Quick Facts
Endolymph is the fluid contained in the membranous labyrinth of the ear; it is entirely separate from the perilymph (Dorland, 2011).
Structure and/or Key Feature(s)
The cochlear duct is filled with endolymph, an extracellular fluid that is similar in consistency to that of cerebrospinal fluid and has a crucial function in auditory detection. This fluid is produced from the marginal cells of the stria vasculairis, which is located on the outer wall of the cochlear duct. It is also produced by ion diffusion from the perilymph, occurring across the vestibular membrane.
The endolymph, along with the perilymph, are the two inner ear fluids. Compared to perilymph, endolymph has lower sodium and calcium ion concentrations, but higher chloride and potassium ion concentrations. There are also higher levels of glucose and total protein in perilymph. The differences in ionic composition between the endolymph and perilymph of the internal ear are what enables the transductive process for auditory processing. Essentially, mechanical vibrations result in waves of the endolymph, which ultimately results in the transmission of nerve signals along the cochlear nerve (Yamashita, 1976).
Anatomical Relations
The endolymph is contained within the cochlear duct. It bathes the apices of the sensory hair cells, as well as other components of the cochlear duct, including the spiral organ (of Corti), stria vascularis, the tectorial membrane, vestibular membrane, and the basilar membrane.
Function
Vibrations are transmitted from the middle ear to the basilar membrane (via the oval window) and disrupts the endolymph in the cochlear duct. This disruption causes a shift in the position of the special stereocilia bundles of the sensory hair cells. This positional change activates positive ion channels in the stereocilia bundles. Once activated, an influx of endolymphatic potassium and calcium ions occurs, causing a depolarization of the membrane and result in the generation of potentials which result in synaptic transmission via neurotransmitter release. Perilymph does contribute to this as well, as it can contribute calcium ions which influx into the channels in these areas (Standring, 2016).
List of Clinical Correlates
—Refractory Meniere’s disease
References
Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.
Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice., 41st edition. Elsevier Limited.
