cochlear fluids
DESCRIPTION
Cochlear FluidsTRANSCRIPT
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Cochlear fluids
KUNNAMPALLIL GEJO JOHN,
BASLP,MASLP
AUDIOLOGIST KUNNAMPALLIL GEJO
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To be Discussed
Origin
Composition
Absorption
Dynamics
Functions
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Inner ear
1. Bony labyrinth: Intricate series of
interconnecting fluid filled tubes.
2. Membranous labyrinth: series of
membranous structures suspended within
the chambers of bony labyrinth.
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The fluid within the membranous labyrinth
is termed as Endolymph.
The fluid that surrounds the membranous
structures is termed as Perilymph.
The endolymph and perilymph
Differ in composition
Play a vital role in physiology of the inner
ear.
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Cochlea
Scala Tympani (ST)
Scala Media (SM)
Scala Vestibuli (SV)
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Points to be noted 1.ST and SV both contain perilymph and they
have openings to the middle ear cavity which
are closed by the round window membrane
and the footplate of the stapes.
2. In the apical turn, the SV and ST are joined
through an opening called the Helicotrema.
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3. In the basal turn, the SV has a wide connection
with the perilymphatic space of the vestibule.
4. The perilymph of ST is connected to the CSF
of the sub arachnoid space by the cochlear
aqueduct.
5. The SM containing endolymph, is present
between the perilymph scalae.
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6. The cells surrounding the endolymphatic
compartment constitute an endolymph-
perilymph barrier.
7. In the basal turn,SM is joined by a narrow duct,
the ductus reuniens to the endolymphatic
compartment in the saccule.
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Scala Media Three major structures bounding endolymph:
1.Reissner‟s membrane
- avascular structure
- composed of two cell layers, forms boundary
between SM and SV.
2.Stria vascularis
- highly vascular structure, multilayered tissue
- forms the lateral wall of scala media
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3. Organ of Corti
- complex structure contains sensory hair
cells, supporting cells and basilar membrane.
- the hair cells with their apical surfaces are in
contact with endolymph and their baso lateral
membrane are in contact with fluid of
perilymph like composition.
- the sensitivity of transduction process
depends on the maintenance of this condition.
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Composition Fluid sampling and analysis techniques:
Glass micropipette (suction) method
Techniques for estimation of ionic concentration
Micro flame photometers or helium glow
photometers
Electrometric titration technique - for chloride
concentration
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Ion selective electrodes – by choice of ion
exchange resin, the electrode can be made
sensitive to K+, Na+, Cl-, H+ or Ca2+.
X- ray microanalysis in the scanning electron
microscope- estimates of elemental
composition of cochlear fluids.
Laser microprobe mass spectrography- to
compare intracellular ion concentrations in
different cell types.
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The ionic composition of the perilymph is similar to that of other extra cellular fluids like CSF.
It has a high Na+ content and low K+ content (Na+ is the predominant cation)
The Na+ content of perilymph of SV(mean 140.6 mM) is slightly lower than that of ST( mean 147.3 mM).
The K+ content of perilymph in SV (6.7mM) is higher than that of ST (mean 3.4 mM).
Therefore the perilymph composition is not homogenous throughout the cochlea.
The osmolarity is similar to that of blood plasma i.e perilymph normally close to osmotic equilibrium with blood.
Perilymph
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BICARBONATE
CHLORIDE
POTASSIUM
SODIUM
ST
PL SV
PL
EL CSF
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Endolymph It has unique ionic composition.
Has high K+ and low Na+ content
( predominant cation is K+)
Na+ concentration in the endolymph is
between 0.5 and 2.0 mM.
K+ concentration is between 150-165mM.
Cl- concentration is in the region of 125-
140mM.
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It is positively polarized by approximately +80mV,the Endocochlear Potential(EP)
(Smith, Lowry& Wu ’67)
The EP and the endolymph K+ concentration are lower in higher (apical) turns of the cochlea than in the basal turn.
