Download The inner ear The inner ear can be divided into

The inner ear
The inner ear can be divided into three parts: the semicircular canals, vestibule and
the cochlea all of which are located in the temporal bone of the skull. Vibrations from
the ossicles are transmitted to the cochlea via the stapes bone. The inner ear
changes mechanical vibrations from the middle ear into electro-chemical impulses by
stimulating hair cells inside the cochlea.
The second step in hearing is the transmission of sound energy from the tympanic
membrane through the middle-ear cavity to the inner ear. The inner ear consists of
a complex system of inter-communicating chambers and tubes called a labyrinth.
There are two such structures in each ear - the osseous and membranous labyrinths.
The osseous labyrinth is a bony canal in the temporal bone; the membranous
labyrinth lies within the osseous one and has a similar shape. Between the osseous
and membranous labyrinths is a fluid, called perilymph, that is secreted by cells in
the wall of the bony canal.
The membranous labyrinth contains another fluid, called endolymph, whose
composition is slightly different. These fluids serve the dual purpose of cushioning
the soft structures and conducting waves from the middle ear to the Organ of Corti,
the actual receptor of sound.
The parts of the labyrinths include a cochlea that functions in hearing, and
three semicircular canals (anterior, posterior and lateral) that function in providing a
sense of equilibrium. A bony chamber, called the vestibule, which is located between
the cochlea and the semicircular canals, contains membranous structures (saccule
and utricle) that serves both hearing and equilibrium.
Cochlea
The cochlea, as its name suggests, is shaped like the coiled shell of a snail. Inside, it
contains a bony core (modiolus) and a thin bony shelf (spiral lamina) that winds
around the core like threads of a screw. The shelf divides the bony labyrinth of the
cochlea into upper and lower compartments. The upper compartments, called the
scala vestibulli, leads from the oval window to the apex of the spiral. The lower
compartment, the scala tympani, extends from the apex of the cochlea to a
membrane-covered opening in the wall of the inner ear called the round window.
These compartments constitute the bony labyrinth of the cochlea, and they are filled
with perilymph. At the apex of the cochlea, the fluids in the chambers can flow
together through a small opening (helicotrema).
The membranous labyrinth of the cochlea is represented by the cochlea duct (scala
media), which is filled with endolymph. It lies between the two bony compartments
and ends as 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 contains many thousands of stiff, elastic fibres, whose
lengths vary becoming progressively longer from the base of the cochlea to its apex.
Vibrations entering the perilymph at the oval window travel along the scala vestibuli
and pass through the vestibular membrane to enter the endolymph of the cochlear
duct, where they cause movements in the basilar membrane.
After passing through the basilar membrane, the sound vibrations enter the
perilymph of the scala tympani, and their forces are dissipated to the air in the
tympanic cavity by movement of the membrane covering the round window. The
Organ of Corti which contains the hearing receptors, is located on the upper surface
of the basliar membrane and stretches from the apex to the base of the cochlea. Its
receptor cells, which are called hair cells, are arranged in rows and they possess
numerous hair like processes that extend into the endolymph of the cochlear duct.
As sound vibrations pass through the inner ear, the hairs shear back and forth
against the tectorial membrane, and the mechanical deformation of the hairs
stimulates the receptor cells. Various receptor cells, however, have slightly different
sensitivities to such deformation of the hairs. Thus, a sound that produces a
particular frequency of vibration will excite certain receptor cells, while a sound
involving another frequency will stimulate a different set of cells.
The cells act very like Neurons in that when it is stimulated appropriately its
membrane becomes depolarised, ion channels open, and the membrane becomes
more permeable to calcium ions. In the presence of calcium ions, some of the
neurotransmitter-containing vesicles in the cytoplasm near its base, fuse with the
cell membrane and release neurotransmitter substance into the outside. This
neurotransmitter simulates the mends of nearby sensory nerve fibres, and I
response they transmit nerve impulses along the cochlear branch of
the vestibulocochlear nerve to the brain. The brain then interprets these nerve
impulses, and the hearing process is complete.