Cochlear fluids

KUNNAMPALLIL GEJO JOHN,

BASLP,MASLP

AUDIOLOGIST KUNNAMPALLIL GEJO

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

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

ineffective. KUNNAMPALLIL GEJO

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.

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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).

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