Lecture 4 Flashcards
Features of sound (4)
Human audible spectrum
Features: amplitude, frequency, waveform, phase
Threshold: 20Hz - 20 kHz
-> SPEECH around 3kHZ **
Mechanoelectric transduction & tonotopy
MT: Sound into neural activity
Tonotopy: cochlear frequency representation
Outer ear (3 structures, functions)
Sound source elevation cues:
- Pinna
- Cochlear
Auditory meatus
-> gathers sound energy for tonotopic membrane
-> boosts sound around 3 kHz
Middle ear (3 structures & function)
Structures: Malleus, incus, staples (OSSICLES)
Function: boots pressure from eardrum 200x
Mechanical processes:
> Lever action of ossicles
>Focus force of large tympanic membrane
(to small diameter oval window)
What regulates sound transmission efficiency in the middle ear
Tensor tympani & stapedius muscles
Bonus: innervated by CN V & VIII
Inner ear main functions
Transform sonic pressure into neural impulses
Mechanical frequency analyzer
Cochlea structure & tonotopy
(see 4.7)
Organ of Corti
(where, # stereocilia, inner vs out hair cells)
Where: between basilar & tectorial membranes
30 - couple hundred stereocilia
Inner hair cells: afferent
Outer hair cells: efferent
Connected by tip-links
Hair bundle displacement
Towards tallest stereocilia = STRETCH TLs
-> K+ enters → depolarization → Ca+ enter → NT release
Other direction = COMPRESS TLs
-> channels close → hyperpolarization
Hai bundle receptor potential (high vs low ƒ ? gating>)
Biphasic (some channels open @ rest)
GRADED potential
-> track displacement!
Low ƒ: hair cells can track sinusoidal motion
High ƒ: hair cells don’t faithfully track
Mechanical gating
Labeled line coding
Tonotopic organization of hair cells corresponds to ƒ
Auditory fibers & stereocilia
Every auditiory fiber innervates
ONE hair cell → specific
Tuning curve & characteristic ƒ
Tuning curve: min. level of sound to increase a fiber’s rate is most “attuned” to (lwoest tuning curve)
Phase locking
< 3kHz → fibers only fire in rising phases
Used for sound localization (temporal info)
Endolymph
Scala media
Rich in K+
Poor in Na+
+80 mV
Drives INFLUX of K+ into stereocilia
-> depolarization then opening of K+ & Ca2+ channels
Perilymph
Scala tympani/vestibuli
Poor in K+
Rich in Na+
0 mV
Drive K+ EFFLUX & repolarization
- Perilymph/endolymph difference: ENDOCOCHLEAR potential
Cochlea to brainstem ascending pathway
Innervates 3 cochlear nucleus parts
-> anteroventral, posteroventral, dorsal
CN VIII in cochlear ganglion
*tonotopic organization maintained
Parts of cochlear nucleus (3)
Superior olivary complex (pons)
Manaural pathway
Inferior colliculus (midbrain)
Interaural time difference (ITD)
Binaural inputs to MSO (from AVCN)
Bipolar dendrites:
> lateral gets ipsilateral input
> medial fets contralateral input
Maps LOCATION - via diff. neurons firing
ONLY SOUNDS < 3kHz ! Need phase locking
(see 4.19)
Interaural intensity difference (IID)
Due to “shadow” of lower intensity at far ear
INPUTS to LSO
1) -> direct EPSP from ipsilateral input
2) -> inhibition from MNTB, contralateral input (weaker than (1))
(see 4.20)
Localization: horizontal vs vertical
Horizontal: ITD (low ƒ) & IID (high ƒ)
Vertical:
> pinna spectral filtering
> processed in dorsal cochlear nucleus
Inferior colliculus
Integrates LSO/MSO + lemniscal complexes + cochlear nucleus directly
Conveys timing, intensity, frequency
Auditory thalamus
All ascending auditory info must pass through medial geniculate complex of the thalamus
Auditory cortex
Ventral MGC → primary AC
Dorsal MGC → secondary AC
Primary AC: precise tonotopic map (EE & EI)
Secondary AC: belt & parabelt: diffused MGC input
Bonus: Wernicke’s area continuous of 2nd AC
