Endolymph is the fluid contained in the

membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses of equilibrium and hearing. It is lodged within the bony labyrinth in the inner ear and has the same general form; it is, howev ...

of the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...

. The major

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

in endolymph is

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

, with the values of

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

and

concentration in the endolymph being 0.91 mM and 154 mM, respectively. It is also called ''Scarpa's fluid'', after

Antonio Scarpa Antonio Scarpa (9 May 1752 – 31 October 1832) was an Italian anatomist and professor. Biography Scarpa was born to an impoverished family in the frazione of Lorenzaga, Motta di Livenza, Veneto. An uncle, who was a member of the priesthood, gav ...

Structure

The

has two parts: the bony labyrinth and the

. The membranous labyrinth is contained within the bony labyrinth, and within the membranous labyrinth is a fluid called endolymph. Between the outer wall of the membranous labyrinth and the wall of the bony labyrinth is the location of perilymph.

Composition

Perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major ...

and endolymph have unique ionic compositions suited to their functions in regulating electrochemical impulses of hair cells. The electric potential of endolymph is ~80-90 mV more positive than perilymph due to a higher concentration of K compared to Na. The main component of this unique extracellular fluid is

, which is secreted from the

stria vascularis The stria vascularis of the cochlear duct is a capillary loop in the upper portion of the spiral ligament (the outer wall of the cochlear duct). It produces endolymph for the scala media in the cochlea. Structure The stria vascularis is part o ...

. The high potassium content of the endolymph means that potassium, not sodium, is carried as the de-polarizing electric current in the hair cells. This is known as the mechano-electric transduction (MET) current. Endolymph has a high positive potential (80–120 mV in the cochlea), relative to other nearby fluids such as perilymph, due to its high concentration of positively charged ions. It is mainly this electrical potential difference that allows potassium ions to flow into the hair cells during mechanical stimulation of the hair bundle. Because the hair cells are at a negative potential of about -50 mV, the potential difference from endolymph to hair cell is on the order of 150 mV, which is the largest electrical potential difference found in the body.

Function

* Hearing:

Cochlear duct The cochlear duct (bounded by the scala media) is an endolymph filled cavity inside the cochlea, located between the tympanic duct and the vestibular duct, separated by the basilar membrane and the vestibular membrane (Reissner's membrane) resp ...

: fluid waves in the endolymph of the cochlear duct stimulate the receptor cells, which in turn translate their movement into nerve impulses that the brain perceives as sound. * Balance:

Semicircular canals The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The ...

: angular acceleration of the endolymph in the semicircular canals stimulate the vestibular receptors of the endolymph. The semicircular canals of both inner ears act in concert to coordinate balance.

Clinical significance

Disruption of the endolymph due to jerky movements (like spinning around or driving over bumps while riding in a car) can cause

motion sickness Motion sickness occurs due to a difference between actual and expected motion. Symptoms commonly include nausea, vomiting, cold sweat, headache, dizziness, tiredness, loss of appetite, and increased salivation. Complications may rarely include de ...

. A condition where the volume of the endolymph is greatly enlarged is called endolymphatic hydrops and has been linked to

Ménière's disease Ménière's disease (MD) is a disease of the inner ear that is characterized by potentially severe and incapacitating episodes of vertigo, tinnitus, hearing loss, and a feeling of fullness in the ear. Typically, only one ear is affected initi ...

Additional images

File:Vestibular system's semicircular canal- a cross-section.jpg, Inner ear illustration showing semicircular canal, hair cells, ampulla, cupula, vestibular nerve, & fluid

References

External links

* https://web.archive.org/web/20051030092447/http://oto.wustl.edu/cochlea/res1.htm Longitudinal Flow of Endolymph at wustl.edu {{Authority control Human head and neck Ear Body fluids