Exam #2
Semicircular canals
Three tiny, fluid-filled tubes in your inner ear that help you keep your balance
Y-axis of a frequency tuning curve
Tone frequency
X-axis of a frequency tuning curve
Tone intensity
Otoacoustic emissions
- Oto = ear - Acoustic = sounds - Emissions = coming out of - Normal ear generates sounds that can be detected in the ear canal - Spontaneous otoacoustic emissions and evoked otoacoustic emissions - "Leakage" of energy back into the ear canal from the inner ear - Something moves in the inner ear, creates a disturbance (reverse traveling wave?) - Stapes moves because the inner ear fluid has been disturbed - Ossicles vibrate, tympanic membrane moves - Middle ear problems interfere with emissions - 70% of normal hearing ears - More common in right ear - Multiple emissions common - Stable in frequency, not level - Essentially "feedback" - Suggestive of high sensitivity - More common in females - More color in skin = more emissions - Emissions larger in infants - Genetic component - Depends on hormones - Ototoxic drugs such as aspirin and quinine make them disappear - A SOUND PRODUCED BY ACTIVE PROCESSES IN THE COCHLEA THAT CAN BE RECODED IN THE EAR CANAL WITH A MINI MICROPHONE
Divisions of the auditory system
- Outer ear - Middle ear - Inner ear - Central auditory system
What innervates the outer hair cells?
- Outer spiral afferents - Outer (tunnel) radial efferents
Deiters' cells and Hensen's cells
- Part of inner ear - Support outer hair cells
Tympanic membrane
- Part of middle ear - Has a curvature - Moves/fluctuates with curve - AKA ear drum - Vibrates in response to sound waves - Causes ossicular chain movement - CLOSES OFF AND PROTECTS THE MIDDLE EAR - SOUND PRESSURE EXERTED ON THIS MEMBRANE CONVERTS AIRBORNE SOUND IN EAR CANAL TO MOTION OF THE OSSICULAR CHAIN
Ossicular chain
- Part of middle ear - Smallest bones in body - Malleus, incus, stapes - CHAIN OF SMALL BONES (MALLEUS, INCUS, STAPES) IN THE MIDDLE EAR THAT SERVES TO CREATE AN IMPEDANCE MATCH BETWEEN THE AIR AND THE FLUIDS OF THE COCHLEA - THE AREA DIFFERENTIAL BETWEEN THE TYMPANIC MEMBRANE AND THE STAPES FOOTPLATE IS THE PRIMARY MEANS BY WHICH THIS MATCH IS ACHIEVED
Inner hair cells
- Part of the inner ear - 1 row for every 3 rows of OHC - U formation - Shaped like a pear or flask - No contact with tectorial membrane
Outer hair cells
- Part of the inner ear - 3 rows for every 1 row of IHC - V or W formation - Hair cells are connected to neurons at the base of the hair cell - Receive signals from the brain - Stereocilia touch tectorial membrane - Shaped like cylinders
Stria vascularis
- Part of the inner ear - Ensures greater concentration of positive K+ ions at stereocilia of hair cells - K+ flow into hair cell stereocilia and out of hair cell at base
Organ of Corti
- Part of the inner ear - Hair cells and their supporting structures - Hair cells are connected to auditory nerve - Produces nerve impulses in response to sound vibrations - In cochlea - INCLUDES THE INNER AND OUTER HAIR CELLS AND THEIR SUPPORTING STRUCTURES - PLAYS A CRITICAL ROLE IN SOUND TRANSDUCTION TO THE INNER EAR
Modiolus
- Part of the inner ear - Nerve fibers (CN VIII) exit cochlea via this - Conical core of the cochlea containing nerve fibers
Stereocilia
- Part of the inner ear - Part of hair cells - Generally arranged in 3 rows of graded lengths - In addition to tip links, they attach with transverse (lateral) links, both in the same row and from row to row - Tip and lateral links between 2 stereo cilia - Tugging of tip links changes permeability of stereocilia - Touches the tectorial membrane - Receive signals from brain - SMALL PROTUBERANCES AT THE TOP OF THE HAIR CELLS THAT ARE SENSITIVE TO THE SHEARING FORCES PRODUCED BY THE MOTION OF THE TECTORIAL MEMBRANE
Border cells of the inner sulcus
- Part of the inner ear - Support inner hair cells
Inner pillars of corti and outer pillars of corti
- Parts of the inner ear - Maintain shape
Outer ear parts
- Pinna (concha, external auditory meatus) - Ear canal
From hair cells to VIIIth CN
- Presssure waves of sound displace the bundle of stereocilia - K+ and Ca++ channels in the hair cell open - Positive ions flow into the cell, resulting in depolarization - Vesicles near the base fuse with the hair cell's surface membrane, releasing a signaling substance called a neurotransmitter - Neurotransmitter diffuses across the bottom membrane of the hair cell and reaches the nerve fiber, which eventually transmits the signal to the brain
Total pressure gain in middle ear
