Unit 2: Part 4-5- Coding in the auditory nerve

sounds transduction

process by which sound waves that have been transmitted to the cochlea are converted into electrical impulses by the inner hair cells of the organ of corti

OHC without motility

only in a damaged cochlea

three tricks of the auditory nerve

-frequency tuning

-timing

-firing rate

frequency tuning

codes frequency via tonotopy PLACE THEORY

timing

codes frequency via phase locking

TEMPORAL THEORY

firing rate

codes level

threshold

minimum SPL required to get the AN to fire

characteristic frequency

the frequency at which the lowest threshold is obtained

apical surface

low characteristic frequency

middle surface

mid characteristic frequency

basal surface

high characteristic frequency

phase locking

neurons tend to fire at a particular phase of the stimulus

the inter spike interval

used to infer the period, frequency and stimulus (the period)

if the stimulus frequency is less than _______Hz the neuron can fire on most stimulus cycles

500

at higher frequencies the nueron will skip stimulus cycles due to

the refractory period

temporal theory

phase locking to a particular phase in the stimulus cycle

volley theory

phase locking summed across many fibers (population response)

how is level coded

- rate coding

-spread of excitation

rate coding

using firing rate to convey stimulus level (higher rate usually means higher stimulus level

saturation of a curve

no info about level above about 50 dB SPL

spontaneous rate

no info about level below about 25 dB SPL

low spontaneous rate = what threshold

high

high spontaneous rate = what threshold

low

spread of excitation

at higher stimulus levels, more of the basilar membrane moves significantly and more neurons will therefore be stimulated

what happens after the auditory nerve

sound information enter the brain: the central auditory nervous sytem

four main parts of the auditory brain

brainstem & midbrain

thalamus

cerebrum

levels of the auditory pathway in ascending order starting at the auditory nerve

AN->CN->LSO/MSO->LL->IC->MGB->A1

auditory information is processed in multiple nuclei and in

multiple parallel streams

the majority of the projections from cochlear nucleus CROSS THE MIDLINE so

right ear primarily activated left midbrain and cortex (and vise versa)

tonotopy starts in teh

cochlea

high frequencies are displayed as the color

blue

low frequencies are displayed as the color

red

the inferior colliculus IC is on the

dorsal surface of the midbrain

frequency is mapped to

depth (you cant view different frequencies on the surface)

in the brainstem

cochlear nucleus CN

superior olivary complex SOC

in the midbrain

inferior colliculus IC

in the thalamus

medial geniculate body MGB

in the cerebrum

auditory cortex A1

the level neural firing code is looking at a neural firing at one instant whereas the frequency encoding requires

observation of neural firing over a stimulus cycle (phase)

Ipsilateral:

parts of the CANS

-cochlear nucleus (CN)

-superior olivary complex (SOC)

-lateral lemniscus (LL)

-Inferior colliculus IC

-medial geniculate body (MGB)

parts of the cochlear nucleus CN

-antero ventral

-postero ventral

-dorsal

cochlear nucleus CN receives input from

-auditory nerve on the ipsilateral side

cochlear nucleus CN sends output to

SOC, LL, IC

main functions of the cochlear nucleus CN

-diversification of AN input

-extract monaural spectral cues for vertical plane localization

-maintains and refines timing information from the auditory nerve

superior olivary complex SOC parts

-medial superior olive MSO

-lateral superior olive LSO

-medial nucleus of trapezoid body MNTB

superior olivary complex SOC recieves input from

-MSO: AVCN on both sides

-LSO: ipsilateral AVCN (directly) & contralateral AVCN

superior olivary complex SOC sends outputs to

afferents to LL

efferents to OHC

superior olivary complex SOC functions

-MSO and LSO involved in binaural processing

-MNTB provides inhibitory input to LSO

-important for specialization

Lateral Lemniscus LL parts

ventral and dorsal

Lateral Lemniscus LL recieves input from

-contraventral CN

-ipsilateral MSo

-bilateral LSO

-contra dorsal CN

Lateral Lemniscus LL sends output to

IC

lateral lemniscus LL functions

-ventral LL resposible for extracting harmonics

-dorsal LL part of the sound localization pathway

Inferior Colliculus IC parts

-central nucleus IC

-external nucleus IC

-dorsal cortex IC

Inferior coliculus IC receievs input from

ipsilateral LL

inferior colliculus IC sends outputs to

-ipsilateral through brachium of IC to MGB

inferior colliculus IC functions

-all major pathways converge here

-multimodal switchboard

-detection of pitch

-provides building blocks to understand phonemes in speech

-involved in localization

medial geniculate body MGB parts

-ventral

-medial

-dorsal

medial geniculate body MGB recieves input from

-ipsilateral IC

Medial geniculate body MGB sends ouputs to

-ventral connects to ipsi A1

-dorsal and medial connect to ipsilateral association cortex

medial geniculate body MGB functions

-multimodal

-involved in directing attention

-has a tight feedback loop with the cortex

the cochlear nucleus CN is made up of

-bushy cells

-octopus cells

-fusiform cells

bushy cells (AVCN)

spherical and globular

enahnce temporal fluctuations for binaural sound localization

spherical bushy cells

receieve infroamtion from endbulbs of Held, the largest synapse in the entire brain

octopus cells (PVCN)

detect onsets

fusiform cells (DCN)

sensitive to spectral notches, convey information about sound elevation

diversification happens in

cochlear nucleus CN

thanks to various types of cells with different response properties

Brodmann area

41 (primary auditory cortex)

42 (secondary auditory cortex)

22 (auditory association cortex)

44/45 (Brocas Area)

OHC shape

cylandrical

IHC shape

flask shaped

how many OHC cilia and how

150 steriocilia

V/W shape

how many IHC cilia and how

50 stereocilia

slightly curved rows

OHC is embedded

in teh tectorial memrbane

IHC does not have contact with

the tectorial membrane

job of IHC

send information to the brain

job of OHC

bosot hte motion of the traveing wave through active process

passive mechanics process

happens in IHC