Auditory and Vestibular system (Biological Aspects 5)

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  • Created by: CanveySam
  • Created on: 30-04-15 13:17

Sound

Sound

Sound is the alternation of compression and rarefaction of air.

The distance between the pressure peaks is called the wavelength.

Humans can detect sound in the frequency range 20 to 20,000Hz (Hz = cycles, or waves, per second).

 The frequency determines the pitch of the sound.

 

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Biology of Ear

mpanicmembraneossicles fluid-filled choclea (through oval window)

Air compression mechanical vibrations electrical potentials 

Tympanic membrane - to ossicles  - to fluid-filled choclea (through oval window)

Air compression  - then mechanical vibrations  - then electrical potentials 

Air compression mechanical vibrations electrical potentials compression mechanical vibrations electrical potentialsossicles fluid-fill   The cochlea traduces sound waves into electrical potentials 

The pressure wave initiates the motion of the basilar membrane and passes to the Organ of Corti 

The pressure wave initiates the motion of the basilar membrane and passes to the Organ of Corti 

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Frequency selectivity

The FREQUENCY SELECTIVITYof sounds is

a fundamental role of the basilar membrane.

Each place along the basilar membrane is more sensitive to a characteristic frequency (where it will vibrate strongly to a pure tone close to its characteristic frequency).

As the characteristic frequency of a particular sound varies from the characteristic frequency, the response diminishes

Oval window (BASE)  ====Basilar membrane======Helicotrema (APEX)

Narrow & thick====Basilar membrane====Wide & thin

High ===========Frequency/Pitch=========Low 

However, consider the non-linearity of basilar membrane vibrationThe FREQUENCY SELECTIVITY
of sounds is a fundamental role of the basilar membrane.

Each place along the basilar membrane is more sensitive to a characteristic frequency (where it will vibrate strongly to a pure tone close to its characteristic frequency). As the characteristic frequency of a particular sound varies from the characteristic frequency, the response diminishes

However, consider the non-linearity of basilar membrane vibration 

Oval window (BASE) ====Basilar membrane======Helicotrema APEX

Narrow & thick====Basilar membrane====Wide & thin

High  ===========Frequency/Pitch=========Low 

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Frequency and amplitude

The AMPLITUDE determines the extend of the vibration.

Loud sounds affect wider portion of the basilar membranemore neurons

The amplitude ofhe wave increases slowly and decreases abruptlyThe AMPLITUDE dete   The amplitude of the wave increases slowly and decreases abruptly

 nds affect wider portion of the basilarembranemore neuronThe amplitude of the wave increases slowly and decreases abruptly 

Our ability to understand that two people are talking at the same time or that two sounds are occurring at the same time depends on the ability of the basilar membrane to perform the frequency selection

This means that the basilar membrane separates out the different frequency components in a complex sound. 

Thus

Frequency determines WHERE the basilar membrane responds more intensively
Amplitude determines how WIDE is the portion of membrane responding to a sound

Multiple sounds:

Two tones with frequency very differenttwo separate patterns of vibrations

Two tones with similar frequencysome point on the basilar membrane will be responding to both of the component tones

Two tones with very close frequency one pattern of vibration. 

The Organ of Corti is sitting on the basilar membrane 

Thus Frequency determines WHERE the basilar membrane responds more intensively
Amplitude determines how WIDE is the portion of membrane responding to a sound

Multiple sounds:

Two tones with frequency very differenttwo separate patterns of vibrations

Two tones with similar frequencysome point on the basilar membrane will be responding to both of the component tones

Two tones with very close frequency one pattern of vibration. 

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Tectorial membrane

The hairs of the hair cell project into the tectorial membrane.

Tectorial membrane does not move to the same degree of the basilar membrane:

shearing motion on the hairs ... change in potential (de- or iper-polarization)  ... auditory nerve

...........

Stereocilia of internal and external hair cells 

sheof the basilar membraneshearing motion on the hairschange in potential (de- or iper-polarization) auditory nerve

 of the basilar membraneshearing motion on the hairschange in

 potential (de- or iper-polarization) auditory nerve

 

of the basilar membraneshearing motion on the hairschange in potential (de- or iper-polarization) auditory nervemembrane.

Tectorial membrane does not move to the same degree of the basilar membrane

shearing motion on the hairs

change in potential (de- or iper-polarization)

auditory nerve 

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Hair cells

HAIR CELL

If stereocilia moves
towards the kinocilium ... increment of firing rate... depolarization ... excitatory effect

 

aways from the kinocilium ...decrement of firing rate ...iperpolarization... inhibitory effect 

Depolarisation of hair cells

Stereocilia move towards the kinocilium:


mechanical opening of ion channel
Positive ions (K+) enter into the cell
Voltage depended ion channels open and other positive ions enter into the cell DEPOLARISATION

Ion channels close
Positive ions are pumped outside the cell. 

