Hearing and Hearing and sense of equilibrium (balance)sense of equilibrium (balance)

Cranial Nerve VIII Cranial Nerve VIII (Acoustic-Vestibular; (Acoustic-Vestibular; Vestibulocochlear) Vestibulocochlear)

Chapter 9Chapter 9

Sensory This nerve is sensory only—with no motor innervation to muscles-- but it does carry some efferent fibers to other parts of the nervous system

VIII (Acoustic-vestibular; vestibulo-cochlear)

Hearing: cochlear nerve branch of VIII receives input from the cochlea

Hearing: cochlear nerve branch carries efferent fibers to cochlea

VIII (Acoustic-vestibular; vestibulo-cochlear)

Balance: vestibular nerve branch of VIII receives input from the vestibular system and cerebellum

Balance: vestibular nerve branch carries efferent fibers to the cerebellum, and to spinal cord via vestibulospinal tract

VIII functions as SSA: Special Sensory Afferent

Vestibulo-cochlear nerve has two Vestibulo-cochlear nerve has two branches branches (#7 in diagram is cranial nerve VIII; see its two branches?)(#7 in diagram is cranial nerve VIII; see its two branches?)• Vestibular nerve (bipolar Vestibular nerve (bipolar

neuron)neuron)– Nerve of equilibriumNerve of equilibrium

• Sensory (afferent) input fromSensory (afferent) input from– utricle, saccule & utricle, saccule &

semicircular canals of inner semicircular canals of inner ear (#8 in diagram)ear (#8 in diagram)

• Efferent fibers to cerebellum, Efferent fibers to cerebellum, spinal cord, & III, IV, VI (Why?)spinal cord, & III, IV, VI (Why?)

• Cochlear nerve (bipolar neuron)Cochlear nerve (bipolar neuron)– Nerve of hearingNerve of hearing

• Sensory (afferent) input from Sensory (afferent) input from cochlea (#6 in diagram) to the cochlea (#6 in diagram) to the brain stembrain stem

• Efferent fibers to cochlea Efferent fibers to cochlea (Why?)(Why?)

Note where VIII enters skull. This is called the INTERNAL AUDITORY MEATUS. Cranial nerve VII (facial) also exits

skull here.

Bipolar sensory cells (SSA)Bipolar sensory cells (SSA)

• Cell bodies of 1Cell bodies of 1stst order sensory order sensory neurons:neurons:– In vestibular In vestibular

ganglion (_____), ganglion (_____), in the case of in the case of the vestibular the vestibular nervenerve

– In spiral In spiral ganglion (_____), ganglion (_____), in the case of in the case of cochlear nervecochlear nerve

• CN VIII enters CN VIII enters the skull at the skull at internal internal acoustic acoustic meatusmeatus

• Vestibular and Vestibular and cochlear cochlear braches braches combine to combine to make up CN make up CN VIIIVIII

www.ece.rice.edu/~dhj/cochlea.jpg

CN VIII enters the skull at CN VIII enters the skull at the same place where CN the same place where CN VII entersVII enters• So what would So what would

happen if the happen if the skull is skull is fractured near fractured near the internal the internal auditory auditory meatus? meatus? – What nerves What nerves

would it would it damage?damage?

– What What functions functions would be would be affected?affected?

• Axonal processes of Axonal processes of first order neurons first order neurons enters brainstem at enters brainstem at “cerebellopontine “cerebellopontine angle” (juncture of angle” (juncture of pons, medulla and pons, medulla and cerebellum)cerebellum)

• Cranial nerve VIII Cranial nerve VIII contains the axonal contains the axonal processes of the processes of the bipolar neurons from bipolar neurons from both the vestibular both the vestibular and cochlear nervesand cochlear nerves

Note: VIII’s site of entry into brainstem at the

juncture of cerebellum, pons and medulla is

common site of acoustic neuroma (a type of brain

tumor)

Auditory pathways from Auditory pathways from cochlear nucleus to cochlear nucleus to primary auditory primary auditory cortex; notice both cortex; notice both ipsilateral and ipsilateral and contralateral pathways.contralateral pathways.

