2.4 - auditory system

glossary

outer ear

• most exterior portion of ear from pinna → eardrum

• channels sound → eardrum

pinna

mostly skin- covered cartilagineous outer shell of ear

• collects sound into acoustic canal

external acoustic (auditory) canal

begins in auricle + ends in eardrum

• channels sound waves to eardrum

cerumen glands

line auditory canal + makes ear wax

tympanic membrane (eardrum)

membrane separates outer ear + middle ear

• vibrates w/ sound waves

middle ear

3 ossicles transmit sound from eardrum → inner ear

malleus

vibrates w/ eardrum

• pulled away from eardrum by Tensor tympani during eating so chewing isn’t too loud

incus

connects malleus + stapess

stapes

tapes onto cochlea

eustachian tube

equilibrates air pressure in middle ear w/ the throat

• drains mucous from the middle ear

inner ear

eval window → auditory nerve

• transmits sound frequency + amplitude info to auditory nerve

semicircular canals

coordinate balance in angular motions (turning around)

• posterior, superior and horizontal

vestibules

coordinate balance in static or linear motions (up/ down)

• utricle

  • saccule

cochlea

transmit sound frequency + amplitude info to the auditory neuron

organ of corti

converts sound vibrations to neuronal signals

• contains basilar membrane, inner + outer hair cells

inner hair cell

depolarize due to K ion influx in response to sound vibration- induced basilar membrane movement

• depolarized cells secrete glutamate to activate auditory nerve

  • transmit sound amplitude + frequency into → auditory nerve

outer hair cell

contract in response to cound vibration induced basilar movement

• believed to amplify basilar membrane movement to soft sounds

  • enhances ability to detect low amplitude sounds!

basilar membrane

membrane at basal side of the organ of Corti

• vibrates to different sound frequencies

• inner hair cells transmit info → auditory nerve

base of basilar membrane

narrower + stiffer

• vibrate to higher frequency

apex of basilar membrane

wider + looser

• vibrate to lower frequency

endolymph

K rich fluid in cochlear duct

• enables K ion to flow into inner hair cells when cilia tilt + open PM cation channels

• influx of K depolarizes, secretes glutamate

perilymph

extracellular fluid that filsl inner ear, except within cochlear duct

• rich in Na

divergent innervation

situation in which 1 presynaptic neuron sends signals to multiple postsynaptic neurons

convegent innercation

situation in which multiple presynaptic neuronns send signals to 1 postsynaptic neuron

2 branches of vestibulocochlear nerve

cochlear and vestibular

cochlear branch

auditory nerve

• innervates cochlea (inner + outer cells)

vestibular branch

innervates semicircular canals + vestibules (saccule + utricle)

interaural time differences

sound localization strategy that uses sound signal arrival time differences from 2 ears to determine sound origin

• uses coincidence detection by medial superior olive neurons to detect sound signal arrival time differences

interaural intensity

sound localization strategy that uses sound signal amplitude differences from the 2 ears to determine sound location

• uses contralateral inhibition mechanism mediated by lateral superior olives

• neurons from ear that recieves the louder sound sitmulus inhibit auditory signal transmission from other ear to brain

tonotopic map

cortical representation at the primary auditory cortex shows the same frequency- dependent organization as that found in the cochlea

conductive hearing loss

due to blockage of sound wave conduction to cochlea

• causes: blockage of auditory canal, damage to eardrum or ossicles

sensorineural hearing loss

due to dysfunction in converting sound vibration to neuronal signals

• cause: damage to/ mutation in inner ear structure/ function

  • ex.) malfunction in cochlea/ auditory nerve

mixed hearing loss

due to malfunctions that affect both sound wave conduction + conversion to neuronal signals

cochlear implant

electronic device placed in cochlea that can stimulate auditory nerce in response to sound stimulation