Perilymph Fluid

 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 cation in perilymph is sodium, with the values of sodium and potassium concentration in the perilymph being 138 mM and 6.9 mM, respectively.^[1] It is also named Cotunnius' liquid and liquor cotunnii for Domenico Cotugno.

Composition[edit]

Perilymph and endolymph have unique ionic compositions suited to their functions in regulating electrochemical impulses of hair cells necessary for hearing. The electric potential of endolymph is ~80-90 mV more positive than perilymph due to a higher concentration of potassium cations (K⁺) in endolymph and higher sodium (Na⁺) in perilymph.^[4] This is referred to as the endocochlear potential.^[5]

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 predominant positive electrolyte) and, via the cochlear aqueduct (sometimes referred to as the "perilymphatic duct"), is in continuity with cerebrospinal fluid.

There are two types of perilymph: the perilymph of the scala vestibuli, and that of the scala tympani. Both have a composition similar to cerebro-spinal fluid (CSF): rich in sodium (140mM) and poor in potassium (5mM) and calcium (1.2mM). The perilymph in the scala vestibuli comes from blood plasma across a hemto-perilymphatic barrier, whereas that of the scala tympani originates from CSF.

Endolymph

Endolymph is created from perilymph.
 The endocochlear potential is the sum of two potentials: a positive potential caused by active secretion of K+ by the stria vascularis (120mV) and a negative potential created by the passive diffusion of K+ ions from the hair cells (40mV), which can be visualised after an anoxia.

Note that the ionic composition of endolymph develops prior to the endocochlear potential. In fact, in the mouse, endolymphatic ion concentrations are finalised during the first postnatal week. In comparison, the mouse endocochlear potential doesn’t develop until the second postnatal week and doesn’t reach its final value until the third week after birth.

Composition and properties of the two cochlear fluids