Internal auditory canal

(through bone)

• 11stst-order sensory -order sensory neurons synapse to neurons synapse to 22ndnd-order sensory -order sensory neurons in nuclei of neurons in nuclei of the brainstem (at the brainstem (at and just below the and just below the juncture between the juncture between the pons and medulla) pons and medulla)

Cochlear complexCochlear complex

Trapezoid bodyTrapezoid body

Superior olivary Superior olivary nucleusnucleus

Lateral lemniscusLateral lemniscus

Inferior colliculus & Inferior colliculus & its brachiumits brachium

Medial geniculate Medial geniculate body & its radiating body & its radiating fibersfibers

Primary (Heschl Primary (Heschl gyri) and secondary gyri) and secondary auditory cortices in auditory cortices in the temporal lobethe temporal lobe

Where is the primary Where is the primary auditory cortex?auditory cortex?

• In In Heschl’s gyrusHeschl’s gyrus, , in the in the superior superior temporal lobetemporal lobe

Locations of Locations of primaryprimary and and higher higher order (association)order (association) auditory auditory corticescortices

Primary: #41 and #42, Heschel’s gyrus, extends especially into lateral sulcus (Sylvian fissure)

Association: #22 (two locations). More posterior of these two locations is called Wernicke’s area (circled)

Auditory information that is received by areas 41/42 is passed on to area 22 for INTERPRETATION of what was heard

Functions of the central auditory Functions of the central auditory systemsystem• Transmission of auditory system’s Transmission of auditory system’s

tonotopic representation i.e., high vs. tonotopic representation i.e., high vs. low frequencies travel separately throughlow frequencies travel separately through– the cochleathe cochlea– the cochlear nucleusthe cochlear nucleus– the higher nucleithe higher nuclei

……into separate locations in primary into separate locations in primary auditory cortexauditory cortex

• Transmission of loudness and timing of Transmission of loudness and timing of auditory signalsauditory signals– For speech and sound processingFor speech and sound processing– For sound localizationFor sound localization

• Integration of auditory input with Integration of auditory input with reticular system and reflexive eye/head reticular system and reflexive eye/head positioningpositioning– For processing of critical signalsFor processing of critical signals

Functions of the central auditory Functions of the central auditory systemsystem•Cochlear nuclear complexCochlear nuclear complex

– Receives afferent signal Receives afferent signal ipsilaterallyipsilaterally

– Transmits signal to ipsilateral and Transmits signal to ipsilateral and contralateral pathwayscontralateral pathways

•Superior olivary nucleusSuperior olivary nucleus– Both ipsilateral and contralateral Both ipsilateral and contralateral

input, important for sound input, important for sound localizationlocalization

•Lateral lemniscusLateral lemniscus– Has stronger contralateral input, Has stronger contralateral input,

but no deficits of hearing in either but no deficits of hearing in either ear if damaged (bilateral)ear if damaged (bilateral)

Fxs of the central auditory system Fxs of the central auditory system (cont.)(cont.)

• Inferior colliculusInferior colliculus– Afferents from lateral lemniscusAfferents from lateral lemniscus– Integrates intensity and timing of Integrates intensity and timing of

input from both ears, for sound input from both ears, for sound localizationlocalization

– Part of tectal (midbrain) circuitry Part of tectal (midbrain) circuitry that integrates eye, head and that integrates eye, head and body movement reflexively body movement reflexively toward visual and auditory stimulitoward visual and auditory stimuli

– Involved in startle reflexInvolved in startle reflex– Works with reticular formation to Works with reticular formation to

select, sequence, analyze, inhibit, select, sequence, analyze, inhibit, and elaborate auditory and elaborate auditory informationinformation

Fxs of the central auditory system Fxs of the central auditory system (cont.)(cont.)

•Medial geniculate bodyMedial geniculate body– Thalamic relay centerThalamic relay center– Possible functions may be Possible functions may be

to to •integrate attention with integrate attention with

auditory afferentsauditory afferents

•Regulate emotions and Regulate emotions and visceral functionsvisceral functions

Fxs of the central auditory system Fxs of the central auditory system (cont.)(cont.)

