AuditionAudition

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Background & Structure of Auditory System Background & Structure of Auditory System

AuditionAudition1. Sense of hearing1. Sense of hearing2. Mechanisms within the ear and brain 2. Mechanisms within the ear and brain that translate sound in our environmentthat translate sound in our environmentinto meaningful neural signals into meaningful neural signals

Sound Sound 1.1.Audible variations in air Audible variations in air pressure (compressions)pressure (compressions)2. Molecules are displaced2. Molecules are displacedforward leaving a correspondingforward leaving a correspondingarea of lower pressurearea of lower pressure

3. Sound waves vary in two ways:3. Sound waves vary in two ways:a.a.Amplitude--intensity; peak to trough;Amplitude--intensity; peak to trough;perceived as differences in loudnessperceived as differences in loudnessb. Frequency: Number of compressions perb. Frequency: Number of compressions persecond; pitch; unit: hertz (1 cycle/second)second; pitch; unit: hertz (1 cycle/second)

Three divisions of the earThree divisions of the ear1. Outer1. Outer2. Middle2. Middle3. Inner3. Inner

Outer earOuter ear1. Pinna1. Pinnaa.a.Funnel shaped outer ear made Funnel shaped outer ear made of skin and cartilageof skin and cartilage2. Auditory canal2. Auditory canala.a.Channel leading from the pinnaChannel leading from the pinnato the tympanic membraneto the tympanic membrane

Middle earMiddle ear

1. Tympanic membrane (eardrum)1. Tympanic membrane (eardrum)a.a.Moves in response to variationsMoves in response to variationsin air pressurein air pressure

2. Ossicles: Series of bones in a small air 2. Ossicles: Series of bones in a small air filled chamber. Transfer the movement offilled chamber. Transfer the movement ofthe tympanic membrane into the movementthe tympanic membrane into the movementof a second membrane covering a hole in of a second membrane covering a hole in the bone of the skull (oval window).the bone of the skull (oval window).The bones of middle ear are malleus The bones of middle ear are malleus (hammer), incus (anvil) and stapes (stirrup)(hammer), incus (anvil) and stapes (stirrup)

3. Functional considerations:3. Functional considerations:a. Cochlea is filled with an incompressible a. Cochlea is filled with an incompressible fluidfluidb. More force is required to displace fluid b. More force is required to displace fluid than airthan air

c. Bones in the middle ear amplify the c. Bones in the middle ear amplify the pressure. Pivot points that act as fulcrums. pressure. Pivot points that act as fulcrums. Malleus is displaced in response to the Malleus is displaced in response to the movement of the tympanic membrane--movement of the tympanic membrane--bottom moves towards the inner ear and bottom moves towards the inner ear and the top moves towards the outer ear. This the top moves towards the outer ear. This pulls the top of the incus towards the outer pulls the top of the incus towards the outer ear and pushes the bottom towards the ear and pushes the bottom towards the inner ear. Stapes is consequently pushed inner ear. Stapes is consequently pushed forward against the oval window which is forward against the oval window which is compressed inwardcompressed inward

d. Oval window is smaller and d. Oval window is smaller and the same pressure across a the same pressure across a smaller area results in a greatersmaller area results in a greaterforce (like a spiked high heel)force (like a spiked high heel)

4. Eustachian tube: Tube that 4. Eustachian tube: Tube that connects the air-filled middle connects the air-filled middle ear to the mouth. It Containsear to the mouth. It Containsa valvea valve

Note: Subjects need to popped their ears Note: Subjects need to popped their ears when they go up in an airplane. On the when they go up in an airplane. On the ground their middle ear is as the same ground their middle ear is as the same pressure as the outside environment. As pressure as the outside environment. As they ascend, air pressure is lower a high they ascend, air pressure is lower a high altitudes. The tympanic membrane will altitudes. The tympanic membrane will bulge out because the pressure in the bulge out because the pressure in the middle ear is greater than the outside middle ear is greater than the outside environment. When they yawn or environment. When they yawn or swallow, the valve in the tube is opened swallow, the valve in the tube is opened and the pressure is relieved.and the pressure is relieved.

Inner earInner ear

1.1.Converts the physical movement Converts the physical movement of the oval window into neural signal of the oval window into neural signal 2. Takes place in the cochlea 3. 2. Takes place in the cochlea 3. Elements a. Elements a. Cochlea b. Cochlea b. Vestibular apparatus Vestibular apparatus i. Not part of the auditory system i. Not part of the auditory system ii. Involved in balance ii. Involved in balance

Basic Auditory PathwayBasic Auditory Pathway

Processes Processes 1.1.Sound waves move the Sound waves move the tympanic membrane.tympanic membrane.2. Tympanic membrane moves2. Tympanic membrane movesthe ossicles.the ossicles.3. Ossicles move the membrane3. Ossicles move the membraneat the oval window.at the oval window.4. Motion at the oval window 4. Motion at the oval window moves the fluid in the cochlea.moves the fluid in the cochlea.