(Sterkers et al ’69)
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Cortilymph
(Perilymph like fluid)
It is found in tunnel of corti (organ of corti).
Is rich in Na+ and poor in K+ but its
composition is different from that of perilymph
Potential of cortilymph is 0mV
Composition is similar to the composition of
perilymph in ST.
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ST Pl. SV Pl. CSF Coc.El. Sac. El. En.
Sac El.
Na+ 149 140 146 1 3 108
K+ 3.7 8 3.2 158 150 14
Ca2+ 0.7 0.6 1.2 0.02 0.09 0.47
Cl- 127 125 131 136 119 98
pH 7.28 7.26 7.28 7.37 - -
Elec.
pot
0 5 0 85 5 13 KUNNAMPALLIL GEJO
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Importance of composition
Provides the charge carrier and the ionic
milieu for the process of transduction.
Acts as a reservoir of metabolic substrates
for the surrounding tissues.
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ORIGIN AND ABSORPTION
Three main principles
Longitudinal flow: The fluids are secreted at a
site spatially separate from the site of resorption
creating a volume flow along the cochlea.
Radial flow: Fluids are secreted and absorbed
in all turns of the cochlea resulting in a radial
flow.
(secreted by RM and absorbed by stria
vascularis)
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Homeostasis: It is the physiological process in an organism that maintain relative stability of its internal environment.
Here its the composition of the fluids with out any volume production or volume flow taking place. That means there is no production or flow, it is homeostatic state.
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Perilymph Hypothesis 1
1.The perilymph is derived from CSF by a
longitudinal flow through the cochlear aqueduct.
This was speculated due to
To the existence of the cochlear aqueduct
The similarity of the ionic composition of
perilymph and CSF.
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Kaupp and Giebel : Perilymph marked with fluorescent rhodamine rapidly appears in the subarachnoid space i.e chemical entered CSF after it was introduced to perilymph.
Calborg and Farmer: Mixing of CSF and perilymph as a result of small cyclical volume movements accompanying pressure change during respiration.
Under normal physiologic conditions, there was no significant pressure difference between perilymph and CSF.
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Hypothesis 2:
1. Perilymph origins in the cochlea by an ultra
filtration mechanism.
2. Hawkins described capillaries in the SV of the
spiral ligament close to the attachment of RM,
which is believed could be site of ultra filtration. It
could also involve longitudinal perilymph flow.
3. If perilymph was resorbed in the lower spiral
ligament of ST near the basilar membrane, then a
volume flow from SV to ST through the
helicotrema could be expected.
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Hypothesis 3:
Perilymph appears to be maintained by local
mechanisms that do not necessarily involve perilymph
secretion at all.
John et al gave alternate mechanisms:
a) Active diffusion
b) Passive diffusion
b) Facilitated transport
d) Exchange across a blood labyrinth barrier comprised
of the pericytes, fibrocytes and endothelial cells
associated with the capillaries of the spiral ligament.
Through this barrier Na+, Cl- and Ca2+ enter into the
perilymph.
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Endolymph Hypothesis I:
1.Stria vascularis is responsible for the active transport
mechanism, which helps in maintaining EP and high
endolymphatic K+ concentration. It is the source of EP.
2. Sellick et al :Showed that EP originates, active secretions of
K+ into the endolymph. Speculated EP could be generated by
conventional Na+/K+ ATPase.
3.If cochlea becomes anoxic or is treated with K+ transport
inhibitors, the EP falls from its normal value of approximate
+85mv, endolymph K+ begins to fall and Na begins to rise.
endolymph equilibrates passively with perilymphatic
compartments.
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Hypothesis II:
The existence of endolymph flow was proposed by
Guild.
1.The existence of the ductus reunions connecting the
cochlear and saccular endolymph, suggests the potential
for longitudinal flow between these compartments.
2.He suggested endolymph was secreted in
cochlea,longitudinally flowed through the ductus
reunions and the saccule and absorbed in the
endolymphatic sac.
3.Variety of traces substance that was injected in the
cochlea, chemical reached endolymphatic sac.
