The membranous labyrinth is a complex system of fluid-filled tubes and sacs found within the inner ear, crucial for both hearing and balance.
Structure
The membranous labyrinth is encased in the bony labyrinth and mirrors its structure but is smaller and has a different composition. It consists of two main parts: the cochlear labyrinth, involved in hearing, and the vestibular labyrinth, involved in balance. The entire system is filled with a potassium-rich fluid called endolymph, contrasting with the perilymph that fills the space between the membranous and bony labyrinths.
Cochlear Labyrinth
This part of the membranous labyrinth includes the cochlear duct (scala media), a spiraling tube that extends from the base to the apex of the cochlea. The cochlear duct houses the organ of Corti, the sensory receptor organ for hearing, which sits on the basilar membrane and is responsible for converting sound vibrations into neural signals.
Vestibular Labyrinth
This includes the utricle and saccule (the otolith organs responsible for detecting linear accelerations and head tilts) and the three semicircular canals (which detect rotational movements). Each semicircular canal ends in a swelling called the ampulla, which contains the crista ampullaris, a sensory organ for detecting angular motion.
Endolymphatic Duct and Sac
Part of the vestibular system, the endolymphatic duct connects the saccule and utricle to the endolymphatic sac. This structure is involved in regulating the volume and pressure of endolymph within the membranous labyrinth.
Function
The primary function of the membranous labyrinth is to convert external stimuli — sound waves for the cochlear part and head movements for the vestibular part — into electrical signals that the brain can interpret. This process involves complex interactions between the endolymph, the sensory cells, and the nervous system.
Hearing: In the cochlear labyrinth, sound waves cause the basilar membrane to vibrate, which in turn stimulates the hair cells in the organ of Corti to create electrical impulses that are transmitted to the brain via the auditory nerve.
Balance and Spatial Orientation: In the vestibular labyrinth, movements of the head cause displacement of the endolymph, which bends the hair cells in the utricle, saccule, and semicircular canals. This mechanical stimulation is converted into electrical signals that inform the brain about the body’s position and motion.
The membranous labyrinth’s role in hearing and balance is essential for daily activities, from walking and maintaining posture to interpreting sounds and speech. Its complex structure and function highlight its significance within the auditory and vestibular systems.
