Neural Basis M14

hearing basics

•Transduction of sound from acoustical to mechanical energy

•Transduction of mechanical energy into electrochemical energy

•Transmission and interpretation of electrochemically produced signal

•Outer ear collects and shapes sound

•Middle ear provides match between the low impedance of air and the high impedance of endolymph

•Inner ear converts sound (now in fluid) to electrical code

•VIII nerve and Brainstem pathways transmit sound and perform rudimentary processing

•Cortex provides speech interpretation and linguistic processing

osseous labyringth

•Consists of

–Vestibular region

•Semicircular canals

–Vestibule

–Cochlear canal

osseous vestibular space

•Semicircular canals

•Ampulae in each

– receptor organ for position in space

cochlear canal

• coiled appearance

• base near  vestibule

• 2-3⁄4 times

• apex

• core of  osseous labyrinth is modiolus

modiolus

– finely perforated bone (the core)

– VIII vestibulocochlear nerve  through  perforations

–Ganglion cells for VIII in modiolus

–Continuous with internal auditory meatus

bony labyrinth of cochlea

•Divided into 2 incomplete chambers

•Scala vestibuli

•Scala tympani

•Separated partially by osseous spiral lamina

osseous spiral lamina

•Spiral shelf that separates scala tympani and scala vestibuli

•Gets progressively smaller as approach apex

•i.e., basilar membrane will get wider…

•At apex, hooklike communication = helicotrema

membranous vestibular labyrinth: semicircular canals

•Anterior (a.k.a. superior): Lateral rotational  movement (i.e., head to shoulder)

•Posterior: anterior-posterior movement (i.e. “yes”)

•Horizontal (a.k.a. lateral): rotational movement in transverse plane (i.e. "no”)

•Utricle and saccule:  acceleration of head in head tilt

scala media

(cochlear duct) is a membranous tube placed between Scala tympani and scala vestibuli

landmarks of cochlear duct

•Osseous spiral lamina

•Reissner’s membrane

•Perilymph in scala tympani, vestibuli

•Endolymph in scala media

•Stria vascularis

•Vascular supply

•Basilar membrane

•Organ of corti

organ of corti

•Four rows of hair cells

•Bed of deiter’s cells (support)

•12,000 Outer hair cells (OHC) = 3 rows

•3500 Inner hair cells (IHC) = 1 row

•Tunnel of corti

•Reticular lamina

•Cilia protrude

on modiolar side

•Spiral limbus

•Tectorial membrane (touching membrane)

•Outer hair cells imbedded in Tectorial membrane

hair cells

•Inner hair cells:  “U” pattern

•40 stereocilia on each HC

•Outer hair cells:  “W” pattern

•120 stereocilia on each HC

•Cilia are all connected by links

•Stereocilia in apex are longer than those in base

afferent innervation

•Each inner hair cell is connected  ten VIII nerve fibers

•= “many-to-one” innervation

•Each outer hair cell shares  innervation with 10 other outer hair cells

• all  innervated by  same VIII nerve fiber

•“one-to-many” innervation

•95% of VIII nerve arises from inner hair cells

•5% of VIII nerve arises from outer hair cells

at habenula perforata of osseous spiral lamina

•Habenula = wispy structure

•Perforata = perforated

•= the most distal part of osseous spiral lamina

•All fibers are myelinated

•Join as VIII nerve

•Nucleus of VIII nerve within modiolus, in spiral ganglion

efferent fibers

•Habenula = wispy structure

•Perforata = perforated

•= the most distal part of osseous spiral lamina

•All fibers are myelinated

•Join as VIII nerve

•Nucleus of VIII nerve within modiolus, in spiral ganglion

cochlear mechanical events: traveling wave

basilar membrane is a tuned structure

•Just as sing into piano

•Strings of resonant frequency related to your speech will vibrate (resonate) with your speech

•So BM is tuned to resonate to frequency differences

–Low frequencies at apex (~20 Hz)

–High frequencies at base (~20,000 Hz)

traveling wave

•Point of maximum perturbation = max excitation on basilar membrane

•Bends stereocillia toward apex, which causes excitation

•(toward base does not stimulate to depolarize)

inner hair cells and the tectorial membrane

•are not embedded in the tectorial membrane, and require turbulence of fluid movement to be depolarized

outer hair cells and tectorial membrane

are embedded in the inferior tectorial membrane, causing a sheering action that depolarizes the hair cell

auditory mechanism: electrical events

•Depolarization of hair cell

•VIII nerve potentials:  resting and evoked potentials

•Intensity coding

depolarizing the hair cell

•Both IHC and OHC have same process

•Traveling wave results in cilia being deflected at point of maximum perturbation

•Cilia are deflected, causing ion channels to open and glutamate is released

•Potassium enters hair cell

•Activates VIII nerve fiber

stria vascularis and depolarization

•Potassium in hair cell migrates into perilymph of scala tympani, and is taken up by stria vascularis

•Potassium is then pumped back into endolymph to re-establish potential gradient

inner hair cell function

•Process frequency of stimulation

Firing rate codes intensity of signal

outer hair cell function

•Serve as sound amplifiers

•When hair cell depolarizes, cell pulses basilar membrane at the frequency of stimulation, thereby amplifying the incoming signal!