•Medial geniculate bodyMedial geniculate body– Thalamic relay centerThalamic relay center– Possible functions may be Possible functions may be

to to •integrate attention with integrate attention with

auditory afferentsauditory afferents

•Regulate emotions and Regulate emotions and visceral functionsvisceral functions

Fxs of the central auditory system Fxs of the central auditory system (cont.)(cont.)

•Primary auditory cortex Primary auditory cortex (area 41, Heschl’s gyrus)(area 41, Heschl’s gyrus)– Maintains tonotopic Maintains tonotopic

organizationorganization– Discriminates timing and Discriminates timing and

intensity of auditory stimuliintensity of auditory stimuli– Gathers “raw data” for Gathers “raw data” for

speech perceptionspeech perception•FrequencyFrequency

•TimingTiming

•intensityintensity

Fxs of the central auditory system Fxs of the central auditory system (cont.)(cont.)• Higher order Higher order

(association) (association) auditory cortexauditory cortex– Integrates raw Integrates raw

data from primary data from primary auditory cortex to auditory cortex to make sense of itmake sense of it•What was that What was that

sound I just sound I just heard?heard?

•What did that What did that person just say?person just say?

•What does the What does the overall intonation overall intonation pattern mean?pattern mean?

• Selective impairments of specific Selective impairments of specific frequencies, due to hair cell damagefrequencies, due to hair cell damage– Noise-induced hearing loss (esp. high Noise-induced hearing loss (esp. high

frequencies) or ototoxic drugsfrequencies) or ototoxic drugs

• Degeneration of spiral ganglion Degeneration of spiral ganglion (progressive hearing loss)(progressive hearing loss)

• Acoustic neuroma: c.n. VIII and cochlear Acoustic neuroma: c.n. VIII and cochlear nuclei damaged, results in deafness in that nuclei damaged, results in deafness in that earear

• Neural problems secondary to genetic Neural problems secondary to genetic disordersdisorders– Moebius syndrome: Moebius syndrome: Congenitally Congenitally

underdeveloped underdeveloped cranial nerves VI and cranial nerves VI and VII, but may also include V and VII, but may also include V and VIIIVIII (hearing affected)(hearing affected)

• Central auditory processing and sequencing Central auditory processing and sequencing disorders; location(s) of underdevelopment disorders; location(s) of underdevelopment or damage not well understoodor damage not well understood

• Demyelinating disease (e.g. multiple Demyelinating disease (e.g. multiple sclerosis) affects pathways in unpredictable sclerosis) affects pathways in unpredictable locationslocations

Disorders at the Disorders at the subcortical levelsubcortical level

Disorders at the cortical levelDisorders at the cortical level

• Central types of deafnessCentral types of deafness– Cortical deafnessCortical deafness: damage to both : damage to both

primary auditory cortices (L & R)primary auditory cortices (L & R)– Auditory agnosiaAuditory agnosia: “What : “What waswas that complex that complex

sound?”; inability to interpret or recognize sound?”; inability to interpret or recognize non-verbal sounds non-verbal sounds from damage to part of auditory association cortexfrom damage to part of auditory association cortex

– Pure word deafness Pure word deafness (rare): (rare): speechspeech cannot be understood cannot be understood through hearing, from damage to part of auditory association through hearing, from damage to part of auditory association cortex, but…cortex, but…

• language can be understood through writinglanguage can be understood through writing

• other types of sounds can be interpretedother types of sounds can be interpreted

• Wernicke’s aphasiaWernicke’s aphasia: Posterior area 22 on : Posterior area 22 on leftleft– Poor Poor languagelanguage comprehension WITH language comprehension WITH language

production, reading, and writing problems as well (overall production, reading, and writing problems as well (overall language deficit)language deficit)

• Receptive aprosodiaReceptive aprosodia: Lesion in : Lesion in rightright temporal-parietal- temporal-parietal-occipital areaoccipital area