If stereocilia moves:

towards the kinocilium:

increment of firing ratedepolarization excitatory effect 

away from the kinocilium:

decrement of firing rateiperpolarization inhibitory effect

Depolarisation of hair cells

Stereocilia move towards the kinocilium
mechanical opening of ion channel
Positive ions (K+) enter into the cell
Voltage depended ion channels open and other positive ions enter into the cell DEPOLARISATION

Ion channels close

Positive ions are pumped outside the cell. 

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Hair cells: Mechanical and electrical resonances

MechaniMechanical resonance:l resonance

Short and rigid towards the base (higher frequencies)

Stereocilia:

Large and flexible toward the apex (lower frequencies)

Electrical resonance:

Membrane potential fluctuations: different in different part of the basilar membrane

...................

Since each inner cell is responding to vibration at a single place of the basilar membrane

each inner cells is responding best to a characteristic frequency

Since each inner cell is innervated by a single neuron (of the 30,000) of the auditory nerve

each neuron is responding to a characteristic frequency

page5image4896 page5image5320

therefore the spatial mapping of frequency along the basilar membrane is now converted into a spatial mapping of frequency in the auditory nerve

Information about sound frequency is maintained at auditory nerve level 

Membrane potential fluctuations: different in different part of the basilar membrane 

......................

Since each inner cell is responding to vibration at a single place of the basilar membrane, each inner cells is responding best to a characteristic frequency 

and

Since each inner cell is innervated by a single neuron (of the 30,000) of the auditory nerve, each neuron is responding to a characteristic frequency 

therefore the spatial mapping of frequency along the basilar membrane is now converted into a spatial mapping of frequency in the auditory nerve

Information about sound frequency is maintained at auditory nerve level 

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Tonotopic representation

Auditory nerve...

cochlear nuclei (decussate)...

Inferior colliculus (through lateral lemniscus)...

Thalamus (medial geniculate nucleus) ...

auditory cortex (BA 41-42) 

From medial geniculate nuclei ... to  IV layer primary auditory cortex

From auditory cortex (V layer ) ... to medial geniculate nuclei

From auditory cortex (VI layer) ... to Inferior colliculi

Primary auditory area comprises BA 41 and 42 

Tonotopic representation

Basal end (towards the oval window) of the basilar membrane is represented most medially in the cortex; whereas apical end isrepresented most laterally in the cortex

Correspondence between location of a neuron and the specific frequency, e.g. a region of the auditory cortex responds to low frequencies whereas another region responds to higher frequencies. 

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Cortical representation

Bilateral cortical representation

Lesions to the central auditory pathway and to the auditory cortex do not give rise to monoaural deficits or compromise auditory perception dramatically. 

Primary auditory cortex is organised in perpendicular columns:

Summation columns and Supression columns:

 

Neurons in SUMMATION columns respond strongly to the stimulation of either ear.

Neurons in SUPPRESSION columns are exited by stimulation of one ear but inhibited by stimulation of the other ear (dominance)

Summation and suppression columns are alternated

and are the basis of ear dominance 

Lesions to the central auditory pathway and to the auditory cortex do not give rise to monoaural deficits or compromise auditory perception dramatically. 

Primary auditory cortex is organised in perpendicular columns: Summation columns and Suppression columns

Neurons in SUMMATION columns respond strongly to the stimulation of either ear.

Neurons in SUPPRESSION columns are exited by stimulation of one ear but inhibited by stimulation of the other ear (dominance)

Summation and suppression columns are alternated and are the basis of ear dominance 

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Auditory fovea

Auditory foImportant sound stimuli are overrepresented

i.e. larger areas to process these stimuliAuditory fovea

Important sound stimuli are overrepresented i.e. larger areas to process these stimuli

In human, stimuli related to speech are overrepresented and lateralised

Musical stimuli are overrepresented on the right belt area

Note that emotional aspects of speech, as well as prosody, are more linked to right hemisphere activity. 

 

overrepresented and lateralised and lateralised 

Musical stimuli are overrepresented on the right belt area

Note that emotional aspects of speech, as well as prosody, are more linked to right hemisphere activity. 

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Descending pathway

Descending pathway

from x to hair cells:

Descending pathway

from cortex to hair cells

Auditory cortex ...

  (V layer) medial geniculate nucleus...

(VI layer) inferior colliculus...

cochlear nuclei (olivary- cochlear path)...

outer hair cells

Function of the descending pathway is not clear.

However, it seems that it may modulate (reducing) sensitivity of the basilar membrane to low sound and sensitivity to frequency selectivity

In turn, modulate attention 

In turn, modulate attentionAuditory cortex

(V layer) medial geniculate nucleus

(VI layer) inferior colliculus

cochlear nuclei (olivary- cochlear path)

outer hair cells

Function of the descending pathway is not clear.

