Exam 2 Flashcards
Outer ear primary role
to create cues for sound localization (binaural cues)
to amplify sound pressure (free field to tympanic membrane)
Outer ear structures
pinna
external auditory canal
tympanic membrane (ear drum)
- connects the outer and middle ear
Pinna
protects the outer ear
gives small boost to sound that falls in resonant frequency range
helps with sound localization (especially high frequency)
external auditory canal
provides boost to sound in the range of resonant frequency
uses cerumen (ear wax) to protect the middle ear from bacteria, debris and provides lubrication
~2.5cm long
Tympanic membrane (ear drum)
Cone shaped structure that completely closes off one end of the ear canal
cone shape funnels the acoustic energy of the sound to its center
Connects to the bones of the middle ear
2 primary acoustic cues from horizontal sound localization
Interaural level difference (ILD)
Interaural time differences (ITD)
Interaural level difference (ILD)
Larger at high frequencies
Lateral Superior Olive (LSO) in the SOC biased to high frequency (ILDs)
Higher level at left ear
Interaural time differences (ITD)
Larger at low frequencies
Medial Superior Olive (MSO) in the SOC biased to low frequency
Middle ear ossicles
Malleus, incus, stapes
Middle ear primary role
Provide an effective and efficient means to deliver sound to the inner ear
Overcome impedance mismatch
- Air filled middle ear → fluid filled inner ear
middle ear is where neural process of hearing begins
Impedance
resistance to movement
High acoustic impedance
hard to move (fluid filled tube)
Small movement for given input
Low acoustic impedance
easy to move (air filled tube)
Large movement for small pressure input
3 ways to get energy from the ear drum to the inner ear
Bone conduction
- The sound could travel via direct vibration of the bones of the skull, bypassing the middle ear and going directly to the inner ear
Air pressure changes in middle ear cavity
- Sound wave would travel through the middle ear without encountering the ossciles and stimulate the oval and round windows directly
Vibration through ossicular chain (main mode for hearing)
- Sound converted into mechanical vibration of the malleus, incus and stapes
What impedance does air-filled ear canal have?
low impedance
What impedance does fluid filled cochlea have?
high impedance
Eustachian tube
Connects middle ear space with nasopharynx (back of nose/mouth)
Opens occasionally, equalizes inside and outside pressure
Stapedius muscle/reflex
Stapedius muscle attached to stapes
Controlled by a reflex loop through brainstem, reduces sound transmissions at high sound levels
Stapedius muscle pulls stapes at a right angle to its typical motion, restricting motion by
- Increasing effective stiffness of ossicular chain
- Increases low-frequency impedance
- Reduces low-frequency energy transmission
Provides limited protection from loud sounds
Middle Ear Pathologies
Otosclerosis
Otitis Media
Cholesteatoma
Otosclerosis
Bone growth around stapes footplate, “locking” stapes in place
Increases stiffness, creating low-frequency hearing loss
Otitis Media
Fluid in middle ear space builds up due to negative pressure
Increases stiffness
- Smaller air space, reduces compliance
Creates low-frequency hearing loss
Cholesteatoma
Skin growth that occurs in middle ear space (extra tissue)
Bad cases can destroy ossicles (or require surgery that destroys ossicles)
Loss of ossicles can create a ~60 dB conductive (outer/middle ear hearing loss)
Structures of Inner Ear
Vestibular system (sense of balance)
Cochlea
- Primary auditory organ of inner ear
Bony labyrinth/spinal lamina
- Series of tunnels within which membranous labyrinth is housed
Semicircular canals
Contain the membranous semicircular ducts
- Sense organs for balance/movement of body in space
Detect angular acceleration (rotation)
Each duct detects motion in a different plane
Cochlear potentials
The hair cells and auditory nerve create biochemical electrical potentials
Relies on the flow of potassium and sodium
The motions and interactions of the cochlear structures create electric potentials
DC (direct current) potentials
Baseline potential changes that do not change once they happen
dominantes at high frequencies
AC (alternating potentials)
Change as a function of the vibrating tissue in the cochlea
dominates at low frequencies
Endolymph and perilymph in cochlea
Produce a +8- mV potential difference
Resting Potential Located in the endolymph of scala media and created by the stria vascularis
+80 mV
Hair cell receptor potential (inside cell)
-40 to -70 mV
Process of increasing afferent activity
When the stapes pulls OUT, the BM pulls UP → hair cells tilt toward the tallest stereocilia → tip links open → depolarizes cell → increases afferent activity
Process of increasing efferent activity
When the stapes pushes IN, the BM pushes DOWN → hair cells tilt away from the tallest stereocilia → top links closed → hyperpolarizes cell → decreases afferent activity
Outer hair cells Method of Shearing