5. Movement of the fluid in the cochlea 5. Movement of the fluid in the cochlea causes a response in sensory neurons.causes a response in sensory neurons.6. Signal is transferred and processed 6. Signal is transferred and processed by a series of nuclei in the brain stem.by a series of nuclei in the brain stem.

7. Information is sent to a relay in the7. Information is sent to a relay in thethalamus(medial geniculate nucleus-MGN)thalamus(medial geniculate nucleus-MGN)8. MGN projects to the primary auditory 8. MGN projects to the primary auditory cortex in the temporal lobe.cortex in the temporal lobe.

CochleaCochlea

BackgroundBackground Cochlea transduces the mechanicalCochlea transduces the mechanicaldisplacement of the oval window into displacement of the oval window into a neural signala neural signal

AnatomyAnatomy1. Cross section 1. Cross section 2. Chambers of the cochlea2. Chambers of the cochleaa. Scala vestibulia. Scala vestibulib. Scala tympanib. Scala tympanic. Scala mediac. Scala media

3. Organ of corti 3. Organ of corti a. Contains auditory receptor cellsa. Contains auditory receptor cellsb. Located in the scala mediab. Located in the scala media

3. Basilar membrane3. Basilar membranea. Separates scala media and scala tympania. Separates scala media and scala tympanib. Properties are very important for auditionb. Properties are very important for audition4. Fluid is continuous between scala 4. Fluid is continuous between scala vestibuli and scala tympani vestibuli and scala tympani a. Physical connection is known as the a. Physical connection is known as the helicotremahelicotrema

Physiology of the CochleaPhysiology of the Cochlea

ProcessProcess1. Mechanical force1. Mechanical force pushespushes on theon the oval window oval window

2. Fluid within the cochlea is incompressible2. Fluid within the cochlea is incompressible3. Fluid pushes forward3. Fluid pushes forwarda.a.Conserves the wave properties of theConserves the wave properties of the sound sound (i.e. the movement of the fluid has frequency (i.e. the movement of the fluid has frequency and amplitude)and amplitude)b. Causes the round window to bulge out b. Causes the round window to bulge out

4. Structures within the 4. Structures within the cochlea are not rigid. Basilar cochlea are not rigid. Basilar membrane is flexible and membrane is flexible and bends in response to soundbends in response to sound

5. Structural properties of the basilar 5. Structural properties of the basilar membrane determine the way it responds to membrane determine the way it responds to soundsounda. Membrane is wider at apex than base (5:1)a. Membrane is wider at apex than base (5:1)b. Stiffness of the membrane decreases from b. Stiffness of the membrane decreases from base to apex (like a diving board)base to apex (like a diving board)

c. High frequency sounds have higher energy c. High frequency sounds have higher energy and can displace the stiffer part of the basilar and can displace the stiffer part of the basilar membrane (near the base)membrane (near the base)d. Lower frequency sounds have lower energy d. Lower frequency sounds have lower energy and displace the apex endand displace the apex ende. Base responds to high frequency and the e. Base responds to high frequency and the apex responds to low frequencyapex responds to low frequency

6. Basilar membrane establishes a place code 6. Basilar membrane establishes a place code in which different locations are maximally in which different locations are maximally deformed in response to different frequency deformed in response to different frequency soundssounds

Transduction of Mechanical DisplacementTransduction of Mechanical Displacement

Organ of Corti Organ of Corti 1. Structure1. Structurea. Outer hair cellsa. Outer hair cellsb. Inner hair cellsb. Inner hair cellsc. Tectorial membranec. Tectorial membraned. Reticular membraned. Reticular membranee. Basilar membranee. Basilar membranef. Stereociliaf. Stereociliag. Spiral gangliong. Spiral ganglion

Auditory receptorsAuditory receptors1. Hair cells1. Hair cellsa. Stereociliaa. Stereocilia

TransductionTransduction1. Bending of these cilia is the critical event in 1. Bending of these cilia is the critical event in the transduction of sound into neural signalthe transduction of sound into neural signal2. Hairs extend above the reticular membrane 2. Hairs extend above the reticular membrane and come in contact with tectorial membraneand come in contact with tectorial membrane3. When the basilar membrane moves in 3. When the basilar membrane moves in response to the motion of the stapesresponse to the motion of the stapesa. Whole complex moves as a unit either a. Whole complex moves as a unit either towards or away from the tectorial membranetowards or away from the tectorial membraneb. Lateral motion of the reticular membrane b. Lateral motion of the reticular membrane bends the stereociliabends the stereocilia