VIII nerve potentials

•Resting potential

•Evoked potentials

•VIII nerve responses and poststimulus time histograms

•Rate-intensity function

VIII nerve resting potential

•Scala vestibuli is +5mV relative to the scala tympani

•Scala media is +80 mV relative to the scala tympani

•Hair cell has -70 mV negative potential

•Total potential difference between endolymph and hair cell is 150 mV

•Stria vascularis is essential for maintenance of potential difference

VII nerve evoked potentials

•Cochlear microphonic:  alternating current artifact that follows basilar membrane movement

•Summating potential:  sustained current shift from HC depolarization

•Whole nerve action potential:  massed response of VIII nerve

intensity coding

•As intensity increases, rate of VIII nerve firing increases

•Codes signal intensity for processing by auditory nervous system

auditory brainstem response

•This is a whole-nerve action potential response of the VIII nerve

•It reflects movement of the impulse up through the auditory pathway (to be discussed).

tonotopic array

•Cochlea is arrayed so frequency decreases as move toward apex

•VIII nerve is also tonotopically arrayed

•Wrapped like a jelly roll; high freq on outside

This tonotopicity is maintained all the way to the cortex

VIII nerve fibers have a characteristic firing signature

•When stimulated they have a high rate of firing

•Then taper off to a plateau

•When stimulus stops, they drop below their “noise” floor

•This is termed “primary”

fibers of VIII nerve have a characteristic frequency

•Reflects tonotopic array

•If hair cell is at the 10000 Hz point, when 10000 Hz is presented, it will fire

•Thus, its characteristic frequency (CF) is 10000 Hz (see CF to in this image)

•If you put an electrode on that fiber, it will only fire when the signal is 10000 Hz

•Sharpness of “tuning curve” reflects how well the fiber “discriminates” its target

tuning curve

•Represents specificity of hair cell activation

•Reflects basilar membrane, hair cell, and VIII nerve activity

•Sharpness of tuning curve reflects frequency specificity of IHC

VIII nerve

•Afferent auditory pathway:

•Cochlear nucleus (CN)

•Superior olivary complex (SOC)

•Inferior colliculus (IC)

•Lateral lemniscus (LL)

•Medial geniculate body

•Efferent pathways

•Crossed and uncrossed olivocochlear bundles

auditory afferent and efferent pathways

•Afferent auditory pathway:

•Cochlear nucleus (CN)

•Superior olivary complex (SOC)

•Inferior colliculus (IC)

•Lateral lemniscus (LL)

•Medial geniculate body

•Efferent pathways

Crossed and uncrossed olivocochlear bundles

afferent auditory pathway

•VIII vestibulocochlear nerve from cochlea

•Major nuclei of pathway

•Cochlear nucleus

•Superior olivary complex

•Lateral lemniscus

•Inferior colliculus

•Medial geniculate body

•Terminates at Heschl’s gyrus of cerebral cortex

cochlear nucleus

first site of signal analysis in the auditory pathway

•This is the first location of significant analysis of the acoustical signal

•Information in the acoustical signal is decomposed into features and elements

•e.g., upsweep, down-sweep, steady-state, onset, etc.

cochlear nucleus: 3 nuclei

anteroventral cochlear nucleus

dorsal cochlear nucleus

posteroventral cochlear nucleus

anteroventral cochlear nucleus

•Output is clean copy of input (no enhancement or analysis)

•Sound localization to superior olivary complex

•Part of dorsal “where” stream

dorsal cochlear nuceleus

•Part of auditory “what” stream

•Octopus cells for timing analysis

•Bushy and stellate cells to sharpen temporal and spectral signal

•Hypothesized bushy cell sharpening function

•Globular cells sharpen high frequency information

•Part of ventral “what” auditory stream

posteroventral cochlear nucleus

•Part of  ventral “what” auditory stream

cell types of cochlear nucleus

•Large spherical cells (AVCN): sensitive to timing of the signal; output to MSO of SOC

•Small spherical cells: sensitive to intensity; output to LSO of SOC

•Octopus cells:  in PVCN, sensitive to timing

•Stellate cells:  send output to periolivary nuclei for efferent system

•Octopus cells:  onset and offset sensitive

•Globular cells:  frequency sharpening

cochlear nucleus responses

•Wide variety of neural responses at cochlear nucleus

•Primary-like:  resemble VIII nerve response; from Bushy cells

•Onset-sensitive: respond to signal onset

•Offset-sensitive: respond to signal offset

•Up-sweep, down-sweep:  respond to transitions

•Pausers (fire, stop, then start firing again)