OHC stereocilia is firmly attached to the tectorial membrane
Movement of the BM physically shears OHC stereocilia
Inner hair cells Method of Shearing
IHC stereocilia is not attached to the tectorial membrane
Fluids trapped between stereocilia and tectorial membrane cause IHC shearing
OHC loss
Causes a significant loss in frequency sensitivity resolution and elevated thresholds
IHC loss
Action potential can’t be sent
Therefore the sound can’t be heard
Otoacoustic emissions (OAE)
With a microphone in the ear canal, you can record sounds that are different than what you put in (or in the absence of sound)
Non-invasive measure of cochlear function in humans
Types of OAEs
Stimulus-frequency OAEs
Transient evoked OAEs
Distortion-product OAEs
Spontaneous OAEs
Stimulus-frequency OAEs
Input: long duration tone
Emission: energy at same frequency
Benefit: place specific on basilar membrane
Disadvantage: hard to separate emissions from stimulus (not used clinically yet)
Transient evoked OAEs
Input: click
Emission: energy at many frequencies
Benefit: easy to seperate emission from stimulus in time
Disadvantage: not place specific on basilar membrane
Distortion-product OAEs
Input: two long duration tones (f1< f2)
Emission: energy at new frequency (2f1-f2)
Benefit: easy to separate emission from stimulus in frequency
Disadvantage: several sources
Spontaneous OAEs
Input: no sound
Emission: energy at particular frequencies
Benefit: presence suggests no gross cochlear pathology
Disadvantage: absence doesn’t say much
Central Auditory Pathway
Auditory cortex (UPPER)
Medial geniculate body (MGB)
Inferior colliculus
Lateral lemniscus
Superior olivary complex
Cochlear nucleus
Auditory nerve (LOWER)
Action potential generation
A stimulus must be intense enough to reach the threshold and an action potential will be generated “all or nothing”
The action potential will have the same duration and intensity
Stages of sodium-potassium pump process for action potential
Depolarization
Repolarization
Hyperpolarization
Need the sodium-potassium pump to change the charge of cell membrane
Depolarization
Goes from resting potential to threshold
Na+ channels open, some K+ channels open
Increacreased impulse frequency
Cell becomes more positive
Repolarization
Going back down to become polarized and overshoots
Na+ channels close, K+ channels all open
Hyperpolarization
Becomes more polarized compared to resting point
K+ channels close, though there is still some K+ leaking in/out
Decreased impulse frequency
Two types of refractory periods
Absolute refractory period (B-C)
Relative refractory period (C-D)
Absolute refractory period (B-C)
After a spike is generated, the neuron must recover
For a short period of time, no additional spikes can be generated
Relative refractory period (C-D)
During hyperpolarization phase
Require a higher intensity stimulus
For a longer period of time, additional spikes are possible but are more difficult to generate and less likely to occur
Spontaneous rate of a neuron
rate that a neuron will fire in the absence of any auditory stimulus
Determined which intensity range that a neuron can respond to with a change in firing rate
Frequency selectivity of AN (tuning curve)
Tuning curve becomes broader for higher ampitudes
High CF-outside
Lower CF- center
Place theory
frequency of the input can be determined by noting which nerve fiber (place) within the AN discharges with the greatest relative discharge rate
Outside of the AN bundle (basal-high frequency)
Middle of the AN bundle (apex-low frequency)
Phase locking
ability of neuron to synchronize firing to a particular phase of stimulus
Neurons will most likely fire at peaks of stimulus
Volley theory
Combining spikes across multiple fibers fills in temporal code
Group of neurons together can fire at each cycle of stimulus
High SR (spontaeouns rate) neurons
low threshold of intensity
Low SR neurons
high threshold of intensity
Auditory Brainstem Responses (ABR)
Sequence of waves generated at increasingly higher levels of the auditory system
Used to diagnose pathologies at different sites
- Based on amplitudes and latencies of each individual wave
Wave I
Auditory nerve
Wave II
Cochlear nucleus
Wave III
Superior olivary complex
Wave IV
Lateral lemniscus
Wave V
Inferior colliculus
Excitation
additive
Reinforce neuron activities
E-E: Add together and increase firing rate a lot
Inhibition
subtractive
Cancel neuron activities
I-E: Final result will depend on magnitude
Cochlear Nucleus
Tonotopic
ventral (front)= low frequencies
dorsal (back) = high frequencies
Outputs to
superior olivary complex (SOC)
lateral lemiscus (LL)
inferior colliculus (IC)
SOC (Superior olivary complex)
Three parts
- Lateral superior olive (LSO)
- Medial superior olive (MSO)
- Medial nucleus of the trapezoid (MNTB)
First point of decussation (crossing over to other hemisphere)
First point of binaural processing
Output
- Efferent to CN
- Afferent to LL and IC
Lateral Limniscus (LL)
Three nuclei
- Ventral (VLL)
- Intermediate (ILL)
- Dorsal (DLL)
Which central auditory system structures are at the pons level?