4. Depending on the direction that the hairs 4. Depending on the direction that the hairs bend, the inside of the hair cells will either:bend, the inside of the hair cells will either:a. Depolarizea. Depolarizeb. Hyperpolarizeb. Hyperpolarize5. Changes in cell potential result from the 5. Changes in cell potential result from the opening of K+ channels on tips of stereociliaopening of K+ channels on tips of stereociliaa. Channels are mechanically gateda. Channels are mechanically gatedb. Flaps that are connected to neighboring b. Flaps that are connected to neighboring cilia by a special protein moleculecilia by a special protein molecule

6. Depending on the direction that hairs bend, 6. Depending on the direction that hairs bend, the channel will either be opened or closedthe channel will either be opened or closeda. Opening the channel allows K+ to enter and a. Opening the channel allows K+ to enter and depolarize the hair celldepolarize the hair cellb. Closing the channel stops the flow of K+b. Closing the channel stops the flow of K+7. In response to be depolarization resulting 7. In response to be depolarization resulting from influx of K+from influx of K+a. Ca++ channel is activateda. Ca++ channel is activatedb. Influx of Ca++ causes the release of b. Influx of Ca++ causes the release of synaptic vesicles from the end of the hair cellsynaptic vesicles from the end of the hair cell

Sequence overview:Sequence overview:1. Physical displacement of the basilar 1. Physical displacement of the basilar membrane bends the stereociliamembrane bends the stereocilia2.Bending of cilia opens or closes K+ channel2.Bending of cilia opens or closes K+ channel3. When K+ enters, the hair cell depolarizes3. When K+ enters, the hair cell depolarizes4. Depolarization activates a Ca++ channel4. Depolarization activates a Ca++ channel5. Ca++ influx causes NT release5. Ca++ influx causes NT release

Generation of Action Potential (AP)Generation of Action Potential (AP)

AP occurs at the level of output ganglionAP occurs at the level of output ganglion1. Multiple outer hair cells make synaptic 1. Multiple outer hair cells make synaptic contact with a single ganglion cell.contact with a single ganglion cell.2. Ganglion make synaptic contact with a 2. Ganglion make synaptic contact with a single inner hair cell (although many ganglion single inner hair cell (although many ganglion cells can contact the same inner hair cell)cells can contact the same inner hair cell)3. 75% of all hair cells are outer hair cells3. 75% of all hair cells are outer hair cellsa. Outer alter the a. Outer alter the stiffness of tectorial membranestiffness of tectorial membrane

4. Only 5% of the fibers in the auditory nerve 4. Only 5% of the fibers in the auditory nerve are from outer hair cellsare from outer hair cells

Auditory PathwayAuditory Pathway

Connections to the brain stemConnections to the brain stem1. Spiral ganglion sends projection to the 1. Spiral ganglion sends projection to the cochlear nucleuscochlear nucleusa. There are two cochlea, each projecting to a. There are two cochlea, each projecting to its cochlear nucleusits cochlear nucleusb. Within the cochlear nucleus this process b. Within the cochlear nucleus this process branchesbranches i. Dorsal cochlear nucleusi. Dorsal cochlear nucleus ii. Posterior ventral cochlear nucleusii. Posterior ventral cochlear nucleus iii. Anterior ventral nucleusiii. Anterior ventral nucleus

c. Dorsal and posterior ventral cochlear nuclei c. Dorsal and posterior ventral cochlear nuclei send efferent projections to the contralateral send efferent projections to the contralateral inferior colliculusinferior colliculus i. Via the nucleus of the lateral leminiscusi. Via the nucleus of the lateral leminiscusd. Anterior ventral cochlear nucleus is a d. Anterior ventral cochlear nucleus is a critical component of a brainstem neural critical component of a brainstem neural circuit that permits the detection of interaural circuit that permits the detection of interaural time differences time differences

Localization of SoundLocalization of Sound

Mechanisms for detecting interaural time Mechanisms for detecting interaural time differencesdifferences1. Two ears separated by about 20 cm1. Two ears separated by about 20 cm i. Diameter of your headi. Diameter of your head2. Detect differences as small as 10 msec2. Detect differences as small as 10 msec

