- Endolymph is the fluid contained in the membranous labyrinth of the inner ear. It is also called Scarpa's fluid, after Antonio Scarpa.
- Perilymph (also known as Cotunnius liquid, and liquor cotunnii) is an extracellular fluid located within the cochlea (part of the inner ear) in two of its three compartments:
- the scala tympani and scala vestibuli.
- The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid.
- The major cation (positively charged ion) in perilymph is sodium: The values of sodium and potassium concentration in the perilymph are 138 mm and 6.9 mm, respectively
- The inner ear has two parts: the bony labyrinth and the membranous labyrinth.
- The membranous labyrinth is contained within the bony labyrinth, and within the membranous labyrinth in scala media 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.
- Endolymphatic duct:
- It is formed by the union of two ducts, one each from the saccule and the utricle.
- It passes through the vestibular aqueduct & drains into Subarachnoid space
- Its terminal part is dilated to form endolymphatic sac which lies between the two layers of dura on the posterior surface of the petrous bone
- Perilymph 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 of endolymph is potassium, which is secreted from the stria vascularis.
- 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
- Perilymph is the fluid contained within the bony labyrinth, surrounding and protecting the membranous labyrinth; perilymph resembles extracellular fluid in composition (sodium salts are the predominate positive electrolyte) and, via the perilymphatic duct, is in continuity with cerebrospinal fluid.
- Fluid waves occur in the endolymph in the various parts of the membranous labyrinth in response to fluid waves in the perilymph.
- Hearing: Cochlear duct: 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: 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.
- Endolymphatic duct drains into Subarachnoid space
- Endolymph in the inner ear Is secreted by Stria vascularis
- Scala media contains endolymph
- The function of stria vascularis is To maintain electric milieu of endolymph
- Endolymph is rich in K+
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