HNS39 Audition Flashcards
Cochlea location
Temporal bone
2 features of sound
- Tone (frequency)
- Nervous system - Location
- Interaural time delay (too short to be distinguished)
- Nervous system
***Ascending Auditory pathway
記: SCSLIM
Cochlea —> ***Spiral ganglion —> Cochlear nerve CN8 —> ***Cochlear nucleus —> ***Superior olive (Medulla) —> ***Lateral lemniscus —> ***Inferior colliculus (Midbrain) —> ***Medial geniculate nucleus (Thalamus) (Lateral geniculate nucleus: Vision) —> Auditory cortex
Central auditory pathway features
Anatomical characteristics:
1. ***Tonotopic projections (Cochlea)
- ***Bilateral, Parallel networks, Crossed connections
—> allow notes to be compared (L/R ear) —> determine location
—> unilateral brainstem lesion —> hearing still ok - Hierarchical organisation
- Expansion in cell population in Inferior colliculus
Common hearing disorder
- Tinnitus
- extreme noise
- infection - Hereditary
- Ototoxicity
- Tumour
- Temporal bone fracture
- head trauma
- location of fracture —> determine conductive / sensorineural hearing loss
2 types of hearing loss
- Conductive hearing loss
- middle ear defect - Sensorineural hearing loss
- sensory pathway / CNS defect
External ear function
Collection / Localisation / Modification of sound
***Middle ear function
- Impedance matching
- Air-borne vibration (External auditory canal) to Peri-lymph vibration (Cochlea)
—> Area-ratio difference (Tympanic membrane (large) vs Oval window (small))
—> Lever action of ossicles (Malleus, Incus, Stapes) - Sound attenuation
Middle ear muscle **reflex —> modify **alignment of ossicles —> alter energy transmission:
—> ***Attenuate low frequency
—> Protect inner ear
—> Improve speech discrimination in noise (Neural mechanism can also help)
***Cochlea structure
3 tubes:
- Scala vestibule (upper)
- Perilymph
- Oval window —> Conduct sound into Cochlea - Scala tympani (lower)
- Perilymph
- Pressure release —> Round window - Cochlear duct
- Endolymph (produced by Stria vascularis, high in K)
- Tectorial membrane (bony structure over hair cells, attach at spiral limbus)
- Organ of Corti: Hair cells (Inner: 1 row + Outer: 3 rows)
- Basilar membrane (attach at osseous spiral lamina + spiral ligament)
***Sound pathway in middle and inner ear
Middle ear:
Malleus —> Incus —> Stapes —> Oval window
Inner ear:
Perilymph vibration (Scala vestibule)
—> Endolymph vibration (Cochlear duct)
—> Basilar membrane displacement
—> Hair cells move (sitting on Basilar membrane, touching upper Tectorial membrane)
—> Shearing force of cilia (Cilia被Tectorial membrane扯向不同方向)
—> Movement of cilia bundle
—> Pressure-sensitive K channels (on tip of cilia) open
—> K enter hair cells (high conc of K in endolymph)
Pressure release:
—> Perilymph vibration (Scala tympani)
—> Round window bulging
***Sensory transduction
Sensory stimulus conveyed to readable message by nervous system
Mechanical displacement:
Air vibration —> Fluid vibration —> Mechanical movement —> Shearing force of cilia
Electrical signals:
Pressure-sensitive K channels (on tip of cilia) open
—> K enter hair cell (high conc of K in endolymph)
—> Depolarisation
—> Voltage-gated Ca channels open
—> Ca rush in
—> Release of Glutamate within synaptic vesicles
—> Excitatory action at CN8
—> Generator potential —> Action potential along CN8
—> Auditory afferent to Cochlear nucleus
Outer and Inner hair cells
Outer hair cells (3 rows):
- 10-100:1 afferent connection (100粒cell:1條nerve)
- innervated by 5% afferent
Inner hair cells (1 row):
- Main transduction of sound
- 1:10 afferent connection
- innervated by 95% afferent
Afferent:
Auditory signals to Cochlear nucleus
Efferent:
Cell bodies in brainstem (Superior olivary nucleus) —> modulate length of hair cells
***Processing of sound frequency in Cochlea
Basilar membrane (~棒球棍):
- Narrow + Stiff + Short stereocilia in base —> High frequency sound cause maximum vibration in base
- Wide + Floppy + Long stereocilia in apex —> Low frequency sound cause maximum vibration in apex
—> **Tonotopic pattern (place-coding): **Frequency of incoming sound determines Peak of travelling wave
—> wave ***subsides rapidly (sharp cut-off) beyond maximum displacement towards apex
Example:
Middle frequency sound —> Maximal displacement of Basilar membrane in middle
**Processing of sound frequency in Cochlea **Nerve
Every nerve has its own receptive field
—> Fire action potential at respective **Frequency + **Intensity of sound
***V-shaped tuning curve (boundary of responsive area)
- Wide top (high frequency cells can still be activated by high intensity sound despite low frequency)
—> relatively poor discrimination
—> for Crude collection of sound
Frequency selectivity
- Characteristic frequency of a single cochlear nerve where it is stimulated even at low sound intensity (好細聲都收到)
- Tip of V-shaped tuning curve
Frequency discrimination up Ascending Auditory pathway
Higher discrimination at Superior olive, Auditory cortex (highest)
—> progressive sharpening of V-shaped receptive field (narrow top, small area)
—> enhances **frequency discrimination
—> information narrows down going up higher centres (i.e. less information can be transmitted upwards)
—> **extraction of information becomes better
Cochlear implant
For patients with hair cells defect
Microphone to collect sound
—> Sound processor
—> Electronic impulses to cochlear implant
—> Stimulating electrodes (Low frequency electrode connect to apex, High frequency electrode connect to base)
—> Activate CN8
Modulation of sound intensity
- Organ of Corti
- sit on Basilar membrane
- hair cells hanging on Organ of Corti
- mechanical layout (intrinsic, cannot be modified) —> Amplifies small vibration -
**Superior olive (Medulla)
- Medial olivocochlear neuron
—> innervate motor proteins of **outer hair cells (NOT inner!!!)
