Endolymph

Endolymph (Scarpa's fluid) is the fluid contained in the membranous labyrinth of the inner ear.

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 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 electrical current in the hair cells. This is known as the mechano-electric transduction (MET) current.

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.