•Build-up neurons have slow onset

hoq bushy cells appear to process frequency specificity

superior olivary complex

second stop in auditory pathway

•First site of binaural interaction

•Specialized for localization of sound in space

•Has phase and intensity sensitive neurons

lateral superior olive

superior olivary complex nuclei

•Uses temporal information (signal phase) to identify signal source location

medial superior olive

superior olivary complex nucli

•Uses frequency information to identify localization of sound source

periolivary nuclei

•Source for olivocochlear bundle

•Efferent system of cochlea

•Maximizes signal-to-noise benefit

lateral superior olive

•Intensity sensitive sense intensity diff between ears (for greater than 1000 Hz sounds)

•Basically compares sound at two ears and tells you where it’s coming from

medial superior olive

•Phase sensitive neurons sense phase difference between ears for low frequency sound (less than 1000 Hz)

lateral lemniscus

3rd stop on auditory pathway

•Consists of

•Stria of Held

•Stria of Monaco

•Trapezoid body

•Receives output of cochlear nucleus and superior olivary complex

•Involved in binaural hearing

inferior colliculus

4th stop in auditory pathway

•Adjacent to superior colliculus

•Involved in localization

•Sharp tuning curves

•Have intensity-sensitive neurons

•Sensitive to interaural time difference

•Site of visual/auditory interaction (superior colliculus is visual relay)

•Output of inferior colliculus to XI accessory for sternocleidomastoid (turn head toward sound source)

medial geniculate body

last stop before cerebral cortex

•Nucleus of thalamus

•Still retains tonotopic array

•Receives input from IC, SOC

•Ventral portion projects to BA 41 of cortex (Heschl’s gyrus; core)

•Dorsal portion projects to BA 42 (belt) of cortex

efferent pathways of audition

rostral system

caudal system

lateral olivocochlear bundle

rostral system

efferent pathway of audition

•Arises from cortex

•Includes SOC, MGB and insula

•Target auditory signal presumably identified by cortex and insula

•Terminates at SOC, which attenuates non-important auditory information from cochlea

caudal system

efferent pathways of audition

•Arises from MSO

•Contralateral, to IHC and OHC

•Attenuates non-essential signal

lateral olivocochlear bundle

efferent pathway of audition

•Arises from LSO

•Ipsilateral to CN and hair cells

•Attenuates non-essential auditory information

vestibular pathway

•Arises from ampullae, utricle, and saccule

•Superior and inferior vestibular branches converge on superior vestibular ganglion in IAM

•Innervates vestibular nuclei of pons and medulla

auditory reception at cerebral cortex

•Receives input from MGB

•Still maintains tonotopic array

•Are neurons with sharp tuning curves; Neurons with CF

•Organized in columns

•All in column have same CF but respond to different parameters

•e.g., up-sweep, down-sweep, onset, offset, steady-state

Clearly capable of processing speech signal

3 major processing regions of auditory reception at the cortex

core

belt

parabelt

the core

•Core:   Heschl’s Gyrus (BA 41)

•Consists of 3 areas, all with tonotopic arrays; all receive the same input from MGB

•A1:  Classically Heschl’s gyrus

•R:  rostral core

•RT:  rostrotemporal core

belt

•Belt: Encircles Core

•8 regions

•CM: caudomedial; auditory and somatosensory sensitive

•CL:  caudolateral; localization sensitive

•ML:  mediolateral; involved in visual processing

•MM:  middle medial; output to frontal eye region

•RL:  rostral lateral; species specific vocalization

•RM:  localization

•RTM and RTL project to parabelt

parabelt

•Receive inputs from belt, MGB, and other regions

•Output to frontal lobe (visual processing, BA8; perhaps for visual localization)

•Output to BA 46 (visual memory); BA 10 (visual-auditory integration)

what and where stream for audition

•Hickock & Poeppel (2004) found evidence for “what” and “where” streams similar to vision

•Dorsal stream (blue)

•Ventral pathway (purple) is the “what” stream of meaning (STS and MTG)

•Dorsal pathway is “where” (blue) but also sound-to-articulation mapping

•This is auditory-to-motor, as in analysis by synthesis

•Dorsal appears to be speech analysis/production related, while ventral appears to be auditory meaning related

cortical auditory reception

•Core: Heschl’s gyrus of temporal lobe (BA 41)

•Three receptive regions: R, A1 and RT

•All have full frequency representation

•Belt: BA 42 higher order processing area

•Processing of species-specific vocalizations (“what” stream)

•Processing of location (“where” stream)

•Signal analysis

•Parabelt

•Higher level processing

•Belt and Parabelt project to

•Superior temporal sulcus of temporal lobe

•Temporal pole

•Parietal lobe: BA 5 & 7

Frontal lobe: BA 10, 12, 8, and 46