neuro 206 lecture 9; hearing

what is the first step of encoding sound?

receiving sound waves

sound is represented by

a sinusoidal wave

the two components of a sound wave are

frequency

amplitude

frequency tells us

how many waves per second (horizontal component)

amplitude tells us

power (vertical component)

which parts of the ear are filled by air?

outer ear

middle ear

which parts of the ear are filled by water?

inner ear

what structure is responsible for amplifying sound in the ear?

middle ear

the middle ear does what?

boosts sound transmission across the air-fluid boundary by 200x

what is the pathway sound travels in the middle ear to be amplified

tympanic membrane → ossicles → oval window of the cochlea

sound amplification involves

sound traveling from large diameter tympanic membrane to small oval window

lever action of three interconnected ossicles

what is the second step of encoding sound?

decomposing natural (complex) waveforms to individual frequencies through Fourier decomposition

two ways to depict sound waves graphically include

power spectrum

spectrogram

what does a power spectrum do?

describes how the variance or power of a time-varying signal is distributed across different frequencies; helps identify which frequencies have highest power

what is a spectrogram?

a visual, three-dimensional representation of a signal's spectrum of frequencies as they change over time

what is true about a complex sound waveform at each slice of time?

it is composed of a range of distinct frequencies

what is the cochlea?

a structure in the inner ear that works as a mechanical frequency analyzer where different frequencies maximally displace the basilar membrane at different locations

the basilar membrane consists of which two areas of importance?

base

apex

the base of the cochlea is located closer to

the middle ear

the base of the cochlea physically is more

narrow and stiff

the base of the cochlea encodes

high frequency sound

the apex of the cochlea is located

on the innermost part of the cochlea spiral

the apex of the cochlea physically is more

wide and floppy

the apex of the cochlea encodes

low frequency sound

what is step three of encoding sound?

Hair cells in the organ of Corti are activated where basilar membrane is flexing

what are the main auditory receptor?

inner hair cells

what do outer hair cells do?

amplify sound

what happens when the basilar membrane moves?

mechanically-gated K+ channels on the stereocilia open/close with bending

what is the significance of the mech-gated K+ channels on the stereocilia?

movement of the basilar membrane can indirectly increase/decrease NT release

what is step 4 of encoding sound?

cochlear nerve afferents have somas located in spiral ganglia

spiral ganglia are

pseudounipolar

the auditory nerve is formed by

The cochlear nerve branching together with the vestibular nerve branch

how is frequency mapped?

tonotopically along the length of the basilar membrane

how do we know that frequency is tonotopically mapped

Cochlea nerve fibers respond preferentially to particular frequencies

has “preferred frequencies”

what is step 5 of encoding sound?

Tonotopic projections from spiral ganglion carry auditory information to the medulla

what receives auditory input from the cochlear nerve?

three cochlear nuclei in the rostral medulla

the cochlear nuclei are characterized by what traits?

tonotopic

monoaural

what does being monoaural mean?

receives auditory input from ipsilateral ear

what is step 6 of encoding sound?

Medulla cochlear nuclei project bilaterally to superior olive complex (a.k.a. superior olivary complex) in the pons

the superior olive complex receives auditory information from _ and thus is _

both ears

binaural

the superior olive complex is important for

ltheocating the source of a sound

which structure does the superior olive complex work with to locate the source of a sound?

medial nucleus of the trapezoid body

what is step 7 of encoding sound?

Several relays in the midbrain -> thalamus -> cerebral cortex

What is the pathway for sound reaching the midbrain?

Nucleus of the lateral lemniscus → Inferior colliculus (tectum) in midbrain → medial geniculate complex (medial geniculate nucleus) in thalamus → primary auditory cortex in temporal lobe

post-superior olive brain regions are all _ and also show some _

binaural

tonotopy

ascending brain regions respond to

more and more specific stimuli

all ascending pathways leading to the cortex converge on the

MGC

the MGC is

the medial geniculate complex

feedback exists at

all levels of the auditory pathway

Interaural timing difference

sounds from the same source will arrive at two ears with a time difference

interaural timing difference is processed in

the medial superior olive