- Pressure gain of a factor of 44 - AKA 33 dB - Even though we will have pressure gains due to middle ear characteristics, some of the intensity in a sound is still lost when the waveform enters the cochlea - If the middle ear is not working properly, a "conductive hearing loss" exists
Traveling wave paradox
- Pressure wave in fluids travels 'instantly' from base to apex - Traveling wave travels from base to apex regardless of where driving force is applied to cochlea - First key to analyzing frequencies
Steps of hearing
- Pressure wave is generated - Wave propagates to the ear and down the ear canal - Tympanic membrane vibrates - Ossicles in the middle ear vibrate - Pressure is delivered to the cochlea - Basilar membrane vibrates - Hair cell stereocilia are deflected - The hair cells generate electrical responses - Basilar membrane motion is amplified - Neurotransmitter is released - Neurotransmitter causes primary neurons to respond - Sound has been "encoded" - Neurotransmitter travels to auditory nerve to be brought to brain
Place and time code
- Principle of resonance revisited - The response of the basilar membrane to a sinusoid driving force at any point is an amplitude-scaled and phase-shifted version of the sinusoid - Place code - Time code
Basilar membrane
- Probably the most important structure in the cochlea - Displaced by the pressure wave in the cochlear fluid - Separates the waveform into frequency components - Transforms the waveform into a spectrum - Apical end: wide, not very stiff (inside of spiral) - Basal end: narrow, stiff - A MEMBRANE THAT EXTENDS FROM THE BASE TO APEX OF THE COCHLEA DIVIDING THE COCHLEA INTO FLUID FILLED CHAMBERS - RESONANT PROPERTIES OF THIS MEMBRANE PROVIDE A PLACE TO CODE FOR FREQUENCY
Outer ear general functions
- Protection - Resonance - Localization
What innervates the inner hair cells?
- Radial afferents - Inner spiral efferents
How does the auditory nerve code intensity?
- Rate of discharge - Rate-level function - Post-stimulus time (PST) histogram (discharge rate vs. time)
Briefly describe the role of the outer ear in sound localization.
- Reflections of the incident sound caused by the folds of the outer ear add back to the incident wavefront coloring the spectrum by destructive and constructive interference - This directionally dependent filtering of the sound provides a cue for the location of the sound source.
Transfer functions
- Relates to outer ear acting as a resonator - The frequencies that benefit us the most are the intermediate temporal frequencies, those that are most important for the perception of speech sounds - Makes hearing easier - we are more sensitive to certain frequencies - Therefore, sound funneling by concha and ear canal produces best sensitivity to sound between about 1-5 kHz
Outer ear as a localization helper
- Result of the transfer function - For a given acoustic source in the free field, there is a difference between the two ears in both the sound pressure level and in the phase - Auditory system can tell a time difference of only 13 ms - The cells within the auditory system which analyze acoustic information are sensitive to these micro changes!
Iso-rate curves in auditory nerve fibers
- Reveals sharp tuning provided by automatic gain control of outer hair cells - Moves onto auditory nerve - Graphs are of auditory nerves responding to patterns
Spontaneous otoacoustic emissions
- SOAE, cochlear emission - Sounds generated when no sounds are played
Central auditory system general functions
- Sensation - Perception
Sensitivity
- Sensitivity enhanced due to outer hair cells - Rocking motion of outer hair cells amplifies low-level sounds - Basilar membrane input-output characteristic: compressive (nonlinear) - Magnitude boosting at lower frequencies - Compressed in the middle
Some differences between IHC and OHC
- Shape (jug vs. cylinder) - Position (sit on shelf versus sit on flexible membrane) - Contact with overlying membrane (OHC only) - Nerve supply is different
Basilar membrane "tuning curve"
- Shows the response of a single place to many frequencies - Which frequencies the place will respond to - Which frequencies it won't respond to - V shaped - Asymmetrical - At best frequency for a point on the basilar membrane, you need just the smallest amount of level to make that point vibrate
Inner ear function
- Sound detection - Balance
Middle ear transformer
- Sound energy must be transferred to fluid filled inner ear - Sound travels from air to water and 99.9% of the energy bounces back - How do we transfer this sound without losing energy?
