|Chapter 1 Human Anatomy Body Parts||Chapter 2 Human organ systems||Chapter 3 Human Skeleton||Chapter 4 Skull|
|Chapter 5||Chapter 6 Throat||Chapter 7 Shoulder Girdle||Chapter 8 Hand - Finger|
An ear is an organ used by an animal to detect sound waves. The term may refer to the entire system responsible for collection and early processing of sound (the beginning of the auditory system), or merely the externally-visible part.
Mammals, including humans, have two ears, one on each side of the head.
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 in order to focus their hearing in a certain direction in much the same way that they can turn their eyes. Humans have generally lost 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, which 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 includes the ossicles (three tiny bones), two muscle tendons (of the stapedius and tensor tympani muscles), and two nerve bundles (the horizontal portion of the facial nerve and a branch of the facial nerve called the chorda tympani). The Latin names of the ossicles are the malleus, incus, and stapes, but they are also referred to by their English translations: the hammer, anvil, and stirrup respectively.
Mammals are unique in having three ear bones. The incus and stapes are derived from bones of the jaw, and allow finer detection of sound. These bones form the linkage between the tympanic membrane and the oval window that leads to the inner ear. The tympanum converts vibrations of air in the ear canal into vibrations of the ossicles. The ossicles in turn transmit the vibrations through the membrane of the oval window into the fluid of the inner ear. The ratio in area between the tympanic membrane and the oval window results in an effective amplification of approximately 14 dB, peaking at a frequency of around 1 kHz. The combined transfer function of the outer ear and middle ear gives humans a peak sensitivity to frequencies between 1 kHz and 3 kHz. The tensor tympani muscle and stapedius muscles of the middle ear contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear. This is called the acoustic reflex.
The middle ear is hollow. If the animal moves to a high-altitude environment, or dives into the water, there will be a pressure difference between the middle ear and the outside environment. This pressure will pose a risk of bursting or otherwise damaging the tympanum if it is not relieved. This is one of the functions of the Eustachian tubes, evolutionary descendants of the gills, which connect the middle ear to the nasopharynx. The Eustachian tubes are normally pinched off at the nose end, to prevent being clogged with phlegm, but they may be opened by lowering and protruding the jaw; this is why yawning helps relieve the pressure felt in the ears when on board an aircraft.
The inner ear comprises both the organ of hearing (the cochlea) and the labyrinth or vestibular apparatus, the organ of balance located in the inner ear that consists of three semicircular canals and the vestibule.
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.
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.
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.
Diseases and medical conditions of the ear and auditory system
Problems with the ear or auditory processing system in the
brain can lead to deafness. Other disorders related to
auditory system are:
Benign Paroxysmal Positional Vertigo
Conductive hearing impairment
Neurofibromatosis Type 1
Neurofibromatosis Type 2
Noise-induced hearing loss
Nonsyndromic hereditary hearing impairment
Sensorineural hearing loss