oil trap the foreign bodies.

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The Middle Ear An air filled cavity within the temporal bone that consist of ( i )Tympanic Membrane.(ear drum) Functions– Pressure Receiver i.e extremely sensitive to pressure changes Resonator i.e. it starts vibrating freely when the sound waves strike Critically dampens the sound waves i.e.as soon the sound will stop T.M. vibrations are stopped immediately

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(ii) Ear Ossicles Malleus-resembles a hammer.The handle of malleus is connected to the inner surface of T.M. Incus-It articulate with head of malleus Stapes- looks like stirrup. The head of stapes articulates with incus & the oval foot plate contact the oval window of the coclea function= to magnify the intensity of sound by 1.2 to 1.3 times by lever action.

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Middle ear mucles (a) Tensor tympani = attached to the neck of malleus .its contraction tenson of tympanic membrane (b) Stapedius : attached to the neck of stapes and on contraction it pulls the foot plate of stapes out from the oval window Function= both the muscles can be reflexley activated by loud sounds amplitude of sound vibration of the tympanic membrane protection of the internal ear from loud sounds( Tympanic Reflex . reaction time40-160sec)

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Middle ear mucles (a) Tensor tympani = attached to the neck of malleus .its contraction tenson of tympanic membrane (b) Stapedius : attached to the neck of stapes and on contraction it pulls the foot plate of stapes out from the oval window

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MIDDLE EAR OSSICLES - malleus , incus and stapes - transmit vibratory motion of the tympanic membrane to the oval window TENSOR TYMPANI MUSCLE - contraction pulls manubrium of the malleus - decreases the vibration of tympanic membrane STAPEDIUS - contraction pulls the footplate of the stapes out of the oval window

Tympanic/Attenuation Reflex: 

Tympanic/Attenuation Reflex function of the muscles and ossicles Functions protect the cochlea from damaging vibrations caused by excessively loud sounds mask low frequency sounds in loud environments decrease persons sensitivity to his or her own speech (muscles)

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latency ---- 40 to 80 milliseconds contraction of tensor tympani and stapedius dampens the movement and ossicles and decreases the sensitivity of acoustic apparatus. reduce intensity of sound transmission by 30 – 40 dB (↓ 1000 cycles/second)

The three auditory ossicles in the right middle ear: 

The three auditory ossicles in the right middle ear Pharyngotym- panic tube Tensor tympani muscle Tympanic membrane (medial view) Stapes Malleus Superior Anterior Incus Epitympanic recess Stapedius muscle

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Role of Middle Ear The ear ossicles result in magnification of sound intensity by 1.2-1.3 times The effective surface area of T.M.=55sq.mm and that of oval window is 3sq.mm,thus reduction of the area is by @17times(55/3) Total magnification=22times(17 Х 1.3)

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Effective transfer of sound energy from air to fluid medium is difficult because most of the sound is reflected as a result of the different mechanical properties of the two media. The middle ear thus act as an impedance matching device by amplifying the sound pressure Amplification of sound intensity is greatest between 1000-3000 Hz .sounds below 16Hz or above 20000Hz are not amplified at all.

Route of sound waves through the ear: 

Route of sound waves through the ear Fluids in cochlear canals Upper and middle Internal ear External ear Pinna External acoustic meatus Air Tympanic membrane Malleus, incus, stapes (ossicles) Oval window Lower Middle ear One vibration Time Spiral organ (of Corti) stimulated Amplification in middle ear Amplitude Pressure

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SOUND TRANSMISSION

Otitis Media: 

Otitis Media inflammation of the middle ear commonly due to infection and common result of a sore throat especially in children eardrum bulges and becomes inflammed and red ------- pain and rupture

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MIDDLE EAR AUDITORY TUBE - formerly called eustachian tube - link the middle ear with the pharynx - opening equalizes pressure in the middle ear cavity with external air pressure

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BONY LABYRINTH MEMBRANOUS LABYRINTH

The internal Ear: 

The internal Ear Boney labyrinth and membranous labyrith Membranous labyrinth comprises one vestibule(utricle and saccule ) and three semicircular canals.-concerned with equilibrium. One Coclea concerned with hearing

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INNER EAR (Labyrinth) COCHLEA - involved in hearing SEMICIRCULAR CANAL - involved in equilibrium - receptors detect rotational acceleration UTRICLE - involved in equilibrium - receptors detect linear acceleration (horizontal direction) SACCULE - involved in equilibrium - receptors detect linear acceleration (vertical direction)

Cochlea cross-section: 

Cochlea cross-section

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COCHLEA

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ORGAN OF CORTI

TRANSDUCTION MECHANISM: 

TRANSDUCTION MECHANISM BASILAR MEMBRANE…..VIBRATES TECTORIAL MEMBRANE STATIONARY STEROCILIA AUDITORY NERVE HAIR CELLS

TRANSDUCTION MECHANISM: 

TRANSDUCTION MECHANISM TECTORIAL MEMBRANE STATIONARY BASILAR MEMBRANE…..VIBRATES STEROCILIA BEND AUDITORY NERVE HAIR CELLS