Place code
- Sound frequency determined by place of maximum vibration along the length of the basilar membrane - High frequencies at basal end - Low frequencies at apical end - A PLACE OF MAXIMUM DISCHARGE RATE IN THE AUDITORY NERVOUS SYSTEM THAT CONVEYS INFORMATION ABOUT THE FREQUENCY OF THE SOUND
Time code
- Sound frequency determined by the frequency of oscillation at any point along the basilar membrane - AKA TEMPORAL CODE - A PATTERN OF NEURAL SPIKE RESPONSES OVER TIME THAT CONVEYS INFORMATION ABOUT THE FREQUENCY OF THE SOUND
Afferent innervation of inner and outer hair cells
- Specificity: many-to-one arrangement - Sensitivity: one-to-many arrangement
Interval histogram
- Spikes are phase-locked to cycles of the sinusoid - Most frequently occurring time interval is equal to the period of the sinusoid - provides a time-code for frequency - Auditory nerve fibers can phase lock, or follow the frequency of a stimulus cycle by cycle, up to 4000 Hz - A PLOT, IN THE FORM OF A HISTOGRAM, OF THE FREQUENCY OF OCCURRENCE OF TIME INTERVALS BETWEEN SUCCESSIVE SPIKES OF AN AUDITORY NERVE FIBER
Basal end of basilar membrane
- Stiff - Near outside of spiral - Narrow - Vibrates at high frequencies - High frequencies will displace the base
Stiffness of the basilar membrane
- Stiffness determines the resonant frequency of a vibrating object because of the change in basilar membrane stiffness from one end to the other - Sine waves of different frequencies cause different segments of the membrane to vibrate - Like any stiff object, the basal end of the membrane will tend to vibrate at high frequencies (high frequencies will displace the base) - Like any object with less stiffness, the apical end of the membrane tends to vibrate at low frequencies (low frequencies will displace the apex) - Like any object with intermediate stiffness, the middle of the membrane tends to vibrate at intermediate frequencies (frequencies between the extremes displace the middle)
Superior olivary complex
- THE FIRST POINT OF CONVERGENCE OF INPUTS FORM THE 2 EARS - NEURONS IN THIS COMPLEX ARE SENSITIVE TO INTRAMURAL DELAY AND SO MANY PLAY AN IMPORTANT ROLE IN SOUND LOCALIZATION
Muscles of the middle ear
- Tensor tympani and stapedius - Add stiffness to the middle ear system and cut down on the transmission of low frequency sounds - This means you can: hear others while you are eating, talking, or in a noisy situation when the noise is in the low frequency region - THE TENSOR TYMPANI AND STAPEDIUS ARE TEH TWO MUSCLES THAT TERMINATE ON THE OSSICULAR CHAIN AND PROTECT THE EAR FROM EXPOSURE TO INTENSE SOUND THROUGH THE 'AUTOACOUSTIC REFLEX'
How is the mapping of frequency to place achieved along the length of the basilar membrane?
- The base of the membrane is narrow and stiff and so has a high resonant frequency - The apex is wide and compliant and so has a low resonant frequency - These physical properties determine the mapping of frequency to place along the length of the basilar membrane
Characteristic frequency
- The frequency at the low-threshold "tip" of the tuning curve - Each fiber is most sensitive to 1 frequency - Responds to a range of frequencies when SPL is high enough - Same nerve fibers, different frequencies
Describe the process by which the mechanical disturbance along the basilar membrane is converted to an electrical potential across the body of a single hair cell.
- The hair cells are sandwiched between the basilar membrane and the tectorial membrane in such a way that the motion of the basilar membrane causes the tectorial membrane to produce a shearing force across the stereocilia at the top of the hair cells - The motion of the stereocilia then cause tip filaments to open channels whereby positively charged calcium and potassium ions flow into the hair cell body
What role does the outer ear play in enhancing sensitivity to sound?
- The large opening at the entrance to the ear, the concha, produces an amplitude gain of about 10 dB at 5000 Hz - The ear canal resonance produces another amplitude gain of about 10 dB at 2000 Hz - Together these structures account for our greatest sensitivity to frequencies between 2000-5000 Hz
Implications of hair cell motion on ear functioning
- The motion of the cells adds energy to the stimulus and appears to enhance motion within the cochlea - Cochlea amplifier!
What is the traveling wave paradox? Why does the wave motion of the basilar membrane always travel from base to apex?
- The wave motion of the basilar membrane always travels from base to apex regardless of where along the bony structure of the cochlea the driving force is applied - this is referred to as the traveling wave paradox - The wave motion is determined by the resonant properties of the membrane, being narrow and stiff at the base, and wide and compliant at the apex
Post-stimulus-time histogram
- Time-pattern of spikes "mimics" time waveform
Cochlear microphonic
- To-and-fro motion of the stereocilia creates pattern of depolarization and hyper polarization of hair-cell potential referred to as the cochlear microphonic - Process is identical in principle to that of a common microphone
Middle ear general functions
- Transformer - Boosts pressure - Controls input
Middle ear parts
- Tympanic membrane - Malleus - Incus - Stapes - Tensor tympani - Stapedius - Eustachian tube
Pressure wave in perilymph
- Very high rate over very short distance - Disturbance same along length (any place) at any given time - Pressure exerted on basilar membrane will depend on mechanical properties of basilar membrane at any given time or space - Mass and stiffness affect resonance
Volley theory
- Volley theory states that groups of neurons of the auditory system respond to a sound by firing action potentials slightly out of phase with one another so that when combined, a greater frequency of sound can be encoded and sent to the brain to be analyzed - THE THEORY THAT SOUND FREQUENCY IS ENCODED IN THE GROUP RESPONSE OF NEURONS PHASE-LOCKED TO THE TIME WAVEFORM
Motility of outer hair cells
- Voltage increase causes the outer hair cell to exhibit "motility" or movement - Motile outer hair cells change shape (lengthen or shorten) - Shrinks with depolarization - Lengthens with hyperpolarization - Motility generates force - Force changes the motion of the traveling wave - This change increases the mechanical input to the inner hair cells - This increase in input is called the "cochlear amplifier" - HAIR CELLS ARE OBSERVED TO BE CONTRACT AND EXPAND - HAIR CELL MOTILITY APPEARS TO PLAY A ROLE IN ACTIVELY REGULATING THE RESPONSE OF THE HAIR CELL TO SOUND
Fluid-filled chambers of the cochlea
- Within the inner ear - Scala vestibuli - Scala media - Scala tympani - Membrane is frequency-selective - Gives info about the content of the sound - Mechanical disturbance (actual fluid not moving) - spirals up and then down again
Georg von Bekesy
- Won Nobel Prize for measuring motion of basilar membrane in elephant cadavers - Cochlea fluid replaced with suspension of powdered aluminum - Light reflected off powder viewed with a microscope - Contemporary attempts to measure in living animals
How do outer hair cells act as amplifiers for basilar membrane and eventually how does that help our hearing?
