VIII functions as SSA: Special Sensory Afferent

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

2. VIII functions as SSA: Special Sensory Afferent

3. Vestibulo-cochlear nerve has two branches (#7 in diagramis 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. CN VIII enters the skull at internal acoustic meatus • Vestibular and cochlear braches combine to make up CN VIII www.ece.rice.edu/~dhj/cochlea.jpg

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. 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 complexTrapezoid bodySuperior olivary nucleusLateral lemniscusInferior colliculus & its brachiumMedial geniculate body & its radiating fibersPrimary (Heschl gyri) and secondary auditory cortices in the temporal lobe

10. Where is the primary auditory cortex? • In Heschl’sgyrus, 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’sgyrus) • 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