Fluid of inner ear

Fluid of inner ear


FLUID OF INNER EAR

ENDOLYMPH:

  • Endolymph is the fluid contained in the membranous labyrinth of the inner ear. 
  • It is also called Scarpa’s fluid, after Antonio Scarpa.

PERILYMPH:

  • 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

STRUCTURE:

  • 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

COMPOSITION:

  • 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.

FUNCTIONS:

  • 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.
Exam Question
 
  • 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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