Transcript The Auditory and Vestibular Systems I. Functional Anatomy of the 2 Systems - Overview A.

The Auditory and Vestibular
Systems
I. Functional Anatomy of the 2 Systems - Overview
A. Parallel ascending auditory pathways.
B. Ascending vestibular system.
II. Regional Anatomy
A. Sensory organs
1. Auditory receptor cells and apparatus.
2. Vestibular sensory organs.
B. Brainstem nuclei
1. Vestibular and its projections.
2. Auditory and its projections.
a. Lateral lemniscus
b. Inferior colliculus
c. Medial geniculate nucleus
d. Primary auditory cortex.
e. Wernike’s Area – a higher-order auditory cortex.
Auditory System
I. Functional Anatomy of the 2 Systems Overview
A. Auditory system.
• Tonotopic basis for frequency coding
along the length of the cochlea.
• Tympanic membrane  transduced info
into neural signals.
(next slides)
II. Regional Anatomy: Hearing - Equilibrium
Hearing
• Sounds are waves of compressed air traveling
through space
- sound intensity wave height
- pitch  wave frequency
Organ of hearing (and equilibrium) – inner ear
• Cochlea
• Vestibular
apparatus
Sensory Organs: Hearing
1- The sound waves enter the
external auditory canal and trigger
vibrations of the tympanic
membrane
2- The tympanic membrane induces a
vibration of the ossicles
3- the last ossicle, the stapes,
transmits amplified vibrations to
the oval window
4- The vibrations induce waves in the
perilymph of the various inner ear
chambers
5- the round window absorbs excess
energy and prevent wave
reverberation
6- the fluid wave is transduced into
an electrical signal by the auditory
receptors, the organs of Corti
located on the basilar membrane
Receptors for sound: the organ of Corti
• The hair cells of the organ
of Corti transduce fluid
wave into an electrical
signal
• The energy of the wave
causes the basilar and
vestibular membrane to
move, thus displacing the
cilia from the organ of
Corti
Organ of Corti
Signal transduction
•
Movements of the cilia open or
close potassium channels 
changes in the state of polarization
of the hair cell
•
Changes in potassium leakage due
to cilia bending trigger changes in
neurotransmitters exocytosis
•
The neurotransmitters send an
electrical signal to an afferent
neuron of the cochlear nerve
•
The louder the sound, the more the
cilia bend, the more numerous are
the APs produced
Coding for pitch
• The location of the organs
of Corti on the basilar
membrane codes for pitch
- Organs of Corti located
near the oval window are
more sensitive to high
pitch sounds while the
ones located toward the tip
of the cochlea respond
more readily to low pitch
sound
Coding for sound intensity
Neural pathway for sounds
• Cochlear nerve  nucleus in medulla
oblongata  thalamus  auditory cortex in
the temporal lobe
• So, how do we perceive the direction from
which a sound is coming from?
B. Brainstem nuclei and their Projections
2. Auditory nuclei:
3 major auditory relay nuclei of the brainstem:
A. cochlear nuclei (same side from cn VIII)
(medulla).
B. superior olivary nuclear complex (integration
from both sides) (pons).
C. inferior colliculus (midbrain).
[2 major divisions we noted earlier].
-superior olivary complex is important in sound
localization (major input from AV cochlear
nucleus).
Auditory System
Overview: Cochlear division of
Nerve VIII  cochlear n. (same
side) in rostral medulla.
1. *Anteroventral cochlear n.
 sup olivary n. (both
sides)  lateral lemniscus
 inferior colliculus.
*Important for horizontal location of
sounds, as well as for other aspects of
sound patterns, other than location.
2. Dorsal posteriorventral
cochlear n.  lat lemniscus
Inferior colliculus (opp.
side).
Note the decussations: Important for
integration.
Clin: one can experience loss in only 1 ear
only if lesion is peripheral.
B. Brainstem nuclei and their Projections
2. Auditory nuclei (cont’d):
• Low-freq sounds are distinguished in space by
interaural time difference.
• High-freq sounds are distinguished by difference
in intensity between the ears.
• Different parts of the superior olivary n. (medial
and lateral) are sensitive to these 2 types of
differences.
• Decussation is visible in trapezoid body.
• Feedback pathway: some superior olivary
neurons project back to the cochlea (both sides).
B. Brainstem nuclei and their Projections
2. Auditory nuclei (cont’d):
• Olivocochlear bundle – regulates flow of
auditory info to the brain (much like inhibitory
dorsal horn n. inhibit somatic sensory info.).
