Concentric wave
- A series of waves that share the same central point, but have different radii (spreading)
Sound wave
- Longitudinal wave
- Compression = Region where particles are crowded together, high pressure
- Rarefraction = Region where particles are spread apart, low pressure
Period
- Time taken for 1 up-down cycle
- Moving a distance of 1 wavelength
- Distance/Time = Speed = Wavelength/Period
Speed depends on the vibrating material
Frequency
- In cycles per second (Hz)
- 1/Period
- Speed = Wavelength * Frequency
Speed depends on the vibrating material
Sound Localization
- Sound arrives first at one side = Louder
- Sound arrives later = Softer (Blocked by the head)
- Connections between neurons on either side enable the comparison of very slight differences in the sound stimuli that arrive at each ear
Medial Superior Olive (MSO)
- Interaural time difference for low-frequency sounds (“which ear heard the sound first”)
Long wavelengths (<2000Hz) = not blocked by the head = no sound shadow = time difference - Each MSO neuron receives excitatory glutamatergic input from both ears
- Auditory nerve fibers synapse in the left and right ventral cochlear nuclei (VCN)
- Both VCN project bilarterally to the MSO
- MSO neuron fires maximally only when inputs from both ears arrive at the same time
- The firing pattern encodes which ear heard the sound first, allowing localization along the horizontal plane
Lateral Superior Olive (LSO)
- Interaural level differences for high-frequency sounds (“which ear heard the sound louder”)
Short wavelengths (>2000Hz) = head creates a sound shadow - Ipsilateral Excitatory Pathway:
1. Sound enters right cochlea, inner hair cells stimulates the auditory nerve (CN VIII)
2. CN VIII fibers synapse in the right ventral cochlear nucleus (VCN)
3. Action potential travels from the right VCN to the right LSO, releasing glutamate (excitatory) - Contralateral Inhibitory Pathway:
1. Right VCN fibers cross the midline
2. Synapse in the left Medial Nucleus of the Trapezoid Body (MNTB)
3. Left MNTB sends glycinergic inhibitory projections to the right LSO
Right LSO neuron computes:
Firing rate = ipsilateral excitation - contralateral inhibition
Outer Ear
Concha: Bowl-shaped cavity that funnels sound inward
External Auditory Meatus: Tube connecting the concha to the tympanic membrane, conducts and slightly amplifies sound
Tympanic Membrane: Thin membrane separating outer ear from middle ear, attached to malleus, converts air pressure waves into mechanical vibrations
Middle Ear
Ossicles (Amplification of sound vibrations)
1. Malleus: Attached to the tympanic membrane
2. Incus: Connects malleus to stapes
3. Stapes: Base is in the oval window
Eustachian Tube: Opening connects middle ear and throat to maintain air pressure, normally closed but opens during swallowing or yawning
Tensor Tympani Muscle: Contracts to tense the tympanic membrane by pulling on the malleus, reducing the transmission of sound vibration (especially self-generated sounds)
Stapedius Muscle: Contracts to stabilize the stapes at the oval window, reducing movement of the stapes and protecting the inner ear from loud sound
Protective Reflex Muscle (But doesn’t protect against sudden, extremely loud sounds due to reflec latency)
Presure Amplification
Mechanisms in the middle ear
1. The tympanic membrane has a much larger surface area than the oval window, so the same force applied to a smaller area at the oval window results in increased pressure
2. The stapes displaces the oval window with about 1/10 the displacement of the tympanic membrane, but with much greater force (The ossicles are a mechanical advantage lever system)
Cochlea
- Inside the cochlea, movement of the stapes at the oval window displaces the cochlear fluid (perilymph), generating a traveling wave along the basilar membrane
- The mechanical properties of the basilar membrane change from base to apex: it is narrow and stiff at the base and wider and more flexible at the apex
- As a result, each sound frequency produces maximum vibration at a specific location along the basilar membrane (tonotopy), corresponding to its resonance frequency (20Hz at the apex to 20kHz at the base)
- High-frequency sounds produce maximal displacement near the base, while low-frequency sounds peak near the apex
- Auditory nerve fibers innervating the base of the cochlea have lower thresholds for high-frequency sounds, while fibers innervating the apex have lower thresholds for low-frequency sounds
Structure inside Cochlea
Upper chamber: Scala Vestibuli
- Contains perilymph fluid
- Low K+ and high Na+ concentrations
Middle chamber: Scala Media
- Contains endolymph
- Has high K+ concentration (150 mM)
- Lower chamber: Scala Tympani
- Contains perilymph
- Has low K+ concentration (7 mM)
Hair Cells
~15,000 Hair cells in each ear
~ 3,500 Inner hair cells
95% of afferent axons innervate inner hair cells
Inner Hair Cells
Sensory transduction
- Converts mechanical input into neural signals
1. K⁺ influx depolarizes IHCs
2. Voltage-gated Ca²⁺ channels open
3. Neurotransmitter released onto auditory nerve fibers
Outer Hair Cells
Amplification
