Download Cranial nerves of hearing and balance, plus auditory pathways

Hearing and
sense of equilibrium (balance)
Cranial Nerve VIII
(Acoustic-Vestibular;
Vestibulocochlear)
Chapter 9
Sensory
VIII (Acoustic- Hearing: cochlear nerve
branch of VIII receives
vestibular;
input from the cochlea
vestibulocochlear)
VIII (Acousticvestibular;
vestibulocochlear)
Balance: vestibular nerve
branch of VIII receives
input from the vestibular
system and cerebellum
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
Hearing: cochlear nerve branch carries
efferent fibers to cochlea
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?
• Axonal processes of
•
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)
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
Auditory pathways from
cochlear nucleus to
primary auditory cortex;
notice both ipsilateral and
contralateral pathways.
Internal
auditory canal
(through bone)
• 1st-order sensory
neurons synapse to 2ndorder sensory neurons
in nuclei of the
brainstem (at and just
below the juncture
between the pons and
medulla)
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?
• Selective impairments of specific
•
•
•
•
•
Disorders at the
frequencies, due to hair cell damage
– Noise-induced hearing loss (esp. high subcortical level
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