AUDITORY CORTEX & VESTIBULAR SYSTEM Flashcards
Auditory cortex
Auditory cortex in humans occupies dorsal and lateral superior temporal gyrus
Auditory cortex can be divided into core, belt and parabelt regions
– Core region is Brodmann area 41 and includes primary auditory cortex (and other areas)
– Belt region surrounds the core region, particularly anterolaterally
– Parabelt region is posterolateral superior temporal gyrus
hierarchically organized: i.e., core projects to belt, and belt projects to parabelt
Auditory cortex is tonotopically organized
Core contains three tonotopic maps
– Core contains 3 areas, i.e, primary auditory cortex (A1), rostral field (R) and rostrotemporal field (RT)
– A1, R and RT are tonotopically organized (orderly arrangement of neurons according to best frequency)
Belt contains several areas and (at least) some of these are tonotopically organized
Parabelt representation of frequency is not well characterized/less clear
Subset of neurons in auditory cortex sensitive to harmonic sounds
Speech and musical sounds contain harmonics
– i.e., sound components at frequencies that are integer multiples of a fundamental frequency (f0)
Harmonics are important for identifying auditory objects
Subset of neurons in auditory cortex respond best to harmonic complex tones (HCTs)
Core, belt and parabelt regions respond to different classes of sounds
Core responds to pure tones
– i.e., sounds comprised of a single frequency
Belt responds to band-passed noise
– i.e., sounds of intermediate complexity between pure tones and vocalizations
Parabelt responds to complex sounds
– e.g., species-specific vocalizations
Auditory, ventral “what” pathway: processes auditory objects
Pathway from core auditory cortex, through belt regions, to inferior frontal cortex
Auditory, dorsal “where” (“how”) pathway: processes sound location
Pathway from auditory cortex, to parietal cortex (intraparietal lobule), to frontal cortex (premotor cortex)
STG has anterior-posterior organization for slow vs fast varying speech
Posterior superior temporal gyrus (STG) represents fast varying speech sounds
– Fast varying speech sounds are on the phonemic time scale
Anterior STG represents slow varying speech sounds
– Slow varying speech sounds are on the syllabic or prosodic time scales
Vestibular labyrinth
Vestibular system uses hair cells and is important for:
– Balance
– Equilibrium
– Posture
– Eye movements
Semicircular canals
– Sensitive to head rotation
Otolith organs
– Sensitive to gravity and tilt
Otolith organs
Macula is sensory apparatus in otolith organs
Hair cells in macula respond to tilt:
– Cells depolarize when cilia bend towards kinocilium
– Cells hyperpolarize when cilia bend away from kinocilium
Otolith organs detect:
– Linear acceleration
– Change in head angle
Otoliths are calcium carbonate crystals (1-5μm diameter)
When your head and macula are tilted, gravity pulls otoliths:
– this deforms gelatinous cap and bends cilia (hairs)
Macula arranged to provide sensitivity to different directions
Macula in “saccule” and “utricle” perpendicular to each other
– Macula oriented vertically in saccule
– Macula oriented horizontally in utricle
Direction preferences of hair cells vary systematically
– Hair cells in each macula cover a range of directions
– Saccule and utricle on each side of head are mirror images
– When cells on one side of head depolarize, the cells on other side at corresponding site hyperpolarize
Semicircular canals
Semicircular canals detect:
– Angular acceleration
– E.g., shaking head from side to side
– E.g., nodding up and down
Hair cells contained in “ampulla”
– Ampulla is a bulge along semicircular canal
– Cilia protrude into gelatinous “cupula”
Cells in ampulla (hyper)depolarize together
– Because kinocilia of hair cells are similarly oriented
Cilia bend when canal is rotated:
– When canal wall and cupula start to rotate, endolymph in canal lags behind due to inertia
– Endolymph exerts force on cupula, causing it to bend
– This bends cilia, causing hair cells to depolarize or hyperpolarize depending on which way cilia bend
Push-pull mechanism for semicircular canals
3 canals on one side of head, together sensitive to all rotation angles
– Horizontal canal – Anterior vertical canal – Posterior vertical canal
Each canal is functionally paired with canal on opposite side
– Left (L) and right (R) horizontal canals: sensitive to rotation in horizontal plane
– L anterior and R posterior canals: sensitive to rotation in vertical plane, ≈45° anteriorly to left
– R anterior and L posterior canals: sensitive to rotation in vertical plane, ≈45° anteriorly to right
Head rotation has opposing effects on canals in functional pair
– Rotation depolarizes hair cells in one canal but Hyperpolarizes cells in other canal
– This is called a “push-pull” mechanism
Vestibular nerve activity during head rotations
Hair cells connected to cells forming the vestibular nerve
– Cell bodies of vestibular nerve cells in “Scarpa’s ganglion”
Depolarized hair cells release glutamate onto vestibular nerve cells
– Glutamate transmitter increases the excitability of the vestibular nerve cells
When the head starts to rotate, vestibular nerve activity changes
– Cupula and cilia bend, changing the membrane potential of hair cells
– Vestibular nerve on one side of head increases activity (upper figure panels)
– Vestibular nerve on other side of head decreases activity (lower figure panels)
Long-lasting head rotation leads to adaptation
– After 15-30s, the endolymph and canal move together, and cupula straightens
– Vestibular nerve activity adapts after 15-30s, returning to original level
When head rotation stops, inertia of endolymph bends cupula
– Cupula now bends in other direction, causing opposite response from hair cells
– Vestibular nerve on each side of head change activity (opposite to initial response)
– This causes temporary sensation of counter-rotation (often reported as dizziness)
Vestibular pathways
Vestibular information combined with other sensory information in the cerebral cortex to plan and execute movements
PIVC plays central role in cortical vestibular network
parieto-insular vestibular cortex
Vestibular cortex (PIVC…) contributes to heading (self-motion) perception
Neurons in PIVC respond to 3D rotation and translation
