1. Hearing and sense of equilibrium (balance) Cranial Nerve VIII (Acoustic-Vestibular; Vestibulocochlear) Chapter 9

2. VIII functions as SSA: Special Sensory Afferent
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
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

3. Vestibulo-cochlear nerve has two branches (#7 in diagram is cranial nerve VIII; see its two branches?)
Vestibular nerve (bipolar neuron)
Nerve of equilibrium
Sensory (afferent) input from
utricle, saccule & semicircular canals of inner ear (#8 in diagram)
Efferent fibers to cerebellum, spinal cord, & III, IV, VI (Why?)
Cochlear nerve (bipolar neuron)
Nerve of hearing
Sensory (afferent) input from cochlea (#6 in diagram) to the brain stem
Efferent fibers to cochlea (Why?)
Note where VIII enters skull. This is called the INTERNAL AUDITORY MEATUS. Cranial nerve VII (facial) also exits skull here.

4. Bipolar sensory cells (SSA)
Cell bodies of 1st order sensory neurons:
In vestibular ganglion (_____), in the case of the vestibular nerve
In spiral ganglion (_____), in the case of cochlear nerve

5. Vestibular and cochlear braches combine to make up CN VIII
CN VIII enters the skull at internal acoustic meatus

6. CN VIII enters the skull at the same place where CN VII enters
So what would happen if the skull is fractured near the internal auditory meatus?
What nerves would it damage?
What functions would be affected?

7. 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)
Axonal processes of first order neurons enters brainstem at “cerebellopontine angle” (juncture of pons, medulla and cerebellum)
Cranial nerve VIII contains the axonal processes of the bipolar neurons from both the vestibular and cochlear nerves

8. Internal auditory canal (through bone)
1st-order sensory neurons synapse to 2nd-order sensory neurons in nuclei of the brainstem (at and just below the juncture between the pons and medulla)
Auditory pathways from cochlear nucleus to primary auditory cortex; notice both ipsilateral and contralateral pathways.
Internal auditory canal (through bone)

9. Cochlear complex Trapezoid body Superior olivary nucleus Lateral lemniscus Inferior colliculus & its brachium Medial geniculate body & its radiating fibers Primary (Heschl gyri) and secondary auditory cortices in the temporal lobe

10. Where is the primary auditory cortex?
In Heschl’s gyrus, in the superior temporal lobe

11. Locations of primary and higher order (association) auditory cortices
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

12. Functions of the central auditory system
Transmission of auditory system’s tonotopic representation i.e., high vs. low frequencies travel separately through
the cochlea
the cochlear nucleus
the higher nuclei
…into separate locations in primary auditory cortex
Transmission of loudness and timing of auditory signals
For speech and sound processing
For sound localization
Integration of auditory input with reticular system and reflexive eye/head positioning
For processing of critical signals

13. Functions of the central auditory system
Cochlear nuclear complex
Receives afferent signal ipsilaterally
Transmits signal to ipsilateral and contralateral pathways
Superior olivary nucleus
Both ipsilateral and contralateral input, important for sound localization
Lateral lemniscus
Has stronger contralateral input, but no deficits of hearing in either ear if damaged (bilateral)

14. Fxs of the central auditory system (cont.)
Inferior colliculus
Afferents from lateral lemniscus
Integrates intensity and timing of input from both ears, for sound localization
Part of tectal (midbrain) circuitry that integrates eye, head and body movement reflexively toward visual and auditory stimuli
Involved in startle reflex
Works with reticular formation to select, sequence, analyze, inhibit, and elaborate auditory information

15. Fxs of the central auditory system (cont.)
Medial geniculate body
Thalamic relay center
Possible functions may be to
integrate attention with auditory afferents
Regulate emotions and visceral functions

16. Fxs of the central auditory system (cont.)
Medial geniculate body
Thalamic relay center
Possible functions may be to
integrate attention with auditory afferents
Regulate emotions and visceral functions

17. Fxs of the central auditory system (cont.)
Primary auditory cortex (area 41, Heschl’s gyrus)
Maintains tonotopic organization
Discriminates timing and intensity of auditory stimuli
Gathers “raw data” for speech perception
Frequency
Timing
intensity

18. Fxs of the central auditory system (cont.)
Higher order (association) auditory cortex
Integrates raw data from primary auditory cortex to make sense of it
What was that sound I just heard?
What did that person just say?
What does the overall intonation pattern mean?

19. Disorders at the subcortical level
Selective impairments of specific frequencies, due to hair cell damage
Noise-induced hearing loss (esp. high frequencies) or ototoxic drugs
Degeneration of spiral ganglion (progressive hearing loss)
Acoustic neuroma: c.n. VIII and cochlear nuclei damaged, results in deafness in that ear
Neural problems secondary to genetic disorders
Moebius syndrome: Congenitally underdeveloped cranial nerves VI and VII, but may also include V and VIII (hearing affected)
Central auditory processing and sequencing disorders; location(s) of underdevelopment or damage not well understood
Demyelinating disease (e.g. multiple sclerosis) affects pathways in unpredictable locations

20. Disorders at the cortical level
Central types of deafness
Cortical deafness: damage to both
primary auditory cortices (L & R)
Auditory agnosia: “What was that complex sound?”; inability to interpret or recognize non-verbal sounds from damage to part of auditory association cortex
Pure word deafness (rare): speech cannot be understood through hearing, from damage to part of auditory association cortex, but…
language can be understood through writing
other types of sounds can be interpreted
Wernicke’s aphasia: Posterior area 22 on left
Poor language comprehension WITH language production, reading, and writing problems as well (overall language deficit)
Receptive aprosodia: Lesion in right temporal-parietal-occipital area