The inner ear translates mechanical vibrations into electrical signals for hearing and maintains balance. This process relies on specialized fluids contained within bony and membranous structures. Perilymph is one of the two primary fluids in this system, playing a role in sound transmission and protecting sensitive neural tissue. Understanding its location is key to grasping how the inner ear functions.
Defining Perilymph
Perilymph is a clear, watery extracellular fluid that bathes the sensory structures of the inner ear. Its chemical makeup is similar to cerebrospinal fluid (CSF) and blood plasma, characterized by a high concentration of sodium ions (Na+) and a low concentration of potassium ions (K+). The fluid typically contains 138 to 150 millimoles per liter (mM) of sodium and 5 to 7 mM of potassium. It communicates with the CSF via the cochlear aqueduct, which opens into the scala tympani.
Specific Anatomical Location in the Inner Ear
Perilymph is housed within the bony labyrinth, the protective shell of the inner ear carved into the temporal bone. This fluid fills the space surrounding the membranous labyrinth. Within the cochlea, the perilymph occupies two main chambers: the scala vestibuli and the scala tympani. The scala vestibuli begins at the oval window, where the stapes transmits sound vibrations. The scala tympani ends at the round window, a flexible membrane that dissipates pressure waves. Perilymph also fills the bony canals of the vestibular system, including the semicircular canals, utricle, and saccule, aiding in balance.
Distinguishing Perilymph from Endolymph
The inner ear contains a second, chemically distinct fluid called endolymph. The two fluids are kept separate by membranes. Perilymph is in the bony labyrinth, while endolymph is contained within the membranous labyrinth, which floats within the perilymph. In the cochlea, this separation is maintained by Reissner’s membrane and the basilar membrane.
The chemical difference is foundational to hearing. Endolymph has an extremely high concentration of potassium ions (K+), up to 150 mM, and a very low concentration of sodium ions (Na+), typically only 1 mM. This ionic composition gives endolymph a positive electrical potential of about +80 millivolts, compared to perilymph’s near 0 millivolts. This electrical gradient, known as the endocochlear potential, powers the sensory hair cells to convert mechanical energy into electrical signals.
Role in Sound Wave Transmission
The perilymph’s location is directly tied to its function as a medium for transmitting sound energy. Sound waves are concentrated by the middle ear bones, and the stapes presses against the oval window. This movement initiates a pressure wave that travels through the perilymph filling the scala vestibuli.
The wave travels down the scala vestibuli, crosses the apex of the cochlea, and returns through the perilymph in the scala tympani, pushing against the round window. As the wave moves, it causes the basilar membrane to vibrate. This mechanical movement, which separates the perilymphatic compartments from the endolymphatic space, stimulates sensory cells. The fluid mechanics of the perilymph translate the initial vibrations into the precise membrane motion required for hearing.