SOC and LL
Input
- From CN and SOC
- Efferent input from inferior colliculus
Output
- Efferent to SOC and CN
- Afferent to Inferior Colliculus (IC)
Inferior Colliculus (IC)
First structure with core and belt organization
Core: auditory- central nucleus
Belt: somatosensory-dorsal cortex and dorsomedial and lateral nuclei
Refining sound localization
Inputs
- Contralateral IC
Outputs
- Efferent to SOC
- Afferent to medial geniculate body
Medial Geniculate Body (MGB)
at level of thalamus
Core: ventral (MGBv)-auditory
Belt: dorsal (MGBd) and medial (MGBm)-somatosensory and auditory
Lateral: high frequency
Medial: low frequency
Input
- From IC
- Core to core, belt to belt
Output
- To auditory cortex
Auditory Cortex
Posterior 2/3 of superior temporal gyrus
Core: primary AC (A1)
Belt: secondary AC (A2)
anterior= responding to sounds in front of us
posterior= responding to sounds behind us
Efferent projections from AC to MGB and IC
Contralateral bias
Majority of auditory nerve fibers project to contralateral structures
Auditory Nerve (CN VIII)
Bilateral structures and pathways with contralateral dominance
Formed by twisting of Type I and Type II neurons
Low frequency neurons in center
High frequency neurons toward periphery (tonotopic)
Hair cells
sensory cells of the inner ear
Tectorial membrane
gelatinious structure that the OHC stereocillia are embedded
Helicotrema
very apex of the cochlea
Stria vascularis
dense layer of blood capillaries on the side of the scala media that supplies metabolic energy to the cochlea
Basilar membrane
stiff structural element within the cochlea that separates the scala media and scala tympani
supports the organ of Corti
floor of scala media
Modiolus
the central axis around which the cochlear spiral winds
Organ of corti
the organ that sits atop of the basilar membrane and contains the outer hair cells and inner hair cells
Reissner’s membrane
Separates perilymph of scala vestibuli and endolymph of scala media
Parts of the Vestibular System (Balance and Movement)
Otolith organs
Saccule
Semicircular canals
Parts of the Auditory System
Cochlea
Organ of Corti
Parts of Both (Vestibular and Auditory Systems)
Endolymph
CN VIII (auditory nerve)
Hair cells
Middle ear function
impedance matching, selective oval window stimulation, pressure equalization
Inner ear function
filtering, distribution, transduction
Inner ear structures
Semicircular canals
Vestibule
Vestibular notch
Cochlea
Round window
Eustachian tube
Impedance factors
stiffness, mass, damping (friction)
stiffness
Most relevant at low frequencies
mass
Most relevant at high frequencies
Damping (friction)
Most relevant at medium frequencies where mass and stiffness cancel each other
scala media
Function: hearing
Located between scala vestibuli and scala tympani
Type I AN afferent fibers
Larger
Myelinated
Innervate IHCs exclusively
Many to one, one to one
Type II AN afferent fibers
Smaller
Unmyelinated
Innervate OHCs (one to many)
5% of afferents forming the auditory nerve
Single-cell cochlear potentials
Voltage inside a single cell
- Hair cells
- Auditory nerve fibers (and other neurons are higher up)
Resting potential
- DC potential in the absence of stimulation (typically -60 mV)
Action potential
- A sharp and rapid peak (depolarization inside auditory nerve fibers that occurs after the potential reaches a threshold)
Gross cochlear potentials
Combined electrical activity from many individual cells (summed currents cause potentials)
Can be measured clinically
Saturation
stage a nerve fibers reaches when the maximum firing rate has been reached and continues to fire at the maximal level without further increasing its potential
Which direction of basilar membrane movement causes OHC depolarization to occur?
upward displacement
Which structures contain perilymph?
scala vestibuli
scala tympani
High sensitivity (feature of auditory nerve fibers)
allows useful spectral features to be discriminated
Sharp tuning (feature of auditory nerve fibers)
allows soft sounds to be detected
Distortion-product OAEs (DPOAEs)
emission that occurs from presenting two tones of different frequencies into the air and getting out a tone of a different frequency
Neural threshold
when this threshold is reached, a neuron will start firing above its spontaneous rate
Stereocilia
hairs that are found on top of the inner and outer hair cells and are bathed in endolymph
Transient evoked OAEs (TEOAEs)
this emission is recorded using a click as a stimulus
Intensity coding
an increase in regular firing rate that occurs with an increase in intensity
Tip links
filaments that connect stereocilia to each other or to the kinocilium in the hair cells of the inner ear
Isointensity curve
a chart measuring an auditory nerve fiber’s firing rate to a wide range of frequencies, all presented at the same intensity level
Y-axis: spikes/per second
X-axis: input stimulus level (dB SPL)
Endocochlear potential
the positive voltage of 80-100 mV seen in the endolymphatic spaces
Round window
structure in the middle ear space that moves opposite the oval window to maintain the cochlear volume
Characteristics of the basilar membrane at the apex level (low frequencies)
loose and wide
processes low frequency sounds
contains auditory nerve fibers of all spontaneous rates
Synaptopathy
phenomenon where there is temporary damage to OHCs but permanent damage to auditory nerve fibers/synapses that likely affects speech intelligibiliy in noise