However, it seems that it may modulate (reducing) sensitivity of the basilar membrane to low sound and sensitivity to frequency selectivity

In turn, modulate attention 

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Sound localisation

SOUND LOCALIZATION
Interaural time differences: a sound coming from the right will 
reach the right ear first.

Interaural time differences: a sound coming from the right will reach the right ear first.

Interaural level difference: a sound coming from the right is more intense in the right ear

Interaural time differences: a sound coming from the right will reach the right ear first. 

Interaural level difference: a sound coming from the right is more intense in the right earInteraural level difference: a sound Lateral Superior Olive (LSO) and Medial Nuclei of Trapezoid Body (MNTB) contain cells sensitive to differences in sound intensity.

 coming from the right is more intense in the right ear

Interaural time differences: a sound coming from the right will reach the right ear first.

Olivary complex (medial-superior) contains cells sensitive to differences in the time of arrival of auditory stimuli to the two ears.

Lateral Superior Olive (LSO) and Medial Nuclei of Trapezoid Body (MNTB) contain cells sensitive to differences in sound intensity. 

Lateral Superior Olive (LSO) and Medial Nuclei of Trapezoid Body (MNTB) contain cells sensitive to differences in sound intensity. 

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Vestibular system 1

VESTIBULAR SYSTEM

The vestibular system detects the position and the motion of the body in space by integrating information from peripheral receptors located in the inner ear on either side of the head.

Two main components:

Vestibular sacs (saccule and utricle) and the Semicircular canals

Receptive tissue with hair cells is placed in the ampulla of the semicircular canals and in the vestibular sacs 

Vestibular sacs and semicircular canals respond to acceleration of the head, or acceleration due to gravity.

The three semicircular canals detect head angular acceleration in the three direction - ROTATION-

PUSH-PULL SYSTEM

The three semicanals works in pair.

When one canal is stimulated, the other side is inhibited. 

Vestibulo-ocular reflex: Signals from the semicircular canals are sent as directly as possible (~10ms) to the eye muscles. 

Two main components:

Vestibular sacs (saccule and utricle) and the Semicircular canals

Receptive tissue with hair cells is placed in the ampulla of the semicircular canals and in the vestibular sacs 

Vestibular sacs and semicircular canals respond to acceleration of the head, or acceleration due to gravity.

The three semicircular canals detect head angular acceleration in the three direction - ROTATION- 

PUSH-PULL SYSTEM

The three semicanals works in pair.
When one canal is stimulated, the other side is inhibited. 

Vestibulo-ocular reflex:

Signals from the semicircular canals are sent as directly as possible (~10ms) to the eye muscles. 

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Vestibular system 2: Three-neuron arc

Three-neuron arc

Information from the right semicircular canal travels :

through the Vestibulocochlear nerve (VIII) ...

Vestibular nucleus (ipsilateral) ....

Abducens nucleus (contralateral - VI) ...

Oculomotor nucleus (III) ...

innervate the left lateral rectus ...
and the right medial rectus muscles of the eyes

Information from the left semicircular canal travels through the same path
BUT it triggers inhibitory process

All this result in a contraction of eyes muscles responsible for turning the eyes to the left 

through the Vestibulocochlear nerve (VIII)

Vestibular nucleus (ipsilateral)

Abducens nucleus (contralateral - VI)

Oculomotor nucleus (III)

innervate the left lateral rectus and the right medial rectus muscles of the eyes

Information from the left semicircular canal travels through the same path

BUT it triggers inhibitory process

All this result in a contraction of eyes muscles responsible for turning the eyes to the left  page9image8656 page9image8816

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Vestibular system 3

Vestibular sacs detect LINEAR ACCELLERATION of head

Otoliths

calcium carbonate crystals contained in the Utricle and Saccule.

They get displaced during linear acceleration, and then deflect hair cells, producing a sensory signal.

In general,
Utricle ....   forward and backward motion of the head .... eye movements

Saccule ... vertical motion of the head ... muscles for posture control. 

Information from receptors travels through the vestibulocochlear nerve,

which is a part of the auditory nerve, 

then information reaches vestibular nuclei in the medulla 

Sacculevertical motion of the headmuscles for posture control.Vestibular sacs detect LINEAR ACCELLERATION of head

Otoliths: calcium carbonate crystals contained in the Utricle and Saccule.

They get displaced during linear acceleration, and then deflect hair cells, producing a sensory signal.

In general,
Utricle  forward and backward motion of the head  eye movements

Saccule  vertical motion of the head  muscles for posture control. 

Information from receptors travels through the vestibulocochlear nerve, which is a part of the auditory nerve then information reaches vestibular nuclei in the medulla 

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