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Hypothesis III:
Endolymphatic homeostasis may occur.If water
equilibrates passively, then endolymph composition
would be maintained without volume flow necessary
taking place. This process is called „local
homeostasis‟.
Hypothesis IV:
The difference in osmolarity between endolymph and
perilymph – leads to bulk flow. Suggests that the
resulting influx of water into endolymph could drive
its movements towards sac.
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Hypothesis V:
Naftalin & Harison (54) : The endolymph flowed
radially, secreted by Reissner‟s membrane and
resorbed by stria vascularis.
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Barrier between fluids 1. Anatomical barrier between endolymph and
perilymph consists of the epithelial lining the SM, the
saccule, the utricle, the three semi circular canals,
endolymphatic duct and endolymphatic sac.
2. Functional barrier for ions and organic compounds
but not for water.
It is a difference in ionic composition that requires
active transport for maintenance.
Glucose, proteins and most amino acids in
endolymph in much lower concentration than in
perilymph.
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It is compatible with selective transport processes operating between these two fluids.
Sterkers et al (82): The kinetics of distribution of traces between blood, CSF, perilymph and endolymph support this concept.
Thalmann,salt & de mott(88): In contrast the apparent water permeability of the endolymph-perilymph barrier remains unsolved since several different approaches did not yield unambiguous and quantitative results.
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Existence of both endolymph-perilymph barrier and
blood perilymph barrier suggest:
Perilymph as a source of endolymph.
1.Konishi,Hamrick et al `82: The compartmental
distribution of radio active tracers is consistent with a
K+ and Cl- exchange occurring between blood and
perilymph and subsequent between perilymph and
endolymph rather than directly between blood and
endolymph.
2. Marcus & Thalmann ’81: K+ free perfusion of
both perilymphatic scales causes a rapid decline of the
EP. Whereas K+ free perfusion of the vasculature is
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Volume of the inner ear fluids
Data for this estimation have been generated by
techniques like:
Serial histological sections
3-D magnetic resonance microscopy.
The total volume of all inner ear fluids spaces
including cochlear and vestibular portions is
approximately 204-228ml in humans and 21ml in
guinea pigs.
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Cochlear
endolymph
volume (μL)
Cochlear
perilymph
volume (μL)
Endolymphatic
sac & duct
volume(μL)
Humans 7.7 75.9 3.92
Guinea pig 1.6 12.1 0.12
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Compartments that influences fluid pressure include:
1. The cranium, via CSF pressure
2. Middle ear cavity
3. Arterial and venous blood pressure transmitted by vasculature of the ear.
Pressure of Inner ear fluids
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The cochlear aqueduct plays a central role in regulation of perilymph pressure.
Two conditions present:
In animals where the cochlear aqueduct is patent, the pressure of CSF that is transmitted through the cochlear aqueduct dominates perilymphatic pressure.
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This includes both the static pressure and the
pressure fluctuation in CSF associated with
respiration, heartbeat, posture changes, coughing
and sneezing.
The CSF pressure averages to 11mm Hg in
humans.
Characteristics of pressure transfer from CSF to
perilymph across the cochlear aqueduct vary in
frequency dependent manner.
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It has been calculated that aqueduct acts as a
low pass filter, attenuating CSF pressure
fluctuations entering the perilymph for
frequency above 20Hz.
Pressure fluctuations below this frequency and
sustained pressures are thought not to be
attenuated by the aqueduct.
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When the cochlear aqueduct is occluded respiration induced pressure fluctuation in perilymph are highly attenuated.
Experimentally induced CSF pressure changes result in smaller, delayed pressure response that is believed to be mediated by endolymphatic sac.
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Perilymph pressure- correlated with the arterial
blood pressure changes during manipulation of
systemic blood pressure.
Manipulation of middle pressure- results in larger
perilymphatic pressure changes that recover slowly
because of the diminished capacity of aqueduct to
shunt pressure changes to the CSF.
