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The second step in hearing was the transmission of sound energy from the eardrum through the bones in the middle ear to the inner ear. The third step is when the sound travels through the inner ear. The inner ear consists of a labyrinth of chambers and tubes. There are two of these labyrinths in each ear — the osseous and membranous labyrinths.
The osseous labyrinth is a bony canal; the membranous labyrinth is inside the osseous one and has a similar shape. Between the two labyrinths there is a fluid, called perilymph, which is secreted by cells in the wall of the canal. The membranous labyrinth contains a different fluid, endolymph, which is slightly different. These fluids both cushion the soft structures in the ear and conduct waves of sound to the organ of Corti which is the actual receptor of sound.
The parts of the labyrinths include a cochlea and three semicircular canals (anterior, posterior, and lateral — named after their locations) that function in providing a sense of balance. A bony chamber called the vestible resides between the cochlea, and the semicircular canals contain membranous structures that serve both hearing and balance.
The cochlea is shaped like a snails’ shell. It contains a bony core (the modiolus) and a thin bony shelf (spiral lamina) that winds around it like a screw. The shelf divides the labyrinth of the cochlea into an upper and lower compartment. The upper compartment (scala vestibuli) leads from the oval window to the apex of the spiral. The lower one (scala tympani) extends from the apex of the cochlea to a membrane covered opening the wall of the inner ear, the round window. These are the bony components of the cochlea, which is filled with perilymph. At the apex of the cochlea the fluids in the chambers can flow together through a small opening (helicotrema).
The membranous part of the cochlea is represented by the cochlear duct (scala media). It is filled with endolymph and lies between the two bony compartments and ends at a closed sac at the apex of the cochlea. The cochlear duct is separated from the scala vestibuli by a vestibular membrane (Reissner’s membrane) and from the scala tympani by a basilar membrane.
The basilar membrane extends from the bony shelf of the cochlea and forms the floor of the cochlear duct. It consists of thousands of stiff elastic fibres, whose lengths vary, getting longer from the base of the cochlea to its apex. Vibrations that enter the perilymph at the oval window travel along the scala vestibuli and enter the endolymph of the cochlear duct where they cause movements in the basilar membrane.
After the vibrations pass through the basilar membrane, they enter the scala tympani, and their forces are dissipated to the air in the tympanic cavity by movement of the membrane that covers the round window. The organ of Corti, which contains the hearing receptors, is located on the top of the basilar membrane. It has receptor cells called hair cells, which are arranged in rows and possess numerous hairs that extend into the cochlear duct. When sound vibrations pass through the inner ear, the hairs shear back and forth against the tectorial membrane, and the movement of the hairs stimulate the receptor cells. Various receptor cells, however, have slightly different sensitivities to the deformation of the hairs. Thus different frequencies of vibration of sound will excite different sets of cells.
These cells act much like neurons in that when they are stimulated its membrane sends nerve pulses through the cochlear nerve to the brain, where the signals are interpreted as sound.