Auditory and Vestibular system (Biological Aspects 5)
- 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.
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
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
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.
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
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.
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
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
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.
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
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.
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
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.
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.
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
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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