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The endolymphatic space (ELS)- incompressible, flexible walled compartment suspended within the perilymphatic space.
No hydrostatic pressure difference in the normal state.
Similar pressure fluctuation associated with respiration and induced pressure changes of CSF in both endolymph and perilymph.
- The static pressure differential across the basilar membrane, would decrease its compliance.
The minimization of pressure differences between endolymph and perilymph is likely to contribute to the maintenance of high cochlear sensitivity to mechanical stimuli.
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Both endolymph and perilymph pressure fell to zero when the round window was perforated.
- These studies confirm that perilymphatic pressure changes are transmitted to endolymph via mechanically compliant boundary membranes.
Such pressure changes occur without endolymph volume change or movements of the membranous walls because endolymph is a fluid and it is thus incompressible.
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Dynamics There are two viewpoints
I. Mass action mechanism: The movement of stapes is transmitted directly to the fluid column in the cochlea, which responds as a whole.
Inward movement of the stapes causes the perilymph to flow up the SV through the helicotrema and then down the ST.
The round window is pushed outward by an amount directly proportional to the inward movement of the stapes.
During the outward movement the direction of flow of the fluid column is reversed.
Sound energy transmitted by the vibrating fluid column is selectively abroad by the B.M.
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II. Alternative point is that the pressure generated in
the SV is transmitted across the SM to the ST.
Such a transmission of pressure result in distortion
of vestibular membrane (Reissner‟s membrane) and
in turn the BM.
There will be displacements of round window that
are out of phase with the direction of movement of
stapes.
The fluid movement may be distinctive for a
particular frequency- this produces the distortion of
BM at the specific frequency.
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Functions of cochlear fluids
1. Transport of dissolved gases and nutrients between
blood and many cell types of cochlea.
2. Transmission of acoustic vibrations from stapes to
sensory structures.
3. Provision of suitable ionic environment for sensory
hair cells, thus helping in physiological process of
cells.
4. Removal of waste products.
5. Provision of chemical environment needed for
transfer of energy from vibration (mechanical) to
neural (electrical) signals.
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ROLE OF FLUID IN TRANSDUCTION.
The Standing Current:
Von Bekesy (1950) : Described a +ve potential in
the endolymphatic space and a – ve potential inside
the organ of corti.
The presence of such potentials would drive a
circulating current.
This is the basis for Davis’s Mechano-electrical
“Battery theory of cochlear transduction” (1957).
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This theory postulates that acoustic stimulus itself
does not need to generate the energy for
transduction.
The energy provided by a standing current
flowing through the hair cells.
Transduction will only change the resistance of
the hair cells and there by the current flow
(Dallos `73).
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The Stria vascularis is believed to be solely
responsible for the generation of the standing
current.
1. It secretes K+ ions into endolymph and generates
the large potential across the epithelial lining the
scala media.
2. This trans epithelial potential of about +80mv is
the EP and drives the standing current in
conjunction with the –ve membrane potential of
the hair cells and steep gradient of K+ between
endolymph and perilymph.
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The standing current generated by Stria
vascularis flows radially through the SM
towards 2 current sinks.
One part of current flows through the organ of
corti containing the sensory hair cells where it
enters the ST
Other part flows through the RM where the
current crosses the SV.
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Zidanic and Brownel`90: Both branches of the current return radially via the spiral ligament to the stria vascularis of the stria vascularis.
The spatial secretion of the current source in the stria vascularis from the sensory cells by almost half a millimeter
It may be pre requisite for the high sensitivity of the auditory system.
This arrangement attenuates the noise originating from blood flow in the highly vascularised stria.
KUNNAMPALLIL GEJO
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Physiologic significance of fluid composition:
The specific ion concentration of endolymph and
perilymph maintain structure and function in the
cochlea.
Variations in endolymphatic K+, Na+ or Ca2+
affect the conformation of the tectorial membrane
(Kron ester frel,1979).
KUNNAMPALLIL GEJO
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KUNNAMPALLIL GEJO
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KUNNAMPALLIL GEJO