The scala vestibuli is one of the three fluid-filled chambers, or scalae, within the cochlea of the inner ear, playing a critical role in the process of hearing.
Location
The scala vestibuli is situated within the cochlea, which is part of the bony labyrinth of the inner ear. It is positioned above the scala media (cochlear duct) and below the scala tympani, making up the uppermost chamber of the cochlea. The scala vestibuli starts at the oval window, an opening at the base of the cochlea that is connected to the middle ear. This is where vibrations from the middle ear’s stapes bone are transmitted into the cochlea.
Structure
The scala vestibuli is a spiraling, tube-like structure that follows the curve of the cochlea from its base to the apex. It is filled with perilymph, a fluid that is similar in composition to cerebrospinal fluid and different from the potassium-rich endolymph found in the scala media. The scala vestibuli is separated from the scala media by Reissner’s membrane, a delicate, thin barrier that helps maintain the distinct biochemical environment necessary for the function of the cochlear hair cells located in the scala media.
Function
The primary function of the scala vestibuli is to conduct sound vibrations from the oval window into the cochlea. When the stapes bone at the oval window vibrates in response to sound waves, it sets the perilymph within the scala vestibuli in motion. These fluid waves travel up the scala vestibuli and, depending on the frequency of the sound, cause specific regions of the basilar membrane (separating the scala vestibuli from the scala media) to vibrate. This vibration stimulates the hair cells in the organ of Corti within the scala media, initiating the process of converting mechanical sound vibrations into electrical signals that can be interpreted by the brain as sound.
The scala vestibuli’s role in transmitting sound vibrations efficiently is essential for the proper functioning of the auditory system, allowing for the precise encoding of a wide range of sound frequencies and intensities. Its structure and fluid dynamics are critical for the sensitive and selective processing of sound that characterizes human hearing.
At the cochlear apex, the scala vestibuli meets the scala tympani at the helicotrema, allowing the perilymph to flow between these two chambers. The walls of the scala vestibuli are made up of bone, lined with a thin layer of cells that contribute to the production and maintenance of the perilymph.
Clinical Significance
Dysfunction or anomalies in the scala vestibuli, such as abnormalities in the perilymph fluid or disruptions in the integrity of the cochlear structures, can lead to hearing impairments or balance disorders. For instance, a perilymph fistula, which is a leak of perilymph from the scala vestibuli into the middle ear, can cause hearing loss and vertigo, significantly affecting an individual’s quality of life. Similarly, conditions like superior semicircular canal dehiscence involve abnormal openings that affect the pressure dynamics between the scala vestibuli and other parts of the inner ear, leading to auditory and vestibular symptoms.
Understanding the scala vestibuli’s anatomy and function is crucial for diagnosing and managing inner ear disorders. Audiological evaluations, such as pure-tone audiometry and vestibular evoked myogenic potentials (VEMPs), can help assess the health of the scala vestibuli and related structures. Additionally, imaging techniques like high-resolution CT scans or MRI can visualize abnormalities in the bony labyrinth and scala vestibuli.