—> efferent-induced depolarisation
—> **shorten cell bodies of outer hair cells
—> ↑ sensitivity of hair cells
—> ↑ displacement of Cilia bundle
—> ↑ sound intensity
—> detection of weak auditory signal against background noise
—> **Selective filtering / Attention (in-build mechanism / cocktail party phenomenon)
Efferent / Descending pathway (掉返轉): Auditory cortex —> Medial geniculate body (Thalamus) —> Inferior colliculus —> Lateral lemniscus —> Superior olive —> Cochlear nucleus —> Spiral ganglion —> Cochlear nerve
***Processing of sound frequency in Cochlear nucleus
-
**Temporal pattern analysis
Cochlea nucleus contain cells of different shapes
—> cell having same neural response as Cochlear nerve + other cells that respond at different time frame
—> sound **segregated into different parts
—> allow higher centre to ***compare notes of original pattern with other different patterns
—> analysis of Temporal pattern - ***Frequency analysis
***Processing of sound frequency in Superior olive
Afferent innervation from left and right side (Directional cues)
時間, 地點, 聲量
- Basic **binaural sound processing
- compare **Time + ***Intensity signals from 2 ears - Extract ***gross spatial information
- higher centres needed for accurate localisation
Inferior colliculus
- ***Reflex centre:
- Novel sound —> Head orientation
- Loud sound —> Startle response
e. g. contract reflex in neck muscles - Detect FM (frequency-modulated) + AM (amplitude-modulated) sounds in speech
Thalamus (Medial geniculate body)
Ascending relay station
Primary auditory cortex
Temporal lobe, Below Lateral sulcus (Sylvian fissure)
Microscopic arrangement:
- Functional columns (areas 41, 42) —> Characteristic frequencies —> Iso-frequency bands (i.e. detect same frequency within same column)
- Rows —> R/L ear alternating —> Binaural bands
—> ***Accurate localisation of sound in space
Wernicke’s area + Broca’s area
Processing of complex sound e.g. language
Wernicke’s area:
- Recognition of language —> send projection to Broca’s area
- Temporal lobe (area 22) (mostly in dominant hemisphere)
Broca’s area:
- Production of speech, Perception of language
- Frontal lobe (area 44, 45) (mostly in dominant hemisphere)
Dyslexia: dysfunction in Broca’s area
***Summary of functions of components of Auditory pathway
- Cochlea:
- **Frequency processing —> Longitudinal variation in Basilar membrane —> **Tonotopic representation in Cochlea
- ***Intensity processing: Organ of Corti + Outer hair cells - Cochlear nerve:
- Receptive field —> **V-shaped tuning curve —> Characteristic frequency (locus of hair cell on Basilar membrane)
- Nerves are sharply tuned to exclude frequencies above CF —> discrimination of very soft (near threshold) tones with slightly different frequencies
- **Progressive sharpening of V-shaped receptive field up ascending pathway —> frequency discrimination —> extraction of information - Cochlear nucleus:
- **Temporal pattern analysis
- **Frequency analysis - Superior olive:
- Basic **binaural sound processing
- Extract gross **spatial information - Inferior colliculus:
- **Reflex centre
- Detect **FM/AM sounds in speech (+ descending projection from cortex to modulate ascending signals) - Thalamus (MGB):
- Ascending relay station (+ descending projection from cortex to modulate ascending signals) - Primary auditory cortex: Accurate ***localisation of sound in space
- Wernicke’s area + Broca’s area: Processing of complex sound
Auditory tests
- Pure tone audiometer:
- measure ***hearing threshold of a conscious subject in terms of intensity (deciBel Sound Pressure Level) and frequency (Hz) - Brainstem auditory evoked response (BAER):
- ***click-evoked neural activity of the auditory pathway for assessing hearing ability of subjects esp. infants
