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The Auditory & Vestibular Systems

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Introduction

Sensory SystemsA. Sense of hearing, audition

1. Detect sound2. Perceive and interpret nuances

B. Sense of balance, vestibular system1. Head and body location2. Head and body movements

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The Nature of Sound

SoundA. Audible variations in air pressureB. Sound frequency: Number of cycles per

second expressed in units called Hertz (Hz)C. Cycle: Distance between successive

compressed patchesD. Range: 20 Hz to 20,000 HzE. Pitch: High and LowF. Intensity: Difference in pressure between

compressed and rarefied patches of air

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The Nature of Sound

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The Nature of Sound

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The Structure of the Auditory System

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The Structure of the Auditory System

I. Auditory pathway stages A. Sound wavesB. Tympanic membraneC. OssiclesD. Oval windowE. Cochlea fluidF. Sensory neuron response

II. Brain stem nuclei outputA. Thalamus to MGN to A1

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Components of the Middle Ear

The Middle Ear

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I. Sound Force Amplification by the OssiclesA. Pressure: Force by surface areaB. Greater pressure at oval window than

tympanic membrane, moves fluidsII. The Attenuation Reflex

A. Response where onset of loud sound causes tensor tympani and stapedius muscle contraction

B. Function: Adapt ear to loud sounds, understand speech better

The Middle Ear

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The Inner EarAnatomy of the Cochlea

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The Inner EarAnatomy of the Cochlea

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Physiology of the CochleaPressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges out

The Inner Ear

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The Organ of Corti

The Inner Ear

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Cilia

The Inner Ear

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Cilia

The Inner Ear

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The Inner Ear

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Transduction by Hair Cells

Sound: Basilar membrane

upwardreticular lamina up stereocilia bends

outward

The Inner Ear

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The Inner Ear

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I. The Innervation of Hair CellsA. One spiral ganglion fiber: One inner hair

cell, numerous outer hair cellsII. Amplification by Outer Hair Cells

A. Function: Sound transductionB. Motor proteins: Change length of outer

hair cellsC. Prestin: Required for outer hair cell

movements

The Inner Ear

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The Inner Ear

The Basilar Membrane Structural properties: Wider at apex, stiffness decreases from base to apex

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The Inner Ear

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Central Auditory Processes

Auditory PathwayA. More synapses at nuclei than visual

pathway, more alternative pathwaysB. Anatomy

1. Dorsal cochlear nucleus, ventral cochlear nucleus, superior olive, inferior colliculus, MGN, lateral lemniscus, auditory nerve fiber

2. Primary pathway: Ventral cochlear nucleus to superior olive to inferior colliculus to MGN to auditory cortex

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Auditory Pathway

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Auditory Pathway

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Response Properties of Neurons in Auditory Pathway

A. Characteristic frequencyFrequency at which neuron is most

responsiveB. Response

More complex and diverse on ascending auditory pathway in brain stem

Central Auditory Processes

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Encoding Sound Intensity and Frequency

I. Encoding Information About Sound IntensityA. Firing rates of neuronsB. Number of active neurons

II. Stimulus Frequency, Tonotopy, Phase LockingA. Frequency sensitivity: Basilar membrane B. Frequency: Highest at base, lowest at

cochlea apexC. Tonotopy: Systematic organization of

characteristic frequency within auditory structure

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Phase LockingConsistent firing of cell at same sound wave phase

Encoding Sound Intensity and Frequency

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I. Techniques for Sound LocalizationA. Horizontal: Left-right, Vertical: Up-down

II. Localization of Sound in Horizontal PlaneA. Interaural time delay: Time taken for

sound to reach from ear to earB. Interaural intensity difference: Sound at

high frequency from one side of earC. Duplex theory of sound localization:

1. Interaural time delay: 20-2000 Hz2. Interaural intensity difference: 2000-

20000 Hz

Mechanisms of Sound Localization

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The Sensitivity of Binaural Neurons to Sound Location

Monaural: Sound in one earBinaural: Sound at both earsSuperior olive: Cochlear nuclei input to superior olive, greatest response to specific interaural delay

Mechanisms of Sound Localization

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Mechanisms of Sound Localization

I. Delay Lines and Neuronal Sensitivity to Interaural Delay

A. Sound from left side, activity in left cochlear nucleus, sent to superior olive

B. Sound reaches right ear, activity in right cochlear nucleus, first impulse far

C. Impulses reach olivary neuron at the same time summation action potential

II. Localization of Sound in Vertical PlaneA. Sweeping curves of outer ear

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Mechanisms of Sound Localization

A given binaural neuron indicates the amount of phase disparity between inputs from the left and right ear.

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I. Acoustic RadiationA. Axons leaving MGN project to auditory

cortex via internal capsule in an arrayB. Structure of A1 and secondary auditory

areas: Similar to corresponding visual cortex areas

II. Neuronal Response PropertiesA. Frequency tuning: Similar characteristic

frequencyB. Isofrequency bands: Similar

characteristic frequency, diversity among cells

Auditory Cortex

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Principles in Study of Auditory CortexTonotopy, columnar organization of cells with similar

binaural interaction

Auditory Cortex

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I. Importance of Vestibular SystemA. Balance, equilibrium, posture, head, body,

eye movementII. The Vestibular Labyrinth

Lateral line OrgansSmall pits or tubes

FunctionSense vibration

or pressure changes

The Vestibular System

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The Vestibular System

Head AngleLinear Acceleration

Head Rotation

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The Vestibular System

The Otolith Organs

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The Vestibular System

The Otolith Organs

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The Vestibular System

I. The Semicircular CanalsA. Function: Detect head movements

II. StructureA. Crista: Sheet of cells where hair cells of

semicircular canals clusteredB. Ampulla: Bulge along canal, contains

cristaC. Cilia: Project into gelatinous cupulaD. Kinocili oriented in same direction so all

excited or inhibited togetherE. Semicircular canals: Filled with

endolymph

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The Vestibular System

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I. Push-Pull Activation of Semicircular CanalsA. Three semicircular canals on one side

1. Helps sense all possible head-rotation angles

B. Canal: Each paired with another on opposite side of head

C. Push-pull arrangement of vestibular axons: Rotation causes excitation on one side, inhibition on the other

The Vestibular System

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I. The Vestibulo-Ocular Reflex (VOR)A. Function: Line of sight fixed on visual

targetB. Mechanism: Senses rotations of head,

commands compensatory movement of eyes in opposite direction

C. Connections from semicircular canals, to vestibular nucleus, to cranial nerve nuclei excite extraocular muscles

The Vestibular System

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The Vestibular System

Vestibular PathologyA. Drugs (e.g., antibiotics) can damage

vestibular systemB. Effects:

1. Trouble fixating on visual targets2. Walking and standing difficult

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Concluding Remarks

Hearing and BalanceA. Nearly identical sensory receptors (hair

cells)B. Movement detectors: Periodic waves,

rotational, and linear forceC. Auditory system: Senses external

environmentD. Vestibular system: Senses movements of

itself

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Concluding Remarks

Hearing and BalanceA. Auditory Parallels Visual System

1. Tonotopy (auditory) and Retinotopy (visual) preserved from sensory cells to cortex code

B. Convergence of inputs from lower levels Neurons at higher levels have more complex responses

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End of Presentation