Chapter 11: Auditory System

Sound wave

Compressed air (increased density)

Rareified air (less dense)

cycle

distance between successive compressed patch

pitch (sound frequency)

number of cycles per second

intensity (loudness) = amplitude

difference in pressure between compressed and rarefied patches of air

phase

location of sine wave relative to some time point

Structure of ear

outer ear

pinna conducts sounds into auditory canal towards tympanic membrane (ear drum)

middle ear

tympanic membrane

ossicles

malleus

incus

stapes

inner ear

cochlea - fluid-filled (where receptor cells live)

Basic auditory pathway

sound waves move tympanic membrane

tympanic membrane moves ossicles

ossicles move membrane at oval window

motion at oval window moves fluid in cochlea

movement of fluid in cochlea causes response in sensory neurons

Middle ear amplifies sound

2 impedance matching mechanisms to overcome energy mismatch between air and water

lever-arm ratio

ossicles act as levers

malleus arm is shorter than incus arm

small movements in malleus --> large movements in incus

Area ratio

amplification due to larger area of tympanic membrane compared to the footplate of the stapes in the oval window

pressure = force/area

3 chambers (scalae) of cochlea

scala vestibuli

scala media

scala tympani

fluid: perilymph

perilymph composition is like extracellular fluid - low K, high Na

fluid: endolymph

composition like intracellular fluid - high K low Na

stria vascularis pumps K into scala media

sound transduction accomplished in the organ of corti

organ of corti sits on basilar membrane

inner ear is the frequency analyzer

cochlea breaks incoming complex sounds into component frequencies

basilar membrane - moves up and down with movement of perilymph

move toward base: narrow/stiff, high frequency

move towards apex: wide/floppy, low frequency

organ of corti: from fluid motion to changes in Vm

hair cells form synapses on neurons whose cell bodies are in the spiral ganglion

spiral ganglion cells receive synaptic input from hair cells, project to the brain in the VIII

Hair cell

sterocilia looks like hair

kinocilium is the tallest sterocilia

displacement of basilar membrane causes deflection of stereocilia

movement of stereo cilia toward kinocilium

opens mechanically gated channels, depolarizes cell

movement away from kinocilium

hyperpolarizes cell

mechanically gated channels are connected by elastic filaments called tip links

resting Vm of cell relative to perilymph: -70mV

very high K in endolymph, Vm=+80mV, Ek=0mV

K moves into cell through mechanically gated channel

depolarization opens V-gated Ca channels

Ca triggers neurotransmitter release

8th nerver afferents depolarized

receptor potential follows stimulating waveform for low frequencies

when it gets too high of a frequency, hair cells can't move that fast

just a constant depolarization

outer hair cells

80% of all hair cells, only 5% of afferent innervation

inner hair cells

20% of all hair cells

95% of afferent innervation

OHCs receive most of the efferents and can modulate sensitivity of IHCs

cochlear amplifier

use prestin

as cell membrane potential fluctuates up and down, pristine molecules expand and contract

cell itself expands and contracts in time

this makes the basilar membrane move, amplifies movements

Auditory pathways (same for VCN and DCN, but DCN passes superior olive)

afferents from spiral ganglion enter brain stem in auditory nerves --> ventral cochlear nucleus (IPSILATERAL)

project to superior olive on both sides of brain

ascend in the lateral lemniscus

innervate inferior colliculus

send axons to MGN of thalamus

projects to auditory cortex

DCN: frequency analysis

VCN: sound localization

each 8th nerve fiber has a characteristic frequency - that frequency with the lowest threshold intensity

Place code: tonotopy

Phase locking

low frequencies phase lock - consistent firing of a cell at the same phase of a sound wave

intermediate frequencies use volley principle - pooled activity of many neurons codes every cycle, response is phase locked but not on every cycle

high frequencies can't phase lock, only use tonotopy

Auditory system encodes Intensity

increase intensity by

increasing firing rate

increase number of active neurons

Horizontal plane: interaural timing differences for low frequencies (wavelength > width of head)

wave diffracts around head with no intensity loss

for continuous sounds

only thing that can be compared is the time at which the same phase of the sound wave reaches each ear

Binaural neurons in medial superior olive are tuned to different intramural time delays, EE cells

delay lines and coincidence detectors

Horizontal plane: interaural intensity differences for high frequencies (wavelength < width of head, sounds>2000Hz)

sound reflects off head, creating sound shadow

binaural neurons in lateral superior olive are tuned to different interaural intensity differences, EI cells

sound is less intense in ear with sound shadow

Ipsilateral ear provides excitatory input to LSO cell

contralateral ear provides inhibitory input to LSO cell

pinna is important for sound localization in vertical plane

detection of sound in vertical plane requires only one ear

sound reaches auditory canal by both direct and reflected pathways

brain localizes source of sound by detecting differences in combined sounds from direct and reflected pathways

auditory cortex

columnar organization

isofrequency bands: sound frequency

binaural bands: sound location