1. Sensitivity 2. Selectivity
For every 1 row of inner hair cells, there are ___ row(s) of outer hair cells.
3
Spike response (action potential)
A DISCRETE (ALL OR NONE) ELECTRICAL POTENTIAL GENERATED BY AN AUDITOYR NERVE FIBER
Iso-intensity curve
A PLOT OF AVERAGE DISCHARGE RATE OF A NERVE FIBER AS A FUNCTION TONE FREQUENCY FOR A FIXED TONE INTENSITY
Plasticity
A TERM USED TO DESCRIBE THE ABILITY OF THE CENTRAL AUDITORY NERVOUS SYSTEM TO CHANGE ITS FUNCTION AND PHYSIOLOGY IN RESPONSE TO ACOUSTIC TRAUMA
Catenary
A curve formed by a wire, rope, or chain hanging freely from two points and forming a U shape
Hair cell regeneration
CONTRARY TO CONVENTIONAL WISDOM, HAIR CELLS OF CERTAIN NONMAMMALIAN SPECIES HAVE BEEN OBSERVED TO REGENERATE
Characteristic (best) frequency
FREQUENCY FOR WHICH THE NERVE FIBER RESPONDS MOST VIGOROUSLY
T/F Inner and outer hair cells are the same size and shape.
False
Radial fibers
MANY OF THESE AFFERENT FIBERS SYNAPSE ON A SINGLE INNER HAIR CELL AND THUS CONVEY INFORMATION ALONG DIVERGENT PATHS REGARDING VIBRATION AT A PARTICULAR PLACE ALONG THE BASILAR MEMBRANE
Outer hair cells are connected to ___ at the base of the hair cell.
Neurons
Inner and outer hair cells
OFTEN REFERRED TO AS THE EAR'S TRANSDUCERS BECAUSE THEY CONVERT THE MECHANICAL DISTURBANCE CREATED IN THE FLUIDS OF THE COCHLEA INTO AN ELECTRICAL POTENTIAL
Outer spiral fibers
ONE OF THESE AFFERENT FIBERS WILL SYNAPSE ON MANY OUTER HAIR CELLS THUS INCREASING SENSITIVITY TO ANY MOTION OF THE BASILAR MEMBRANE
Feature analysis
PROPERTY OF THE CENTRAL AUDITORY NERVOUS SYSTEM THAT ALLOWS US TO BREAK DOWN A COMPLEX SOUND (E.G. MUSIC) INTO SIMPLER ACOUSTIC FEATURES (E.G. PITCH, TIMBRE)
Cochlea
Sense organ that translates sound into nerve impulses to be sent to the brain
As the pressure wave in the perilymph builds up, the scala media is compressed, which pushes on the ___ .
Tectorial membrane
Describe two mechanisms by which the middle ear maintains an impedance match between the airborne sound and the compression wave propagated through the fluid of the cochlea.
