FUNCTIONAL ANATOMY

HUMAN EAR

PINNA EXTERNAL EAR - funnels sound wave to the external auditory meatus - plays a role in sound localization

EXTERNAL AUDITORY CANAL - transmits sound pressure waves to the tympanic membrane - contains glands that secrete cerumen

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

TENSOR TYMPANI MUSCLE - contraction pulls manubrium of the malleus - decreases the vibration of tympanic membrane

MIDDLE EAR

OSSICLES STAPEDIUS - malleus, incus and stapes - contraction pulls the footplate of the stapes out of- transmit vibratory motion of the tympanic membrane the oval window to the oval window

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)

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)

Impedance Matching

function of the tympanic membrane and the ossicles (Impedance Matching Device) depends on the following

ratio of the surface area of large tympanic membrane to that of the smaller oval window mechanical advantage of the lever system formed by the ossicle chain increases the efficiency of energy transfer by 30 dB

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

SACCULE - involved in INNER EAR (Labyrinth)CANAL equilibriumSEMICIRCULAR - receptors detect linear - involved in equilibrium acceleration (vertical - receptors detect rotational direction) acceleration

COCHLEA - involved in hearing

UTRICLE - involved in equilibrium - receptors detect linear acceleration (horizontal direction)

BONY LABYRINTH

MEMBRANOUS LABYRINTH

COCHLEA

ORGAN OF CORTI

HAIR CELLS

ELECTRICAL RESPONSES OF HAIR CELLS

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) -20o C at sea level ( with temperature and altitude) solid > liquid > air

speed of sound

SOUND
SOUND WAVES

Amplitude - height of the wave

S shaped Curve or Sine Wave

Frequency - number of waves that pass a given point in a given time (cycles/sec)

PITCH - correlated with frequency

LOUDNESS - correlated with amplitude

- 120160 dB (painful), 90110 extremely high , 60- 80 very loud 40-50 moderate and 30 faint

CHARACTERISTICS OF SOUND WAVES

SOUND TRANSMISSION

Conduction of sound waves

Ossicular Conduction

main pathway for normal hearing unimportant for normal hearing initiated by vibration of round window

Air Conduction

Bone Conduction

involves skull bones plays a role in transmission of extremely loud sounds

Masking

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

Other Cortical Areas Concerned with Audition

Brodmann area 22

concerned with the processing of auditory signals related to speech right side -- melody, pitch and sound intensity portion of posterior superior temporal gyrus involved in language-related auditory processing.

Planum Temporale

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

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)

PLACE THEORY OF HEARING

Central Auditory Pathway

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

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. functions best at frequencies above 3,000 cycles per

second neural analysis ---- lateral suprior olivary nucleus

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

Presbycusis

gradual hearing loss associated with aging probably due to gradual cumulative loss of hair cells and neurons.

Instruments used to measure auditory acuity

Audiometer Tuning fork