The outer ear is the external portion of the ear and includes the eardrum. The visible part is called the pinna, or auricle, and functions to collect and focus sound waves. Many mammals can move the pinna (with the auriculares muscles) in order to focus their hearing in a certain direction in much the same way that they can turn their eyes. Humans, as opposed to other mammals, do not have this ability. From the pinna the sound pressure waves move into the ear canal, a simple tube running to the middle ear. This tube amplifies frequencies in the range 3 kHz to 12 kHz.
The human ear has earlobes at the bottom that are vestigial but are used by many people to provide an attachment point for earrings. The earlobe is usually formed cleft from the side of the face and hangs from the rest of the ear but occasionally will be found looking fused and "lobeless" due to a recessive gene. The helix is the outer edge of the outer ear
The middle ear is the hollow portion of the ear behind the eardrum. The middle ear contains one or more ossicles, which amplify vibration of the eardrum into pressure waves in the fluid in the inner ear. The middle ear has also been called the tympanic cavity, or cavum tympani. The middle ear makes contact with the outside via the eardrum, the brain via the oval window, and the nasal cavity via the eustachian tube.
The movement of the ossicles may be stiffened by two muscles, the stapedius and tensor tympani, which are under the control of the facial nerve and trigeminal nerve, respectively. These muscles contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear. This is called the acoustic reflex.
Of surgical importance are two branches of the facial nerve, which also pass through the middle ear space. These are the horizontal and chorda tympani branches of the facial nerve. Damage to the horizontal branch during surgery can lead to partial, unilateral facial paralysis.
Inner Ear or Cochlea
The cochlea (Latin for "snail") is a spiraled, hollow, conical chamber of bone filled with perilymph and endolymph (in the scala media), a fluid medium that receives the sound vibrations transmitted from the air to the oval window through the ear drum and ossicles of the middle ear (see above). Running through its centre is the cochlea duct, which contains the spiral Organ of Corti, the receptor organ responsible for hearing. The bony cavity of the cochlea is divided into three separate chambers: the scala vestibuli, which lies superior to the cochlea duct and abuts the oval window; the scala media, which is the membranous cochlea duct containing endolymph and the organ of Corti; and the scala tympani, which lies inferior to the scala media and terminates at the round window. The two bony chambers (scala vestibuli and scala tympani) both contain perilymph and join together at the cochlear apex, a region called the helicotrema. Separating the scala vestibuli from the scala media is the Reissner's membrane. The basilar membrane separates the scala media from the scala tympani. Sitting on top of the basilar membrane is a cellular layer known as the Organ of Corti, which is lined with hair cells — sensory cells topped with hair-like structures called stereocilia. The vestibular apparatus is filled with the same endolymph as the cochlea, but instead of detecting sound, it detects rotation of the head. If a line is drawn through the middle of each of the three semicircular canals, perpendicular to the plane in which the canal lies, the three lines would be perpendicular. They would represent three axes of rotation. Any rotation could be represented as three simultaneous rotations about the three axes.
Ear is the organ for hearing and balance (equilibrium) in the body. The structure of the car converts vibrations of air into f1uid vibrations and then into sensory impulses. These sensory impulses are carried through the auditory nerve or the vestibulocochlear nerve to the centers in the brain for interpretation for hearing and equilibrium.
External auditory meatus conveys vibrations of sound to the tympanic membrane and is about 1 inch long. A waxy secretion called cerumen is produced in this part. Its main function is to prevent dust particles, foreign bodies, etc., to reach the delicate part of middle ear or inner ear. Tympanic membrane or eardrum marks the beginning of the middle ear (tympanic cavity). Tympanic cavity is the site for the eustachian tube. From here, the tube runs forward into nasopharynx, where it opens. This tube helps in equalizing air pressure on each side of the tympanic membrane. This tube is also called a pharyngotympanic tube. The sound waves collected from the pinna and auditory meatus strike against the tympanic membrane. From the tympanic membrane the sound waves pass through bones called auditory ossicles. This chain of bones (ossicles) transmits the vibrations from tympanic membrane to inner ear. The inner ear is the most delicate part, consisting of several cavities, which channel in the temporal bone. Its two main parts (cavities) are a bony labyrinth and a membranous labyrinth inside it. Both are fluid-filled cavities. The fluid inside the bony labyrinth and outside membranous labyrinth is called perilymph. The fluid inside the membranous labyrinth is called endolymph. Bony labyrinth is divided into three regions, namely, semicircular canals, vestibule and cochlea. Vestibule is the central part with which all other parts communicate. It contains two sacs, saccule and utricle. The utricle is connected to semicircular canals, and from the saccule arises the cochlea. Fluid within saccule and utricle passes freely among the sacs and excess fluid is stored in the endolymph sac for return to circulation. Receptors (hair cells) on both these structures, along with semicircular canals, help in maintaining equilibrium. Movement of the endolymph within the membranous labyrinth caused by head movement and head position stimulates the hair cells, which generate impulses through the vestibulocochlear nerve and in response to this we make appropriate movements to co-ordinate the head’s activity and maintain balance.
The third area of inner ear is cochlea, which is a spiral tube twisted on itself. The cochlea has a membranous labyrinth inside it, which is called cochlear duct and contains endolymph. The surrounding cochlear duct on the inside is the basilar membrane, which has many sensory structures called organ of Corti, which contain hair cel1s that are re-ceptors for auditory stimuli. When three ossicles vibrate, they transfer the vibrations to the fluid within the cochlea, which is then set into motion. This moving fluid (endolymph) within the cochlea moves the basilar membrane and, as a result, the hair cells attached to the organ of Corti are stimulated. These impulses (stimuli) are transmitted to the brain for interpretation as sound.
The eighth cranial nerve or the auditory nerve supplies the special sense of hearing and the olfactory or first cranial nerve supplies the sense of smell.
Stirrup, found in ear, is the smallest bone in the human body
The inner ear comprises both:
The organ of hearing (the cochlea)
The labyrinth or vestibular apparatus, the organ of balance located in the inner ear that consists of three semicircular canals and the vestibule.
As the stapes oscillates against the oval window in response to sound, the perilymph within the scala vestibuli also oscillates. For very low frequencies (below 20Hz), the pressure waves propagate along the complete route of the cochlea - up scala vestibuli, around helicotrema and down scala tympani to the round window. Frequencies this low do not activate the organ of Corti and are below the threshold for hearing. Higher frequencies do not propagate to the helicotrema but are transmitted through the endolymph in the cochlea duct to the perilymph in the scala tympani. The hair cells in the organ of Corti are tuned to certain sound frequencies, being responsive to high frequencies near the oval window and to low frequencies near the apex of the cochlea.
All excited hair cells send nerve impulses to the brain, which are perceived as a sound of whatever pitch the hair cell is associated with. A very strong movement of the endolymph due to very loud noise may cause hair cells to die. This is a common cause of partial hearing loss and is the reason why anyone using firearms or heavy machinery should wear earmuffs or earplugs.
Function and Mechanism of Hearing