- 1. The area differential of 15-to-1 between the tympanic membrane and oval window produces a 20log(15/1)=23 dB gain in pressure exerted by the stapes footplate at the oval window - 2. The leveraging action of the ossicular chain produces another 2 dB gain in pressure at the stapes footplate
Afferent nerve fibers
- A basic type of nerve fiber that innervates the organ of Corti - "Ascending" - Carry sensory information to the brain - (Inner) radial - Outer spiral - Low level to high level
Efferent nerve fibers
- A basic type of nerve fiber that innervates the organ of Corti - "Descending" - Carry "commands" from auditory CNS to hair cells - Tunnel (outer) radial - Inner spiral - High level to low level
Round window
- A membrane-covered opening in the inner wall of the middle ear that compensates for changes in cochlear pressure - Allows fluid in the cochlea to move, which in turn ensures that hair cells of the basilar membrane will be stimulated and that audition will occur
Oval window
- A membrane-covered opening that leads from the middle ear to the vestibular canal of the inner ear (cochlea) - Contracted by stapes - THE MOTION OF THE OSSICULAR CHAIN IS CONVERTED INTO A MECHANICAL DISTURBANCE IN THE FLUIDS OF THE COCHLEA VIA THE ATTACHMENT OF THE STAPES FOOTPLATE TO THIS MEMBRANE
Stapedius
- A muscle of the inner ear - Innervated by the facial (CN VII) cranial nerve - Pulls so that stapes rotates out of the oval window - Adds to stiffness of ossicular chain - A in diagram - Responds to: loud sound, pharynx preparing for speech production
Tensor tympani
- A muscle of the inner ear - Innervated by the trigeminal (CN V) cranial nerve - Pulls "inward" on the malleus - "Tenses" the tympanic membrane - B in diagram - Responds to: loud sound, swallowing, touching face, pharynx preparing for speech production
Osseous (bony) labyrinth
- A part of the inner ear - Rigid outer wall of inner ear - Secretes perilymph which conducts sound vibrations - Blue in diagram - Contains vestibule, semicircular canals, cochlea - Houses structures of inner ear
Membranous labyrinth
- A part of the inner ear - Tube within osseous labyrinth - Filled with endolymph which conducts sound - Houses receptor cells for hearing and equilibrium - Orange in diagram - Contains semicircular canals, cochlea duct, utricle, saccule - Houses structures of inner ear
Eustachian tube
- A part of the middle ear - Function is to equalize middle ear pressure with ambient pressure - Connects middle ear with the nasal passages - When open, equalizes pressure across the tympanic membrane - AKA equalizes middle ear pressure with ambient pressure - Transmission through the middle ear is best when pressure is same on both sides of the tympanic membrane - When a pressure imbalance exists, swallowing or yawning or chewing might help to open the Eustachian tube - PART OF THE MIDDLE EAR THAT SERVES AS A 'RELEASE VALUE' SO AS TO EQUALIZE PRESSURE INSIDE AND OUTSIDE OF THE MIDDLE EAR CAVITY
Ear canal
- A part of the outer ear - A "closed tube" - Resonates to frequencies with a wavelength of about 3000 Hz - Lots of standing waves inside - Produces a boost in SPL in the mid frequency region (2500-3500 Hz) of about 15 dB - STRUCTURE OF THE OUTER EAR - ITS RESONANCE PRODUCES A GAIN OF ABOUT 10-15 DB IN THE REGION OF 2 KHZ
Pinna
- A part of the outer ear - Can be mobile (bats, kangaroos) or stationary - Landmarks: concha, external auditory meatus - Shaped like a bugle or trumpet and works in the same way - Helps to capture sound - Adds gain to sound in the 1500-7000 Hz (high frequency sounds) due its shape and size - THE VISIBLE PORTION OF THE OUTER EAR - SERVES TO FUNNEL SOUND INOT THE EAR CANAL - PLAYS AN IMPORTANT ROLE IN SOUND LOCALIZATION THROUGH DIRECTIONALLY DEPENDENT FILTERING
Concha
- A part of the pinna (in the outer ear) - Deep center of the pinna - Produces a boost in SPL in high frequency region (5000 Hz) of about 10 dB - LARGE CAVITY AT ENTRANCE TO EAR CANAL - FUNNELS SOUND INTO THE EAR CANAL, PRODUCING ABOUT A 15 DB GAIN IN THE REGION OF 5 KHZ
External auditory meatus
- A part of the pinna (in the outer ear) - Opening to the auditory canal
Tinnitus
- A ringing or buzzing in the ears - A variety of possible cause - High incidence among those with sensorineural hearing loss - Possible mechanisms: change in efferent control of AGC lowers the threshold for OHC oscillation, damage to hair cells cause 'leaking' electrical current that stimulates nerve) - Common treatment is masking noise - Separate from OAEs
Frequency tuning curve
- A way for the auditory nerve to code frequency - Tuning by place - Each neuron innervates a small "point" along the basilar membrane - Auditory nerve fibers are "tuned" - each fiber responds to some frequencies but not others - Tuning curve (TC) also called "isorate curve" - a plot of threshold as a function of frequency (V-shaped, similar to basilar membrane tuning curve) - AKA PHYSIOLOGICAL TUNING CURVE - A SINGLE CURVE GIVING ALL COMBINATIONS OF TONE INTENSITY (X-AXIS) AND TONE FREQUENCY (Y-AXIS) THAT PRODUCE THE SAME AVERAGE DISCHARGE RATE OF THE AUDITORY NERVE FIBER
Phase looking
- A way for the auditory nerve to code frequency - Tuning temporally - For frequencies below 4 Hz, spikes are "synchronized" or "phase-locked" to the stimulus waveform - Spikes occur during 1/2 of the cycle (peak) - Spikes do not occur during the other half-cycle (trough) - This is caused by the way K+ flows into inner hair cells: stereocilia bend voltage increases, neurotransmitter is released during one-half cycle only - So, a spike will occur during each cycle, and it is synchronous with the stimulus - Ex: for a 500 Hz stimulus, each cycle is completed in 2 sec (this means a spike will occur ever 2 msec)
