Within the intricate, snail-shaped structure of the inner ear, known as the cochlea, lies a world of fluid dynamics fundamental to our sense of hearing. This auditory portion of the inner ear is not a hollow space but is filled with specialized liquids. These fluids are contained within the cochlea’s spiraled chambers, playing a part in the process of converting sound waves into electrical signals our brain can understand. The function of these fluids involves both mechanical and electrical processes that form the basis of auditory perception.
The Two Main Cochlear Fluids
The cochlea is partitioned into three distinct, fluid-filled chambers that spiral around a central bony core. The two largest chambers are the scala vestibuli, which connects to the oval window, and the scala tympani, which terminates at the round window. Both of these chambers are filled with a fluid called perilymph.
Perilymph surrounds and protects a smaller, triangular-shaped chamber that runs between them, known as the cochlear duct or scala media. This central cochlear duct contains a different fluid called endolymph. The cochlear duct is separated from the scala vestibuli by Reissner’s membrane and from the scala tympani by the basilar membrane. It is upon the basilar membrane that the organ of Corti, the sensory organ for hearing, is situated.
Unique Chemical Makeup
The functional differences between perilymph and endolymph are a direct result of their unique chemical compositions. Perilymph is chemically similar to extracellular fluid found elsewhere in the body and cerebrospinal fluid (CSF), characterized by a high concentration of sodium (Na+) ions and a low concentration of potassium (K+) ions. Specifically, perilymph contains approximately 140 millimoles per liter (mM) of sodium and only about 5 mM of potassium.
In contrast, endolymph is compositionally more like intracellular fluid, with a high concentration of potassium and a low concentration of sodium. Endolymph contains around 150-154 mM of K+ but only about 1 mM of Na+. This ionic difference is actively generated and maintained by a specialized tissue in the lateral wall of the cochlear duct called the stria vascularis. This structure functions to secrete potassium ions into the endolymph, creating a positive electrical potential of approximately +80 to +100 millivolts (mV) within the scala media, known as the endocochlear potential. This electrical potential is a source of energy for hearing.
How Cochlear Fluids Enable Hearing
The fluids within the cochlea contribute both mechanically and electrochemically to auditory function. The process begins mechanically when vibrations from the middle ear bones are transferred to the oval window. This action creates pressure waves in the perilymph of the scala vestibuli. Because fluids are incompressible, these waves travel up the cochlear spiral, are transmitted across the cochlear duct, and then descend through the perilymph of the scala tympani, ultimately causing the round window to bulge outwards.
As these pressure waves move through the perilymph, they cause the basilar membrane to vibrate at specific locations corresponding to the frequency of the sound. The movement of the basilar membrane deflects the stereocilia, or hair-like projections, on the sensory hair cells of the organ of Corti. This deflection opens mechanically-gated ion channels at the tips of the stereocilia. Due to the high potassium concentration and positive potential in the endolymph, K+ ions rush into the hair cells. This influx of positive ions depolarizes the hair cells, triggering the release of neurotransmitters that generate an electrical signal in the auditory nerve, which is then sent to the brain for interpretation as sound.
Maintaining Fluid Balance
The volume and composition of cochlear fluids are actively regulated through a dynamic process of production, circulation, and absorption to maintain a stable inner ear environment, or homeostasis. The stria vascularis requires a significant amount of energy for this regulation, supplied by a rich network of capillaries.
Perilymph is understood to be derived primarily as an ultrafiltrate of blood plasma from capillaries in the cochlea, with some contribution from cerebrospinal fluid via a small channel called the cochlear aqueduct. A structure known as the endolymphatic sac, connected to the cochlear duct via the endolymphatic duct, is believed to play a part in endolymph volume regulation and reabsorption. This process helps to clear waste products and excess ions to ensure the ionic and pressure gradients necessary for hearing are preserved.
Consequences of Fluid Imbalance
Disruptions in the balance of cochlear fluid volume, pressure, or chemical composition can have significant consequences for hearing and balance. When the mechanisms that regulate fluid homeostasis fail, it can lead to a condition known as endolymphatic hydrops, an excessive accumulation of endolymph in the cochlear duct. This distension of the endolymphatic space is the characteristic pathological finding in Meniere’s disease.
The excess pressure and volume associated with endolymphatic hydrops are thought to be responsible for the symptoms of Meniere’s disease, whose unpredictable nature reflects the unstable fluid state within the inner ear. These symptoms include:
- Episodes of spinning vertigo
- Fluctuating hearing loss that often affects low frequencies initially
- Tinnitus (ringing in the ear)
- A sensation of pressure or fullness in the affected ear