Endolymph is a clear potassium-rich lymphoid fluid that fills the cavities of the membranous labyrinth in the inner ear. Separated by the Reissner membrane, the membranous labyrinth is surrounded by the sodium-rich perilymph. For hearing, the different ion concentration between perilymph and endolymph plays a major role, while the mechanical-physical properties (principle of inertia) are used to generate feedback from the vestibular organs.
What is endolymph?
The inner ear contains the organs within the membranous labyrinth that convert mechanical sound waves and rapid head movements or rotational and linear accelerations of the entire body into electrical nerve impulses and transmit them to the CNS via the vestibulocochlear nerve. The organs communicate with each other via the endolymph, a lymphatic fluid rich in potassium and low in sodium. The membranous labyrinth is surrounded by another lymphatic fluid, the perilymph, which is high in sodium and low in potassium. The membranous labyrinth is floating, so to speak, in the perilymph. However, the volume ratios are extremely small. The total volume of endolymph in each inner ear is only about 0.07 ml. The voltage potential that exists between the endolymph and perilymph due to the difference in electrolyte composition is used to convert the mechanical sound waves within the cochlea, the auditory cochlea, into electrical nerve impulses. In contrast, the physical-mechanical properties of the endolymph play the main role in the conversion of accelerating stimuli into electrical nerve impulses.
Anatomy and structure
The endolymph consists of a clear fluid, a potassium-rich electrolyte similar in composition to the intracellular fluid (cytoplasm). The endolymph is produced by the epithelial cells of the stria vascularis within the cochlea and reabsorbed by the saccus endolymphaticus, where the ductus endolymphaticus terminates, so that there is constant renewal and a dynamic balance between secretion and reabsorption of the endolymph. The epithelium of the stria vascularis is one of the few epithelia that is permeated by supplying and disposing blood capillaries to fulfill its function of secreting endolymph. At the same time, the epithelial cells ensure the constancy of the composition of the endolymph. In addition to the high potassium concentration of 140 – 160 meq/l (milliequivalent per liter), the endolymph also contains a similarly high concentration of chlorine (120 – 130 meq/l) as the perilymph. The protein content reaches only a value of 20 – 30 mg/100 g and is thus less than half of the protein content of the perilymph. The pH of 7.5 is slightly more basic than perilymph, which has an average pH of 7.2.
Function and tasks
The two main functions of the endolymph are to enable the conversion of mechanical sound waves and the conversion of head or body accelerations into electrical nerve impulses. For the conversions of sound waves into electrical impulses, depending on the frequency and strength of the sound pressure, the electrical potential difference of sometimes more than +150 mV between the endolymph and the surrounding perilymph is primarily used. The conversion of physical sound waves into electrical nerve impulses takes place under energy consumption by mechanoreceptors in the cochlea. Mechanoreceptors in the arcades and in the macular organs Sacculus and Utriculus are responsible for the generation of electrical nerve impulses analogous to rotational or linear accelerations on the head or body. Important for a correct conversion of the acceleration impulses are specific gravity and viscosity of the endolymph, which significantly determine the physical-mechanical properties. In a broader sense, it is also important that the volume or pressure of the endolymph in the endolymphatic system remains constant, i.e. that the secretion and resorption rates correspond to each other. Deviations from the normal values immediately trigger unusual acceleration sensations that make coordinated movements difficult. Alcohol ingestion, for example, leads to a change in endolymph viscosity that can last up to 36 hours, i.e., until the blood alcohol content has long since been reduced. Another task of the endolymph is to supply proteins to certain tissues with which it is in direct contact.
Diseases
The sense of hearing and vestibular sensation can be affected by a number of complaints and diseases caused by abnormalities of the endolymph. One well-known disease is Meniere’s disease, which results in altered composition of the endolymph and perilymph, such that the electrolytic properties are altered and there is increased accumulation of endolymph throughout the endolymphatic system (endolymphatic hydrops). The dynamic balance between secretion and adsorption is disturbed. Meniere’s disease usually results in the symptoms of vertigo, tinnitus, and hearing loss (Meniere’s triad). An endolymphatic hydrops can lead to leaks in the Reißner membrane with the effect that perilymph and endolymph are partially mixed and severe dizziness with malaise up to vomiting develops as well as abnormal hearing sensation up to shrill tinnitus symptoms. Complaints of sudden spinning vertigo are often caused by benign paroxysmal positional vertigo (BPPV). The condition is essentially benign, but can be uncomfortable if left untreated. Symptoms are caused by a tiny calcium carbonate crystal that has become dislodged from saccule or utriculus and lodged in the endolymph in one of the arcades, causing peculiar sensations of movement and positional vertigo. The problem can be solved naturally by a sequence of certain body positions. The tiny crystal granule can thus be transported out of the archway again. The exact causes for the development of endolymphatic hydrops have not (yet) been adequately clarified. It can be assumed as certain that permanent stress and psychological permanent strain either directly cause the formation of endolymphatic hypertension or promote it as a co-factor.
Typical and common ear diseases
- Ear flow (otorrhea)
- Otitis media
- Ear canal inflammation
- Mastoiditis
- Ear furuncle