Post-stimulus time (PST) histogram
- A way for the auditory nerve to code intensity - Discharge rate vs. time - The PST histogram shows how discharge rate changes while a sound is on - Discharge rate: peaks just after the onset of the sound declines gradually during the sound may drop below SR at offset - A PLOT, IN THE FORM OF A HISTOGRAM, OF THE FREQUENCY OF OCCURRENCE OF TIME INTERVALS BETWEEN A SPECIFIC PHASE OF THE TIME WAVEFORM AND THE SPIKE RESPONSE OF AN AUDITORY NERVE FIBER
Rate of discharge
- A way for the auditory nerve to code intensity - However, no single neuron has a dynamic range that will cover the dynamic range of hearing (120 dB or so) - Individual neurons usually "saturate" 20-60 dB above threshold - So, other neurons must be "recruited" to allow intensity coding beyond saturation
Rate-level function
- A way for the auditory nerve to code intensity - Sounds above some amplitude cause discharge rate to increase above the spontaneous discharge rate - Further increases in level produce still more spikes - But at some level, discharge rate "saturates" (further increases in level do not cause discharge rate to increase) - "Threshold": the amplitude or level at which discharge rate just begins to increase - "Neural dynamic range": the range of SPLs over which changes in stimulus amplitude produce changes in discharge rate - When a fiber's discharge rate is saturated, it no longer provides information about the sound's level - The dynamic range of individual auditory nerve fibers is much smaller than the dynamic range that we can hear - Increased neural activity usually leads to increased sensation so the perception of intensity and loudness must involve some mechanism other than discharge rate in a ingle auditory nerve fiber
Electrical spikes
- Action potential - Myelin sheath speeds up neural conduction because the action potentials literally jump from one node to the next - All or nothing - Depolarization: Na+ in (graph rising) - Hyperpolarization: K+ out (graph lowering) - Spike duration is 1-2 ms - Can also be really slow along unmyelinated axons (0.5 m/sec)
The "middle ear reflex"
- Activation of the middle ear muscles by sounds that are intense, low frequency, long duration - Not activated by low intensity sounds, high frequency sounds, transient sounds (such as clicks) - Provides some protection against damage from some intense sounds
2 basic types of nerve fibers innervate the organ of Corti
- Afferent "ascending" - Efferent "descending"
Specificity (detection)
- Afferent innervation of inner and outer hair cells - Many-to-one arrangement - Convey info along divergent paths regarding vibration at a particular place along the basilar membrane - Can pick up on specific frequency - Inner radial nerve fiber --> 1 IHC
Sensitivity (identification)
- Afferent innervation of inner and outer hair cells - One-to-many arrangement - Increasing sensitivity to any motion of the basilar membrane - Can pick up on motion - 1 outer spiral nerve fiber --> many OHC
Innervation of the ear
- Afferent neurons carry information from the ear to the brain - The afferent neurons are bipolar, which means that there are 2 axonal projections leading away from the cell body - Efferent neurons carry information from the auditory CNS to the ear - The efferent neurons are unipolar, which means that there is one axonal projection leading away from the cell body and projecting to hair cells
Traveling wave characteristics
- Always starts at the base of the cochlea and moves toward the apex - Amplitude changes as it traverses the length of the cochlea - The position along the basilar membrane at which its amplitude is highest depends on the frequency of the stimulus - AKA when sound pressure is transmitted to the fluids of the inner ear by the stapes, the pressure wave deforms the basilar membrane in an area that is specific to the frequency of the vibration - Higher frequencies cause movement in the base of the cochlea - Deeper frequencies work at the apex - This characteristic is known as COCHLEAR TONOTOPY
Inner ear general functions
- Analysis - Transduction to neural impulses
Cochlear function: summary
- Analyzer: conducts an analysis of frequencies and intensities and represents the sound in a tonotopic manner - Amplifier: adds energy to low intensity signals (below about 60 dB) - Transducer: converts the mechanical activity in the cochlea to electrical stimulus for nerve fibers
Transformer action 1 of middle ear
- Area ratio of tympanic membrane to stapes - Area ratio about 17:1 - Stapes pressure is 17 times tympanic membrane pressure because area is smaller - Pressure increase is about 25 dB
3 transformer actions of middle ear
- Area ratio of tympanic membrane to stapes - Lever due to malleus/incus length - Curvature of tympanic membrane (catenary)
Transmission
- As the basilar membrane bounces up and down, the fine stereocilia are sheared back and forth under the tectorial membrane - When the stereocilia are pulled in the right direction, the hair cell depolarizes - This signal is transmitted to a nerve process lying under the organ of Corti - This neuron transmits the signal back along the auditory nerve to the brainstem
Function of the afferent auditory nerve
- Auditory nerve fiber discharges - Responses of auditory nerve are measured as discharge rate, in spikes/second - Many auditory nerve fibers respond even when no sound is present (spontaneous activity) - 3 types of auditory nerve fibers (low, medium, or high spontaneous rate)
What happens when the sound is not a sine wave?
