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HEARING AND SENSE OF BALANCE

Peter Århem

Department of neuroscience

Karolinska institutet

Characteristics of sound - loudness Loudness is usually given as sound pressure level (Lp) which is a logarithmic measure of the pressure relative pressure at the human audibility limit = 20 micropascal (Pa). Dimension is usually decibel (dB) Lp = 20 log(p/p0) dB where p0 = 20 Pa Human range: 0-120 dB (1 000 000 times)

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Characteristics of sound - pitch

Frequency ranges (measured in Hz): Human 20 - 20 000 Dog 40 - 60 000 Mouse 1000 - 70 0000 Bat 15 000 - 90 000 Dolphin 75 - 150 000

THE EAR

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Outer, middle and inner ear

Middle ear

Air filled cavity in the temporal bone. Transmits vibrations of the tympanic membrane to the inner ear. Amplification mechanism

1.Tympanic membrane larger area than oval window

2. Lever effect of ossicles; hammer (malleus), anvil (incus) and stirrup (stapes)

Attenuation mechanism Protects cochlea from loud sounds Contraction of m. stapedius and m. tensor tympani (the acoustic reflex)

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Inner ear

The inner ear – both hearing and vestibular function

The inner ear membrane labyrinth

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Inner ear

Liquid-filled, complex cavity in temporal bone. Hearing part = cochlea. Three rooms, scala vestibuli (upper), scala media (middle) and scala tympani (lower). Endolymph (high [K+]) in scala media, and perilymph (high [Na+]) in scala vestibuli and tympani. Transduction of mechanical vibrations to electrical energy: vibrations of oval window vibrations of basilar membrane activation of hair cells on basilar membrane in organ of Corti.

Cross section of the cochlea

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Organ of Corti

Function of the organ of Corti

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Von Bekesy’s idea - frequency as labeled line code

Cochlear implant

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When mechanosensitive channels on stereocilia activate • inflow of K+ • depolarization • Ca channel activation • neurotransmitter release

Hair cell transduction mechanism

Hair cell transduction mechanism

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Electric potential conditions in the cochlea

Mechanism of hair cell receptor potential

Apical part of hair cells (-45 mV) surrounded by endolymph (+80 mV) membrane potential of -125 mV.

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CENTRAL PATHWAYS

Auditory pathways

• Hair cells in cochlea • spiral ganglion

• cochlear nuclei in medulla • superior olive in pons • inferior colliculus in midbrain

• medial geniculate nucleus (thalamus) • auditory cortex (A1; area 41 and 42)

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Auditory pathways in the brain stem

Function of superior olive – sound localization

• Lateral part (LSO) – interaural intensity difference (at high frequencies)

• Medial part (MSO) – interaural time difference (at low frequencies)

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Medial superior olive -

estimating interaural time difference

Auditory pathways in thalamus and cortex

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Primary auditory cortex (A1) is tonotopically

organized

Higher order auditory areas

(secondary and Wernicke’s area)

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Questions (hearing)

1. Normal auditory frequency range?

2. The three ossicles?

3. Where do we find auditory hair cells?

4. Two types of hair cells in the cochlea?

5. Which cochlear hair cells are most sensitive to low-frequent sound?

6. Where is sound localization processed in the auditory system? 7. Two principal ways to localize sound?

8. Auditory thalamic nucleus?

9. Where is auditory cortex localized?

THE VESTIBULAR SYSTEM

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Figure 14.1 The labyrinth and its innervation The vestibular system is part of the inner

ear

The vestibular system consists of • two otolith organs (utriculus and

sacculus) and • three semicircular canals

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The hair cells of the otolith organs sense the linear acceleration (gravitational force) due to their organization and the otolithic membrane

Figure 14.7 The ampulla of the posterior semicircular canal The hair cells of the semicircular canals sense rotational movements of the head

due to the geometry of the canals

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Summary

Two otolith organs (utriculus and sacculus) Hair cells on macula; direction sensitive. Sensitive to

• position of head in gravitational field • linear acceleration

Three semicircular canals

Hair cells on crista in ampulla; direction sensitive. Sensitive to

• head rotation - angular acceleration

Hair cell transduction mechanism

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Summary on transduction mechanism of vestibular hair cells

• K channel activation of apical stereocilia • inflow of K+ • depolarization • Ca channel activation • neurotransmitter release

CENTRAL PATHWAYS

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Vestibular pathways

• Hair cells in otholith organs and semicircular canals

• Scarpa’s ganglion • vestibular nucleus in medulla/pons (four

major nuclei)

• VPM (thalamus) • somatosensory cortex (S1/area 3a and

neighbouring regions)

Different functions of vestibular system – key point: vestibular nucleus

• Stabilizing gaze (the vestibulo-ocular reflex, VOR): Vestibular nucleus eye muscle nuclei

• Stabilizing posture (integrating afferent signals from cerebellum): Vestibular nucleus spinal motoneurons

• Informing cortex: Vestibular nucleus VPM (thalamus) somatosensory cortex (SI/3a and neighbouring regions)

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Stabilizing gaze (vestibulo-ocular reflex, VOR) – eye muscle nuclei

Nystagmus Nystagmus – rhythmic eye movements with a slow and a fast phase. Fast phase defines direction. Physiological and pathological.

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Figure 14.11 Pathways mediating the VCR and VSR

Stabilizing posture (vestibulo-spinal reflex, VSR) – spinal motorneurons

Figure 14.12 Thalamocortical pathways carrying vestibular information Informing cortex - S1 and posterior parietal cortex

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Questions (vestibular system):

1. The two vestibular organs? 2. Main function of the two vestibular organs?

3. Vestibular thalamic nuclei?

4. Which component in the vestibular system coordinates head and eye movements and where is it located?

5. What is VOR? 6. The primary cortical vestibular area?