The Auricle is what is commonly referred to as the ear. It is composed of irregular plate of elastic cartilage covered with skin, and an occasional hair. Its rim, the helix, is somewhat thicker, and its fleshly, dangling lobule (earlobe) lacks supporting cartilage. The size and shape of the auricle vary considerably from individual to individual, usually being larger in the male than in the female. Though a considerable number of muscle fibres are attached to the auricle, modern man has all but lost the ability to wiggle his ears. The external auditory meatus passes into the temporal bone. Near its opening the tube is guarded by hairs. It is lined with skin that contains numerous modified sweat glands called ceruminous glands, which secrete wax (cerumen). The hairs and wax help to keep relatively large foreign objects, such as insects, from entering the ear.
Sounds generally are created by vibrations of objects that are transmitted
through matter in the form of sound waves. For example, the sounds
of some musical instruments are produced by vibrating strings or reeds,
and the sounds of the voice are created by vibrating vocal folds in the
larynx. The auricle of the ear helps to collect sound waves travelling
through air and directs them into the auditory meatus. Sound waves entering
the external auditory canal eventually hit the tympanic
membrane, or eardrum (tympanum = drum), the boundary between the outer
and middle ears. The eardrum is a thin, translucent, connective tissue
membrane, covered by skin on its external face and by a mucosa internally.
It is shaped like a flattened cone, with its apex protruding medially into
the middle ear. Sound waves make the eardrum vibrate; the eardrum,
in turn, transfers the sound energy to the tiny bones of the middle ear
and sets them into vibration.
The middle ear communicates with the air cells (sinuses) of the mastoid process of the temporal bone and with the nasopharynx via a tube called the pharyngotympanic (auditory) tube (Eustachian tube). Air enters the middle ear through the pharyngotympanic tube to ensure that atmospheric pressure is maintained either side of the tympanic membrane; the equalisation of pressure to vibrate correctly.
The auditory ossicles being the smallest bones in the body are named according to their shape - the malleus (hammer), incus (anvil), and stapes (stirrup). The handle of the malleus is secured to the eardrum, and the base of the stapes fits into the oval window. tiny ligaments suspend the ossicles, and minisynovial joints link them together into a chain that spans the middle ear cavity. The ossicles transmit the vibratory motion of the eardrum to the oval window, which in turn sets the fluids of the inner ear into motion, eventually exciting the hearing receptors.
In addition to transmitting vibrations, the auditory ossicles form a lever system that helps increase (amplify) the force of vibrations as they are passed from the eardrum to the oval window. Since the ossicles transmit vibrations from the relatively large surface of the eardrum to a much smaller area at the oval window, the vibration force becomes concentrated as it travels from the external to the inner ear. As a result of these two factors, the pressure applied by the two stapes at the oval window is about twenty-two times greater than that exerted on the eardrum by sound waves.
The middle ear also contains two small skeletal muscles that are attached to the auditory ossicles. The tensor tympani, is inserted on the medial surface of the malleus, and when it contracts it pulls the bone inward. The other muscle, the stapedius, is attached to the posterior side of the stapes and serves to pull it outward. These muscles are the effectors in the tympanic reflex, which occurs when the ears are assaulted by very loud noises. When the reflex occurs, the muscles contract and the malleus and stapes are moved. As a result, the bridge of ossicles in the middle ear becomes more rigid, and its effectiveness in transmitting vibrations to the inner ear is reduced.
The tympanic reflex is a protective mechanism that reduces pressure
from loud sounds that might otherwise damage the hearing receptors.
The tensor tympanic muscle also functions to maintain a steady pull on
the eardrum. This is important because a loose tympanic membrane
would not be able to transmit vibrations effectively to the auditory ossicles.
When the air pressure difference is great enough, some air may force
its way up through the eustachian tube into the middle ear. At the
same time, the pressure on both sides of the eardrum is equalized, and
the eardrum moves back into its regular position. The person usually
hears a popping sound at this moment, and normal hearing is restored.
A reverse movement of air ordinarily occurs when a person moves from low
altitude into a higher one. The eustachian tube is usually closed by valve
like flaps in the throat, which may inhibit air movements into the middle
ear. Swallowing, yawning, or chewing may aid in opening the valves,
and these actions can hasten the equalisation of air pressure if discomfort
is experienced during altitude changes.
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.
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.
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