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

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

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.

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

Vestibular and cochlear braches combine to make up CN VIII CN VIII enters the skull at internal acoustic meatus www.ece.rice.edu/~dhj/cochlea.jpg

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?

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

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)

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

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

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

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

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)

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

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

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

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

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?

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

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