- Increases sensitivity and frequency selectivity
1. Stereocilia deflection opens K⁺ channels, causing depolarization
2. Depolarization activates prestin, the motor protein in outer hair cell membranes
3. OHCs shorten (electromotility), feeding mechanical energy back into the basilar membrane
4. This amplifies basilar membrane motion, increasing sensitivity and sharpening frequency selectivity
(Hyperpolarizatioin causes the OHC to lengthen, reducing basilar membrane motion)
Movement of Basilar Membrane
Upward Movement
- Causes a shearing force between the basilar membrane and the tectorial membrane, deflecting stereocilia toward the tallest row, stretching tip links, opening K⁺ channels, and depolarizing the hair cell
Downward Movement
- Produces shearing in the opposite direction, deflecting stereocilia toward the shortest row, relaxing tip links, closing K⁺ channels, and hyperpolarizing the hair cell
Toward tall = Turn ON
Toward short = Turn OFF
Mechano(acoustical) Transduction
Depolarization
1. Sound causes the basilar membrane to vibrate
2. This vibration produces a shearing force between the basilar membrane and the tectorial membrane, bending the hair bundles
3. Deflection toward the tallest stereocilia stretches tip links (gating springs), opening mechanically gated ion channels
4. K⁺ flows into the hair cell from the high-K⁺ endolymph (in the scala media), driven by the concentration gradient and the positive endocochlear potential
5. The hair cell depolarizes
6.Depolarization opens voltage-gated Ca²⁺ channels at the base of the inner hair cell
7. Ca²⁺ influx triggers synaptic vesicle fusion at the ribbon synapse, releasing glutamate
8. Glutamate activates receptors on auditory nerve (CN VIII) afferent fibers, increasing their firing rate
9. Signals are transmitted to the brain via the auditory pathway
Hair bundle displacement toward the tallest stereocilia is positively related to hair cell depolarization: larger deflections open more K⁺ channels, causing greater depolarization and increased neurotransmitter release onto auditory nerve fibers
Hyperpolarization
1. A shearing force causes stereocilia to deflect toward the shortest stereocilia
2. Tip links relax, closing mechanically gated K⁺ channels at the stereocilia
3. K⁺ influx from endolymph decreases, while K⁺ efflux at the base of the cell into perilymph continues
4. The hair cell membrane hyperpolarizes
5. Hyperpolarization closes voltage-gated Ca²⁺ channels at the basal synapse
6. Ca²⁺ influx decreases, leading to reduced synaptic vesicle fusion
7. Neurotransmitter (glutamate) release decreases
8. Auditory nerve firing rate decreases
Sensorineural Hearing Loss
Can be caused by：
1. Hair cell damage (cochlea, inner or outer hair cells)
2. Auditory nerve (CN VIII) pathology
Examples:
- Occupational deafness: Usually noise-induced hearing loss (OHC loss, base of cochlea, high-frequency loss)
- Presbycusis: Age-related high-frequency hearing loss (Usually OHC loss at the cochlear base)
- Antibiotic ototoxicity: Infection by antibiotics (OHC/inner hair cell damage)
- Acousitc neuroma/Vestibular schwannoma: Tumor of Schwann cells on CN VIII, can block action potential transmission, cause hearing loss, tinnitus, dizziness, facial numbness (CN VII compression sometimes)
Conductive Hearing Loss
Due to mechanical blockage or dysfunction in the outer or middle ear
Examples:
1. Ear wax: Blocks the external auditory canal, preventing sound waves from reaching the tympanic membrane
2. Otitis media: Infection/inflammation of the middle ear, often causes fluid buildup, reducing tympanic membrane and ossicle vibration, most common in children
3. Otosclerosis: Stapes footplate becomes fixed to the oval window, prevents transmission of sound to the cochlea
Hearing Loss
- Often cumulative: repeated exposure to loud noise over time or age-related degeneration of hair cells
- Can involve “hidden” damage
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Neuroscientists And Their Accomplishments14
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Positional Terms21
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Localization of Function6
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Voltage-gated Na+ Channel Blockers3
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Action Potential Conduction12
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Disease3
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Neurotransmitters11
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Diving Force of Synaptic Current2
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Dermatomes14
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Somatosensory Receptors14
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Eye and Retina20
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Central Vision33
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Auditory System20
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Vestibular and Motor Systems40
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Synaptic Plasticity and Memory22
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Technique of the Week3
