AUDITION auditory pathway endococlear potential

Audition

1. •These tip links are attached with
mechanosensitive cation
channels
•They get open up when the
shorter cilia moves towards the
stereocilia.
•When stereocilia is pushed
towards the higher neighboring
ones the ion channel get open
up causing entry of K
+ (mainly)
and Ca
2+

2. •Upward displacement bends
the stereocilia toward the
tallest cilium, which leads to K+
influx through K+ channels and
depolarization of the hair cells;
•downward deflection bends the
stereocilia in the opposite
direction, which closes K+
channels and hyperpolarization
of the hair cell

4. •Very fine processes called tip links tie the tip of each stereocilium to
the side of its higher neighbor, and mechanically sensitive cation
channels are at the junction in the taller process.
•When the shorter stereocilia are pushed toward the taller ones, the
channel open time is increased.
• K+, the most abundant cation in endolymph, and Ca2+ enter via the
channel and induce depolarization.
•A myosin-based molecular motor in the taller neighbor then moves
the channel toward the base, releasing tension in the tip link.
•This causes the channel to close and restores the resting state.
•Depolarization of hair cells causes them to release a neurotransmitter,
probably glutamate, which initiates depolarization of neighboring
afferent neurons.

5. •Whentheorganofcorti
movesup, tectorial
membranemoves forwards
relative to basilar
membrane.
•This bends stereocilia towards
highestcilia---produces
depolarization.
•Bending of stereocilia away
from highest cilia produces
hyper polarization.

6. •TheK+thatentershaircellsarerecycled
•It enters supporting cells and then passes on toother
supportingcellsbywayofgap junctions.
•Inthecochlea,iteventuallyreachesthestria vascularis
andissecretedbackintothe endolymph,completing
thecycle.
WhathappenstoK+?

7. Auditorypathway

8. •Receptors–haircells
•Innervatedbytheauditory
portionof8
th cranialnerve
whosecellbodiesarepresent
in thespiralganglionlocatedin
themodiolus.

9. Spiralganglion→CochleardivisionofVIIInerve
↓
Dorsal&ventralcochlearnuclei
↓
Superiorolivarynucleus(contralateral)inpons
↓
Lateralleminiscus
↓
Inferiorcolliculusinmidbrain
↓
Medialgeniculatebodyinthalamus
↓
Auditoryradiations→Primary
auditorycortex

11. •Axons of spiral ganglion of
internal ear (that innervate
thehaircells)formcochlear
(auditory) divisionofVIII
nerve.
•Auditorynerveentersthe
medulla,endsin ventral&
dorsalcochlearnuclei.First
synapseoccurshere.
•Secondorderneuronswhich
arisein cochlearnuclei end
variouslyinsuperior olivary
nucleus, nucleus of lateral
leminiscus&theinferior colliculi.

12. •Third order neurons arise in these
nuclei & end in medial geniculate
body(thalamus).
•Some fibres also send collaterals
to reticular formation of both
sides.

13. •Frommedialgeniculatebodyanotherset
ofneurons start&theirprocessesvia
auditory radiation go to thecerebral
cortexSuperior temporal gyrus (area
41,42).
•Herenerveimpulsesperceivedas
sound.
•Auditory informationsuchas
loudness,pitch, source & directionare
perceived.

14. Featuresofauditorypathway
•Bilateralrepresentation
•Descendingpathway:olivo-cochlearbundle-outer haircells
•Roleingeneralarousal-reticularformation
•Spatialorganization/Tonotopicorganization-from cochleato
auditorycortexthe fibers are tone/pitch specific

15. •Air/Ossicularconduction
•Mainpathway
•ViaTM→auditoryossicles→fluidininnerear
•Boneconduction:
•Transmissionofvibration ofbonesofskull→fluid in
innerear
Transmissionofsound

16. •Because the inner ear, the cochlea, is embedded in a bony cavity in
the temporal bone, called the bony labyrinth, vibrations of the entire
skull can cause fluid vibrations in the cochlea.
•Therefore, under appropriate conditions, a tuning fork or an
electronic vibrator placed on any bony protuberance of the skull, but
especially on the mastoid process near the ear, causes the person to
hear the sound.
•However, the energy available even in loud sound in the air is not
sufficient to cause hearing via bone conduction unless a special
electromechanical sound-amplifying device is applied to the bone.