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ELECTRICAL RESPONSES OF HAIR CELLS GENESIS OF ACTION POTENTIALS IN AFFERENT NERVES

Special Senses: Sound Transmission and Transduction: 

Special Senses: Sound Transmission and Transduction Sound waves Tympanic membrane vibrations Ossicles transmit & amplify vibration Via oval window to perilymph then endolymph

Sound Transmission cont.: 

Sound Transmission cont. Vibrations in perilymph are transferred across the basilar membrane to the cochlear duct Vibrations in endolymph stimulate sets of receptor cells Receptor (hair) cells release NT which stimulates nearby sensory neuron Impulse to auditory cortex of temporal lobe via Cochlear nerve to Vestibulocochlear N. (VIII)

Characteristics of Basilar Membrane : 

Characteristics of Basilar Membrane apex is wider than the base tension is higher at the base than at the apex base vibrate at higher frequency than the apex (frequency analyzer)

Characteristics of Basilar Fibers : 

Characteristics of Basilar Fibers length of the fibers is greater at the apex than at the base fiber diameter is greater at the base than at the apex base -- shorter and wider apex – taller and slender high –frequency resonance (base), low frequency resonance (apex)

Resonance of the basilar membrane and activation of the cochlear hair cells: 

Resonance of the basilar membrane and activation of the cochlear hair cells (a) (b) (c) Stapes Oval window Scala vestibuli Cochlear duct Scala tympani Basilar membrane Round window Base Hz 20,000 (High notes) Hz 1500 Hz 500 500 Hz 4000 Hz 24,000 Hz Hz 20 (Low notes) Apex Perilymph Cochlear nerve Relative lengths of basilar fibers within different regions of basilar membrane Basilar membrane

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PLACE THEORY OF HEARING

Central Auditory Pathway: 

Central Auditory Pathway the frequency of action potential in single auditory nerve is proportional to the loudness of the sound stimuli.

Neural Pathways for Hearing: 

Neural Pathways for Hearing 10-60

Sound Localization : 

Sound Localization time lag between the entry of sound into one ear and its entry into the opposite ear. functions best at frequencies below 3,000 cycles/sec. neural analysis ---- medial superior olivary nucleus difference between the intensities of the sounds in the two ears. f unctions best at frequencies above 3,000 cycles per second neural analysis ---- lateral suprior olivary nucleus

Types of Deafness: 

Types of Deafness Conductive Deafness Sensorineural Deafness due to impaired sound transmission in external and middle ear impacts all sound frequencies Causes plugging of the EAC with cerumen of foreign bodies otitis externa and otitis media perforation of eardrum osteosclerosis due to loss of cochlear hair cells (common), problems with the eight cranial nerves or within central auditory pathways (nerve deafness) impairs the ability to hear certain pitches (permanent) causes aminoglycoside antibiotics (streptomycin and gentamycin) prolonged exposure to noise tumors and vascular damage

Hearing Impairments: 

Hearing Impairments Conduction deafness: Transmission of sound waves through middle ear to oval window impaired. Impairs all sound frequencies. Hearing aids. Sensorineural (perception) deafness: Transmission of nerve impulses is impaired. Impairs ability to hear some pitches more than others. Cochlear implants.

Structure of the middle ear: 

Structure of the middle ear (b) Pharyngotympanic (auditory) tube Auditory ossicles Entrance to mastoid antrum in the epitympanic recess Tympanic membrane Semicircular canals Cochlea Cochlear nerve Vestibular nerve Oval window (deep to stapes) Round window Incus (anvil) Malleus (hammer) Stapes (stirrup) Internal jugular vein Vestibule External acoustic meatus

Conduction of sound waves: 

Conduction of sound waves Ossicular Conduction main pathway for normal hearing Air Conduction unimportant for normal hearing initiated by vibration of round window Bone Conduction involves skull bones plays a role in transmission of extremely loud sounds

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Table 9–1. Common Tests with a Tuning Fork to Distinguish between Nerve and Conduction Deafness. Weber Rinne Schwabach Method Base of vibrating tuning fork placed on vertex of skull. Base of vibrating tuning fork placed on mastoid process until subject no longer hears it, then held in air next to ear. Bone conduction of patient compared with that of normal subject. Normal Hears equally on both sides. Hears vibration in air after bone conduction is over. Conduction deafness (one ear) Sound louder in diseased ear because masking effect of environmental noise is absent on diseased side. Vibrations in air not heard after bone conduction is over. Bone conduction better than normal (conduction defect excludes masking noise). Nerve deafness (one ear) Sound louder in normal ear. Vibration heard in air after bone conduction is over, as long as nerve deafness is partial. Bone conduction worse than normal.

Masking: 

Masking presence of one sound decreases an individual’s ability to hear other sounds due to the relative and absolute refractoriness of previously stimulated auditory receptors and nerve fibers to other stimuli.

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Audiometry Auditory acuity is commonly measured with an audiometer. This device presents the subject with pure tones of various frequencies through earphones. At each frequency, the threshold intensity is determined and plotted on a graph as a percentage of normal hearing. This provides an objective measurement of the degree of deafness and a picture of the tonal range most affected.