chapters 9 and 10- anatomy and physiology of hearing

audition

the process associated with hearing

- a vital part of verbal communication

- the ear changes acoustic energy into electromechanical energy

hearing in 6 months of gestation

infant can hear in utero

as infants develop:

- can differentiate intensity, frequency changes, and volume at an early age through environmental stimuli to produce speech (cooing, variegated babbling, jargon)

- can listen to sounds that they are making and compare them to the sounds that they are hearing to create new sounds (mimicking the sounds around them)

structures of the ear

outer

middle

inner

eustachian tube

more lateral in infants

- hard to drain fluid when tube is not slanted

- susceptible for muddle ear infections (otitis media)

- no cone of light= middle ear infection

sloped in adults

- eustachian tube can open and drain fluid

the outer ear

shapes the frequency components of sound (air-filled)

- curves in outer ear to funnel the sound vibrations into the external auditory meatus (EAM)

pinna of outer ear

- funnels acoustic information to the EAM

- structure provided by a cartilaginous framework

- collector of sound that is processed in the middle ear and the cochlea

external auditory meatus (EAM) (ear canal)

funnels sounds to tympanic membrane (eardrum)

- terminates at tympanic membrane

- 2/3 of TM is housed in bone

- 1/3 is housed in cartilage

auditory pathway of sound

when info enters the brain it is processed on the opposite side that it is received

middle ear structure

air-filled

- increases pressure arriving at cochlea

- acts to overcome impedance

1. tympanic membrane

2. the ossicles

3. the entry of the cochlea

4. the oval window

impedance

resistance to the flow of energy

tympanic membrane

separates outer ear from middle ear

- responsible for initiating the mechanical impedance matching process of middle ear

first layer- outer (cuticle) layer

second layer- intermittent (fibrous) layer

third layer- inner layer (lining of the middle ear)

middle ear function

impeding matching device

- increases pressure of signal arriving at cochlea

- matches the impedance of two conductive mechanisms: outer ear (air-filled), and inner ear (fluid-filled)

three impedance-matching mechanisms that cause a signal to gain 31 dB

conversational speech 30-50dB

1. area ratio (TM and oval window)

2. level advantage

3. buckling of TM

area ratio (TM and oval window)

provides a 25 dB gain

- makes signal more intense

level advantage

manubrium is 9mm long

process of stapes- 7mm long

- provides a 2mm gain

buckling of TM

provides a 4-6 dB gain

disorders that impede middle ear function

otitis media

otosclerosis

tumors

the ossicles

3 of the smallest bones in the body located in the middle ear

1. malleus

2. incus

3. stapes

malleus

- largest of the ossicles (9mm long, weighs 25 mg)

incus

- weighs 30 mg and 7mm long

stapes

- third bone of ossicular chain

- weighs 4mg with area of 3.5 mm

- helps to transmit sound vibrations from eardrum to oval window

- articulation of incus and stapes into a ball and socket joint

tympanic muscles

- muscles in the middle ear attached to the ossicles

- smallest muscle of the human body

stapedius muscle (6mm long)

tensor tympani (25mm long)

stapedius muscle

- embedded in the posterior part of the middle ear

- inserts into the posterior neck of the stapes

- stapes is rotated posteriorly

- CN VII stapedial branch - acoustic reflex (helpful but slow)

tensor tympani

- inserts into upper mandibular malli (malleus)

- pulls malleus anteromedially

- tympanic reflex

- CN V trigeminal: mandibular branch

landmarks of the middle ear

medial wall

anterior wall

medial wall of the middle ear

oval window

round window

promontory of the cochlea

prominence of the facial muscle

anterior wall of the middle ear

eustachian tube (auditory tube 36 mm)

- responsible for aeration of middle ear

- leads to nasopharynx

posterior wall of the middle ear

prominence of the stapedial pyramid

the inner ear

- it contains sensors for balance (the vestibular system)

- it contains sensors for hearing (the cochlea)

vestibule - entryway for these structures

osseous labyrinth- osseous vestibule

- embedded within the temporal bone

- epithelial lining secretes perilymph (fluid found within the superficial cavities of the labyrinth)

- the oval window within the lateral wall

- vestibular aqueduct within the medial wall

osseous labyrinth - osseous semicircular canals

- canals of the vestibular system

- contains sense organs for movement of the head and body in space

osseous cochlear labyrinth

- looks like coiled snail shell

divided by the (incomplete) osseous spinal laminae into:

scala vestibuli

scala media

scala tympani

the round window

cochlear aqueduct

scala media

houses sensory organ for hearing

filled with fluid called endolymph (between scala vestibuli and scala tympani)

scala typmani

filled with perilymph

the round window

communicates between scala tympani and middle ear space

cochlear aqueduct

connects scala tympani and subarachnoid space

membraneous labyrinth (cochlear duct)

scala media

organ of corti

stereocilia

organ of corti

- sits on basilar membrane

- sensory organ of hearing within scala media

four rows of hair cells

- 3 rows of outer hair cells

- 1 row of inner hair cells

tunnel of corti separates inner and outer cells

stereocilia

protrude from surface of hair cells

- inner hair cells are innervated by many nerve fibers

- outer hair cells are innervated by one nerve fiber

auditory mechanism

- processes the acoustic signals of speech

- has amazing range of sound pressure

- has a frequency range from 20Hz to 20,000 Hz

outer ear

collects sound and shapes frequency components

middle ear

matches airborne acoustic signals with fluid medium of cochlea

inner ear

performs temporal and spectral analysis on ongoing acoustic signals

auditory pathway

conveys and further processes the signal

cerebral cortex

interprets the signal

the cochlea

establishes first level of auditory processing of incoming acoustic signals

- determines frequency components of signal

- determines amplitude of signal

- identifies temporal aspects of signal

the traveling wave

wave-like action of basilar membrane

- determines frequency data going to the brain

- arises from stimulation of perilymph of vestibule

- moves along basilar membrane from base to apex until it reaches point of maximum growth

- wave damps after reaching maximum growth/excursion

basilar membrane

determination of ability to analyze frequency

- graded thickness, stiffness, and width

- base is narrow and stiff

- apex is wider, less stiff, and more massive

tonotopic arrangement

- high frequency sounds resolved at the base

- low frequency sounds processed at the apex

electrical events

initiation of electrical events in cochlea

- depends on stereocilia

- minute cilia protruding from surface of hair cells

- basilar membrane displaced

- electrical potentials are intiated