17. •Sound entering to the inner ear through oval window
spreadalongthescala vestibuliasatravellingwave.
•Most of the sound energy passesfromscalavestibuli to
endolymph.
•Thiswavecausesvibration ofbasilarmembrane.
•Thiscausesvibrationof organofcorti.
Vibrationofbasilarmembrane

18. Cochlearmicrophonics
•Thesumofreceptor potentialsofthehaircells(outer hair
cells)recordedextracellularly.
•Aresimilartogeneratorpotential.
•Thiscanberecordedbyplacingandelectrodein the scala
mediaandtheotherelectrodein scalatympani.
•Itformsanoscillatorywavepattern.
•Themicrophonicpotentialsrecordedarehavingsame polarity
ofacousticstimulus(cochlear microphonics)

19. Cochlear microphonics
•Cochlea can function as a microphone.
•Keep an active electrode on the cochlea and an indifferent electrode
on some other part of the ear and connect these electrodes to an
audio amplifier.
•The cochlear microphonic potentials recorded have the same
frequency and intensity of the sound fed into the ears during
recording.
•The source of the cochlear microphonic potentials is the Outer hair
cells.

20. •These potentials are not action potentials.
•The cochlear microphonic potential is resistant to ischemia and
anesthesia and it shows no latency or refractory period and does not
obey all-or-none law.
•Thus these potentials are similar to the generator potentials in the
hair cells.
•Degeneration of organ of Corti abolishes this potential.

21. Theoriesofhearing/pitchdiscrimination
•Travellingwavetheory
•/PlaceTheory/VonBekesy
•Volleytheory
•Telephonetheory
•Resonancetheory

22. •Sound→distortionof Basilarmem.
•Siteofmax distortion dependsonfrequency
•Highpitch→nearbase
•Lowpitch→nearapex
Travellingwavetheory

23. •High-frequency sound wave travels
only a short distance along the
basilar membrane before it reaches
its resonant point and dies
•Medium-frequency sound wave
travels about halfway and then dies
•Very low-frequency sound wave
travels the entire distance along the
membrane.

24. •Another feature of the traveling wave is that it travels fast along the initial
portion of the basilar membrane but becomes progressively slower as it
goes farther into the cochlea.
•The cause of this difference is the high coefficient of elasticity of the basilar
fibers near the oval window and a progressively decreasing coefficient
farther along the membrane.
•This rapid initial transmission of the wave allows the high-frequency
sounds to travel far enough into the cochlea to spread out and separate
from one another on the basilar membrane.
•Without this rapid initial transmission, all the high-frequency waves would
be bunched together within the first millimeter or so of the basilar
membrane, and their frequencies could not be discriminated

25. DETERMINATION OF SOUND FREQUENCY—THE
“PLACE” PRINCIPLE
•low-frequency sounds cause maximal activation of the basilar membrane near
the apex of the cochlea, and high-frequency sounds activate the basilar
membrane near the base of the cochlea. Intermediate-frequency sounds activate
the membrane at intermediate distances between the two extremes.
•There is spatial organization of the nerve fibers in the cochlear pathway, all the
way from the cochlea to the cerebral cortex.
•Recording of signals in the auditory tracts of the brain stem and in the auditory
receptive fields of the cerebral cortex shows that specific brain neurons are
activated by specific sound frequencies.
•Therefore, the major method used by the nervous system to detect different
sound frequencies is to determine the positions along the basilar membrane that
are stimulated the most, called the place principle for the determination of sound
frequency.

26. •Therefore, the major method
used by the nervous system to
detect different sound
frequencies is to determine the
positions along the basilar
membrane that are stimulated
the most, called the place
principle for the determination
of sound frequency

27. •The distal end of the basilar membrane at the helicotrema is stimulated by all sound
frequencies below 200 cycles/sec.
•Therefore, it has been difficult to understand from the place principle how one can
differentiate between low sound frequencies in the range of 200 down to 20 cycles/sec.
These low frequencies have been postulated to be discriminated mainly by the so-called
volley or frequency principle. That is, low-frequency sounds, from 20 to 1500 to 2000
cycles/ sec, can cause volleys of nerve impulses synchronized at the same frequencies,
and these volleys are transmitted by the cochlear nerve into the cochlear nuclei of the
brain.
•It is further suggested that the cochlear nuclei can distinguish the different frequencies
of the volleys. In fact, destruction of the entire apical half of the cochlea, which destroys
the basilar membrane where all lower frequency sounds are normally detected, does not
totally eliminate discrimination of the lower frequency sounds.

29. Deafness
•Clinical deafness may be
due to impaired sound
transmissionin the
external or middle ear
(conduction deafness)
•Ortodamagetothehair
cells or neural
pathways(nerve
deafness).
Degree of
hearingloss
Hearingthreshold
0notsignificant0-25dB
1mild 26-40dB
2moderate 41-55dB
3Moderately
severe
56-70dB
4severe 71-91dB
5profound Above91 dB

30. Conductivehearingloss
•Anydiseaseprocesswhichinterfere withthe conductionofsound
fromexternaleartocochlea causesconductionhearingloss.
•Causes-
❖Externalear: obstructionintheearcanal Eg:foreignbody,
wax,tumors
❖Tympanicmembrane–perforation
❖Middleearcavity-fluid in middle ear(otitismedia)
❖Otosclerosis
❖Eustachiantubeobstruction.