HEARING AND SENSE OF BALANCE - KTH · 2015-04-16 · 1 HEARING AND SENSE OF BALANCE Peter Århem...
Transcript of HEARING AND SENSE OF BALANCE - KTH · 2015-04-16 · 1 HEARING AND SENSE OF BALANCE Peter Århem...
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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?