Cranial Nerve VIII: Auditory

What is the first thing to ask if a child comes in with delayed speech?

have they had a hearing test?

What is the auditory stimulus?

created by compression of air molecules - sound creates waves in the air molecules that have these characteristics

What are the parameters of the auditory stimulus?

pitch (frequency)

loudness (intensity)

timbre

location (orientation)

Characteristics of Pitch

measured in Hz

normal range is 20-20,000 Hz

less than 20 Hz called “infrasound” - elephants can hear

above 20,000 Hz called “ultrasound”

Characteristics of Loudness

measured in decibels

normal range is 0-130

prolonged exposure over 85-90 is damaging

Characteristics of Timbre

overtones

distinguish one sound from another at same frequency

a piano and hap both playing middle “c”

Characteristics of Location

want to know where the sound comes from

if an emergency vehicle is coming, you want to know where it is to pull over

As amplitude gets smaller…

the loudness decreases and vice versa

What makes hearing challenging?

transmission of sound waves through air YET they have to transmit accurately through fluid

Outer Ear Characteristics

from pinna of ear to outer membrane

main purpose: localization and directing sound to the ear

protected from debris by hair and earwax

What is earwax called?

cerumen

Middle Ear Characteristics

chamber that is filled with the bones

continuous pathway from middle ear to Eustachian tube into the pharynx to equalize pressure

part that looks like a snail is hearing system

Inner Ear Characteristics

where the neural ear is - sensory receptor and all the action in terms of changing stimulus into electrical signal

sound waves are compressing air and move into outer ear

press on eardrum - tympanic membrane

Cochlea characteristics

about the size of a pea

like a straw wrapped around tip of pencil

pencil tip - modals (bony framework)

straw - membranous apparatus filled with fluid

What is in the modialis?

contains neurons in spiral ganglion (neurons of CN VIII) - around bony framework that contains bipolar sensory cells

What are the layers of the membranous apparatus?

perilymph - outside of membranous apparatus

endolymph - inside

sensory receptor for hearing - organ of corti

What is helicotrema?

scale vestibule and scala tympani connects to each other at the apex of cochlea

What are the inner hair cells responsible for?

deflecting the stimulus - fires CN VIII and heads into brain for coding

What are the outer hair cells responsible for?

amplifying stimulus by changing flexibility in the system

What is tonotopic representation?

regions of the basilar membrane respond differently to different frequencies or tones

high frequencies - at the base

low frequencies - at the apex

What are the ventral and dorsal cochlear nuclei critical to?

SOUND LOCALIZATION - has to do with communication between the two sides

peripheral hearing is…

BILATERALLY REPRESENTED IN THE BRAIN!!!

What is the clinical implication of peripheral hearing being bilaterally represented?

unilateral brain CNS lesion does NOT cause hearing loss

If someone has hearing loss, where is the problem likely?

middle or inner ear, NOT brain

do NOT hear person had a stroke and lost hearing in contralateral ear

Where does everything eventually join?

in an ascending fiber bundle - lateral lemniscus

What is the primary termination site?

transverse temporal gyri of temporal lobe

What does a typanogram do?

give information about prescence of fluid in the middle ear, mobility of middle ear system - fluid built up when there is infection

What is the most common hearing testing done?

otoacoustic emission testing/newborn screening

What is the gold standard for hearing testing?

auditory evoked response testing

What is conductive hearing loss?

hearing problem in EXTERNAL or MIDDLE ear

if BONE conduction is impaired

What is sensorineural hearing loss?

hearing problem in INNER ear

if AIR conduction is impaired