The Auditory System

Csilla Egri, KIN 306 Spring 2012

The system that allows you to hear the most annoying sound in the world

Outline

Anatomy of the ear Sensory transduction

Cochlea Organ of Corti

Frequency tuning Auditory pathways

Localization of sound Auditory disorders

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Anatomy of the Ear3

B&B Figure 13-6

Anatomy of the Ear: External Ear

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folds of pinna provide cues about vertical location of sound gathers and focuses

sound energy on tympanic membrane

Anatomy of the Ear: Middle Ear5

amplifies vibrations of tympanic membrane to oval window matches low acoustic

impedance of air to high acoustic impedance of fluid of the inner ear

Eustachian tube

Anatomy of the Ear: Inner Ear6

connected to the middle ear via two openings: oval window (entrance)

& round window (exit) contains the cochlea

a coiled tube separated into three fluid filled partitions

location of the Organ of Corti, the sensory organ responsible for sound transduction

perilymph

endolymph

B&L Figure 8-17Eustachian tube

Cochlea – partially unravelledHelicotrema or apex

Perilymph ≈ CSFWhat is the composition of CSF compared to plasma?

Sensory Transduction: Overview

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1. Ossicles amplify sound wave from tympanic membrane to oval window

2. Creates pressure wave in cochlear fluid

3. Deforms basilar membrane causes round window to bulge out

4. Motion of basilar membrane is transduced into action potentials by hair cells in organ of Corti located along the basilar membrane

Cochlea: cross-section

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Reissner’s membrane separates scala media

from scala vestibuli Basilar membrane

separates scala media from scala tympani

Tectorial membrane attached along one edge

of the wall to scala media Organ of Corti

Located in scala media on top of basilar membrane

B&B Figure 13-9

Cochlea: Organ of Corti9

contains hair cells with sterocilia embedded into the tectorial membrane outer hair cells:

stereocilia are arranged in V-like structure in three parallel rows

inner hair cells: stereocilia are arranged linearly in single row

90% of afferents synapse on inner hair cells

As basilar membrane moves up/down with pressure waves, stereocilia of hair cells bend against tectorial membrane

Sensory Transduction: Hair cells

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Stereocilia coupled by “tip links” Upward displacement of basilar

membrane bends stereocilia towards kinocillium Opens mechanically gated non-

selective cation channels = depolarization

Excitatory neurotransmitter released = AP in afferent nerve fibre

Not all afferent fibres fire AP in response to particular sound frequency

B&B Figure 13-15

EndolymphHigh [K+]

PerilymphLow [K+]

+80mV

-60mV

0mV

Frequency Tuning of Basilar Membrane

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Due to geometry of basilar membrane Base: narrower & stiffer

Vibrates at higher frequencies

Apex: wider & more flexible Vibrates at lower

frequencies

Maximal firing of afferent fibers depends on location along basilar membrane

Frequency tuning: tonotopic mapping of frequency in the basilar membrane and organ

of Corti

Auditory Pathways12

Cochlear afferents synapse on cochlear nuclei in the medulla on the same side Most second order neurons cross over and synapse in the superior olivary nucleus

Receives input from both ears (binaural) Medial and lateral superior olive (MSO and LSO) important in computing location

of sound

Auditory Pathways13

Ascend via the lateral meniscus tract on ipsilateral side to the inferior colliculus in the midbrain

Fourth order neurons travel to the medial geniculate nucleus (MGN) in the thalamus Terminate in the primary auditory cortex in the temporal lobe Tonotopic organization maintained from cochlear nuclei to auditory cortex

Sound Localization14

two strategies to localize the horizontal position of sound sources, depending on the frequency frequencies above 3 kHz use interaural intensity differences

computed by neural circuitry in the lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB)

frequencies below 3 kHz use interaural time differences computed by neural circuitry in the medial superior olive (MSO)

B&B Figure 14-15

Interaural Intensity Differences: LSO and MNTB

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cochlear nucleus on same side as location of sound directly excites LSO

LSO excites inhibitory interneurons in contralateral MNTB

Contralateral LSO inhibited Excitation/inhibition arrangement

sends maximal signal to auditory cortex on same side as sound

Computes interaural intensity difference

Interaural Time Differences: MSO

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MSO neurons are coincident detectors: respond only when excitatory signals arrive simultaneously Anatomical differences in connectivity

allow each MSO neuron to be sensitive to sound source from particular location

Auditory Cortex17

Primary auditory cortex corresponds to Broadmann’s area _____ and ______

Sends projections to auditory association area Discrimination of sound patterns Wernicke’s area: language comprehension

Lesion to this area results in receptive aphasia

Wernicke’s area

Auditory Disorders18

Sensorineural hearing loss Dysfunctions of the inner ear, vestibulocochlear nerve or

auditory cortex Most common cause is damage to hair cells. Two

examples include: Noise induced hearing loss

Caused by auditory trauma or long term exposure to loud sounds

Leads to structural damage or complete degeneration and loss of hair cells

Sensory presbycusis Age related hearing loss Hair cell damage caused by factors other than auditory

trauma Often occurs in both ears

WebCT readings: Sensorineural Hearing Loss

Objectives

After this lecture you should be able to: Describe the structure and function of the outer, middle,

and inner ear Relate the anatomical organization of the cochlea and

associated structures to sensory transduction of sound Explain how damage to these structures can cause hearing

loss Differentiate between the mechanisms for localization of

horizontal sound above and below 3kHz Outline the neuronal pathway from the cochlea to the

auditory cortex

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1. What is the first location in the auditory pathway that receives input from both ears?

2. What does tonotopic organization mean?3. Endolymph closely resembles the ionic composition of

intra or extracellular fluid of a typical neuron?

Test your knowledge