- Complex sound - Each frequency component in the spectrum displaces the membrane at the same place as a sine wave of the same frequency - So, a hypothetical sound with spectral peaks at 700, 1500, and 2500 Hz might produce a displacement pattern with 3 reps at different places along the membrane - Basilar membrane acts as a spectrum analyzer
Review of auditory transduction
- Compression wave in the cochlear fluids produces traveling wave of basilar membrane. Frequency is mapped to a place of maximum wave amplitude - Motion of basilar and tectorial membranes produce shearing force across stereocilia causing them to be displaced - Displacement of stereocilia opens ion channels resulting in less negative potential in the body of the hair cell, causing hair cell depolarization - The depolarization of hair cell in turn causes neurotransmitter release at the basal end of the hair cell - Neurotransmitter reaches the nerve fiber, which eventually transmits the signal to the brain
Outer ear functions
- Concha and ear canal funnel sound to middle ear - Captures sound and acts as a "resonator": directly dependent constructive (sound increases) and destructive (sound decreases) interference - Localization helper
Total boost in resonance
- Concha produces a boost in SPL of about 10 dB - External ear canal produces boost in SPL of about 15 dB - These and other resonance effects combine to produce an SPL boost of about 20 dB in the response of the ear
Middle ear function
- Conserve sound energy as sound passes from air to liquid medium - Protector of inner ear - Sound transmission
Hair cell transduction
- Conversion of mechanical disturbance in fluids of cochlea to change in electrical potential across hair body - Conversion of one form of energy to another - In the cochlea, mechanical energy (vibration) becomes electrical energy - THE PROCESS BY WHICH 1 FORM OF ENERGY IS CONVERTED TO ANOTHER, AS IN THE TRANSDUCTION OF THE MECHANICAL DISTURBANCE IN THE FLUIDS OF THE COCHLEA TO AN ELECTRICAL POTENTIAL ACROSS THE HAIR CELL MEMBRANE
Transformer action 3 of middle ear
- Curvature of tympanic membrane (catenary) - Because of the complicated way its shape changes, the tympanic membrane itself amplifies force as it moves - A tympanic membrane that is flat will collect energy - A tympanic membrane that is shaped like a catenary will increase the force delivered to the ossicles - 6 dB increase
First recording
- David Kemp - Ear probe with mic in ear - Sound generated in ear even without sound input - Graph still showing variations - At the time didn't know about outer hair cells - Thought equipment was malfunctioning - Cautious to say anything - Means ear is generating sound on its own - People skeptical - Eventually named autoacoustic emission
Nerve cell anatomy
- Dendrites - Soma - Axon (myelinated with nodes of Ranvier or unmyelinated) - Terminal boutons - Neurotransmitter - Synapses
Displacement of the basilar membrane toward the scala vestibuli
- Depolarization of the outer hair cell - Associated with decrease in outer hair cell length
Hair cell: endocochlear potential
- Difference in electrical charge inside and outside the hair cell body - Huge positive potential outside of cell (+80) - Huge negative potential inside of cell (-70) - Huge mV difference (150 mV)
Tonotopic arrangement of basilar membrane
- Different areas along basilar membrane respond to different frequencies - Stapes (base) picks up on higher frequencies - Apex picks up on lower frequencies - AN ARRANGEMENT MAINTAINED THROUGHOUT THE AUDITORY NERVOUS SYSTEM WHEREIN FIBERS CREATE ORGANIZED MAPS BASED ON THEIR CHARACTERISTIC FREQUENCIES
Inner spiral fibers
- EFFERENT NERVE FIBERS - SYNAPSE ON THE DENDRITES OF RADIAL FIBERS INNERVATING THE INNER HAIR CELLS
Outer (tunnel) radial fibers
- EFFERENT NERVE FIBERS - SYNAPSE DIRECTLY ON THE BODY OF OUTER HAIR CELLS - CARRY INFORMATION FROM HIGHER CENTERS SO AS TO REGULATE THE RESPONSE OF THE OUTER HAIR CELLS
Evoked otoacoustic emissions
- EOAE - Sounds generated by other sounds
Tectorial membrane
- Extends along longitudinal length of cochlea parallel to basilar membrane - Creates shearing force across stereocilia of inner and outer hair cells - PIVOTS ON THE SPIRAL LIMBUS AND SO CAUSES A SHEARING OF THE HARI CELL STEREOCILIA WHEN IN MOTION - THIS SHEARING FORCE IS IN TURN TRANSDUCED INOT A MEMBRANE POTENTIAL BY THE HAIR CELLS
Selectivity
- Frequency selectivity enhanced due to outer cells - Small displacements of basilar membrane are amplified locally by automatic gain control mechanism of outer hair cells
How does the auditory nerve code frequency?
- Frequency tuning curves: place coding - Phase locking: temporal coding
Hair cell polarization
- Hair cells, like all excitable nerve cells, are tiny batteries, with an excess of negatively charged ions inside and an excess of positively charged ions outside - Inside vs. outside cell "polarized" (difference in charge) - Stria vascularis ensures greater concentration of positive K+ ions at stereocilia of hair cells - K+ flow into hair cell stereocilia and out of hair cell at base
Displacement of the basilar membrane in the opposite direction
- Hyperpolarization of the outer hair cell - Associated with increase in outer hair cell length
Endolymph
- In the inner ear - An extracellular fluid contained in the scala media
Perilymph
- In the inner ear - An extracellular fluid located within the cochlea - In two of its three compartments: the scala tympani and scala vestibuli
Functional role of efferent pathways of the olivocochlear bundle
- Inhibits outer hair cell response, possibly affecting the sharpness of mechanical tuning and operation range - Permits one cochlea to influence the response of the other possibly as a mechanism for selective attention to one or the other ear
Efferent innervation
- Inner spiral: connected to IHC, thin unmeylinated axons - Tunnel (outer) radial: connected to OHC, thick, myelinated axons
Middle of basilar membrane
- Intermediate stiffness - Near middle of spiral - Not wide, not narrow - Vibrates at intermediate frequencies - Frequencies between the extremes will displace the middle
Infant hearing screening
- Intermodulation distortion measured as an otoacoustic emission in ear canal - Used to screen infants for hearing loss - Pass/fail - Easy = you don't have to do anything
What is another name for a frequency tuning curve?