Lateral Lemniscus
Most auditory path neurons course in lateral
lemniscus  inferior colliculus.
Some synapse on nucleus of lateral lemniscus 
contralateral inferior colliculus.
Another important site of decussation (Probst’s
Commisure)
Probst’s commisure
B. Brainstem nuclei and their Projections
2. Auditory nuclei (cont’d):
Inferior Colliculus
•
•
•
•
•
Within the midbrain tectum.
The central nucleus within the inf colliculus receives the
auditory info, which will proceed to the medial geniculate
nucleus of the thalamus and the 1° auditory cortex.
Laminated – neurons in a single lamina are maximally
sensitive to similar tonal frequencies.
Receives input from superior olivary n., n. of lateral
lemniscus (both sides), and dorsal n. + pv cochlear
(direct).
Projects to thalamus through the brachium of the inferior
colliculus.
B. Brainstem nuclei and their Projections
2. Auditory nuclei (cont’d):
Medial geniculate nucleus
• Thalamic auditory relay nucleus.
• The major part (ventral division) is tonotopically
organized (receiving its input from the central n.
of the inf coll, which is also tonotopically
organized.
• Therefore, the MGN is also laminated – layers
maximally sensitive to similar frequencies.
• Thalamocortical auditory projections are called
auditory radiations  1° auditory cortex with 2
gyri within sulcus of temporal lobe: Heschle’s Gyri
B. Brainstem nuclei and their Projections
2. Auditory nuclei (cont’d):
Medial geniculate nucleus
• Columnar organization of neurons sensitive to
tones of similar frequencies (isofrequency
columns).
• Also binaural columns (similar interaural
intensity differences – for localization of highfrequency sounds).
• Like other 1° sensory cortices, this has a
prominent layer 4.
Thalamic nuclei
B. Brainstem nuclei and their Projections
2. Auditory nuclei (cont’d):
Wernicke’s Area
• A higher-order auditory cortex for the
interpretation of language. (language on L side of
brain; interpreting emotional content of language
on R side of brain).
• One projection of Wernicke’s area is to Broca’s
motor speech area in the frontal lobe.
Primary Auditory cortex
Higher order auditory cortices
Auditory System
Overview: Cochlear division of
Nerve VIII  cochlear n. (same
side) in rostral medulla.
1. *Anteroventral cochlear n.
 sup olivary n. (both
sides)  lateral lemniscus
 inferior colliculus.
*Important for horizontal location of
sounds, as well as for other aspects of
sound patterns, other than location.
2. Dorsal posteriorventral
cochlear n.  lat lemniscus
Inferior colliculus (opp.
side).
Note the decussations: Important for
integration.
Clin: one can experience loss in only 1 ear
only if lesion is peripheral.
Vestibular System
Sensory Organs: Vestibular:
Ampulae of semicircular canals
Maculae of utricle (linear acceleration) + saccule
Endolymph – gel-like fluid flows over the hair cells with movement
and deflects them – Ca carbonate crystals (otoliths)
Equilibrium
• Ability to detect head
position and movement
(or acceleration)
- Change of speed = linear
acceleration (utricle and
saccule)
- Turning = rotational
acceleration (semi-circular
canals)
Utricle and saccule
• Sensory cells have cilia
extending into a
gelatinous material topped
by otoliths
• Saccule detects backwardfrontward movement
• Utricle detects changes
relative to gravity
Semi-circular canals
• The receptors in the
ampulla are hair cells with
cilia extruding into a
gelatinous mass (cupula)
• When the head rotates, the
cupula moves  cilia
pulled APs (vestibular
nerve  cerebellum …)
• So why does a person become dizzy after
he/she stops spinning?
B. Brainstem nuclei and their Projections
1. Vestibular nuclei – on floor of 4th ventricle.
4 nuclei: inferior, medial, lateral, superior
 ascending projections to VPN of
thalamus  1° vestibular cortex in
parietal lobes (just behind the 1° somatic
sensory cortex).
Can project to nearby parietal areas for
integration of info regarding head motion
with info from somatic sensory receptors
in trunk and limbs.
Vestibular Nuclei:
Vestibular System
Overview:
Head motion  vestibular hair
cell receptors  4 vestibular n. in
rostral medulla and caudal pons:
2 Descending projections to sc
and extraocular muscles (control
movements)  cerebellum.
2 Ascending projections  VPN
of thalamus  1° vestibular ctx
in parietal lobe (for conscious
awareness of orientation and
motion).