CircuitCircuit1. Medial superior olive (MSO) has cells that 1. Medial superior olive (MSO) has cells that receive coincident innervation from the right receive coincident innervation from the right and left anteroventral cochlear nucleusand left anteroventral cochlear nucleusa. Cells within the MSO are organized such a. Cells within the MSO are organized such that the distance from the respective cochlear that the distance from the respective cochlear nuclei varies systematicallynuclei varies systematically i. Length of the axonal connections i. Length of the axonal connections determine which MSO cell receives coincident determine which MSO cell receives coincident activation by action potentialactivation by action potential

LimitationsLimitations1. System works well for sounds that have 1. System works well for sounds that have frequencies below 3 kHzfrequencies below 3 kHz

Localization of sounds above 3 kHzLocalization of sounds above 3 kHz

1. Sound does not bend around the head1. Sound does not bend around the head2. Directed to one side or the other and an 2. Directed to one side or the other and an intensity difference resultsintensity difference results

3. Circuit: Anteroventral cochlear nucleus 3. Circuit: Anteroventral cochlear nucleus projects directly to the ipsalateral lateral projects directly to the ipsalateral lateral superior olive (LSO)superior olive (LSO) i. Indirectly to the contralateral lateral i. Indirectly to the contralateral lateral superior olive via an inhibitory neuron superior olive via an inhibitory neuron originating in the medial nucleus of the originating in the medial nucleus of the trapezoid body (MNTB)trapezoid body (MNTB)

4. Mechanism: Anteroventral cochlear 4. Mechanism: Anteroventral cochlear nuclear firing rate is greater for sound with nuclear firing rate is greater for sound with higher intensitieshigher intensities i. Sound arising directly lateral to the i. Sound arising directly lateral to the listener, LSO firing will be highest on that listener, LSO firing will be highest on that side side ii. Excitation from the ipsilateral ii. Excitation from the ipsilateral anteroventral cochlear nucleus will be anteroventral cochlear nucleus will be maximal maximal iii. Inhibition from the contralateral MNTB iii. Inhibition from the contralateral MNTB will be minimalwill be minimal

Higher Brain Activities Involved Higher Brain Activities Involved in Auditory Perceptionin Auditory Perception

Monoaural systemsMonoaural systems

1. Dorsal and posteroventral cochlear nuclei 1. Dorsal and posteroventral cochlear nuclei send projections to the inferior colliculussend projections to the inferior colliculusa. Pathways respond to sound arriving at a. Pathways respond to sound arriving at one ear onlyone ear only2. Auditory maps2. Auditory mapsa. Auditory space is not mapped at the a. Auditory space is not mapped at the cortical level. Tonotopic maps are created at cortical level. Tonotopic maps are created at the levels of the inferior colliculusthe levels of the inferior colliculus3. Highly processed auditory information is 3. Highly processed auditory information is then relayed to the medial geniculate then relayed to the medial geniculate nucleus of the thalamusnucleus of the thalamus

4. From the thalamus, this information 4. From the thalamus, this information ascends to the primary auditory cortex (A1) ascends to the primary auditory cortex (A1) located in the temporal lobelocated in the temporal lobea. A1 has a topographical map of the cochleaa. A1 has a topographical map of the cochlea i. Specific regions (isofrequency bands) of i. Specific regions (isofrequency bands) of A1 are activated in response to acoustical A1 are activated in response to acoustical stimulation of the basilar membranestimulation of the basilar membrane

Coding of Intensity and FrequencyCoding of Intensity and Frequency

IntensityIntensity1. Firing rate of individual hair cells1. Firing rate of individual hair cells2. Activation of multiple hair cells.2. Activation of multiple hair cells.3. Wave of higher amplitude has 3. Wave of higher amplitude has more width and activates more hair more width and activates more hair cells in a given areacells in a given area

FrequencyFrequency1. Consequence of the mechanics of the 1. Consequence of the mechanics of the basilar membranebasilar membrane2. Different portions of the basilar 2. Different portions of the basilar membrane are maximally deformed by membrane are maximally deformed by sound of different frequenciessound of different frequencies3. Hair cells that are selectively activated 3. Hair cells that are selectively activated project to cochlear nuclei (brain stem) project to cochlear nuclei (brain stem) with tonographic specificitywith tonographic specificitya. Specificity is conserved all the way to a. Specificity is conserved all the way to the cortexthe cortex

Cortical ResponsesCortical Responses

Neuronal organizationNeuronal organization1. Isofrequency bands 1. Isofrequency bands a. Temporal lobea. Temporal lobe2. Neurons within these bands respond to 2. Neurons within these bands respond to fairly similar characteristic frequenciesfairly similar characteristic frequencies