- Isorate curve - Physiological tuning curve - A plot of threshold as a function of frequency - V-shaped - Similar to basilar membrane tuning curve
Transformer action 2 of middle ear
- Lever due to malleus/incus length - Malleus is 9 mm, incus is 7 mm - Malleus length is greater than incus length, so force is increased by 1.3:1 at stapes (about 2 dB) - Transformer action lever due to malleus/incus length
What does the stapedius respond to?
- Loud sound - Pharynx preparing for speech production
What does the tensor tympani respond to?
- Loud sound - Swallowing - Touching face - Pharynx preparing for speech production
General method for evaluating acoustic properties
- Measure SPL in sound field (input) - Measure SPL with probe near tympanic membrane in ear canal (output) - Compare
Effects of death on tuning curves
- More overlap on the graph - As an animal dies, the basilar membrane has higher thresholds and broader tuning curves
Cochlear amplifier
- Motility of outer hair cells generates force - Force changes the motion of the traveling wave - This change increases the mechanical input to the inner hair cells - This increase in input is called the "cochlear amplifier" - The cochlear amplifier improves the sensitivity of the basilar membrane - Makes thresholds lower - Improves its frequency selectivity - Makes the tuning curve sharper - For inner hair cells: current causes the cell to release neurotransmitter which elicits responses in primary auditory neurons (inner hair cells are not motile)
Review of cochlear mechanics
- Motion of ossicles creates compression wave of perilymph in cochlea - Compression wave in perilymph produces 'instantaneous' pressure wave across the length of the basilar membrane - Pressure wave creates 'traveling wave' on the basilar membrane - Frequency is mapped to place of maximum wave amplitude on the basilar membrane by principle of resonance - Frequency of vibration is the same each point along the length of the basilar membrane, also by the principle of resonance
Hair cell depolarization
- Moving the stereocilia causes tiny pores on the stereocilia to open, allowing positive ions to rush into the cell, which causes depolarization - Depolarization = less difference between charge on inside and outside of cell - Influx of Ca++ and K+ ions depolarize cell body (enter stereocilia tips) - Voltage-gated calcium channels open - Calcium influx triggers release of neurotransmitter - Neurotransmitter bonds with receptor channels, opening them
Shearing force
- Not actual contact - Basilar membrane displacement causes a searing at the hair cell stereocilia - Stimulus to inner and outer hair cells - Tectorial membrane displacement - THE FORCE EXERTED ACROSS THE HAIR CELL STEREOCILIA BY THE TECTORIAL MEMBRANE THAT ACTS AS THE 'EFFECTIVE' STIMULUS TO THE HAIR CELLS
Apical end of basilar membrane
- Not stiff - Near inside of spiral - Wide - Vibrates at low frequencies - Low frequencies will displace the apex
Distortion product OAEs
- OAE evoked by intermodulation distortion - Can easily be separated from input - Very widely used for research - Occurs during stimulation with 2 pure tones - Easier because input and output frequencies are different - Called distortion because it originates in cochlea, not stimulus - Provides information at discrete frequencies - Very useful clinically
Conductive hearing loss
- Occurs because intensity is not transmitted or "conducted" into the cochlea - Caused by infection, tumors, middle ear fluid from infection or Eustachian tube dysfunction, foreign body, or trauma (as in a skull fracture) - Middle ear infections: diseases that affect the ossicles or their joints (otosclerosis)
Stimulus frequency otoacoustic emissions
- Occurs during stimulation at the frequency of the stimulus - Most obvious for low intensity stimuli - Difficult to separate from the stimulus - Not useful clinically
Who has OAEs?
- Only occur in healthy cochlea - Outer hair cells need to be functioning to produce emissions - OAEs will be reduced or absent in significant cochlear disorders (e.g. ototoxicity, noise induced) - OAEs are cochlear responses, but not a measure of hearing threshold - In a central pathology, OAEs may be present but the person may not "hear" sounds - OAE may be produced but not recorded in a conductive pathology
Inner ear parts
- Osseous and membranous labyrinth - Perilymph - Endolymph - Basilar membrane - Scala vestibuli - Scala tympani - Scala media - Helicotrema - Mediolus - Organ of Corti - Tectorial membrane - Inner hair cells - Outer hair cells - Hair cell stereocilia - Hensen's cells - Reticular lamina - Spiral limbus - Osseous spiral lamina
