The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system.
Often the entire organ is considered the ear, though it may also be considered just the visible portion. In most mammals, the visible ear is a flap of tissue that is also called the pinna (or auricle in humans) and is the first of many steps in hearing. Vertebrates have a pair of ears placed somewhat symmetrically on opposite sides of the head. This arrangement aids in the ability to localize sound sources.
The outer ear is the most external portion of the ear. The outer ear includes the fleshy visible outer ear, called the pinna or auricle, the ear canal, and the outer layer of the tympanic membrane, also known as the ear drum. The outer ear is the only visible portion of the ear in humans and almost all vertebrates, and consequently the word "ear" may be used to refer to the pinna alone.
Human ear (from Descent of Man)
The auricle consists of the curving outer rim (the helix), the inner curved rim (the antihelix), and opens into the ear canal, properly called the external acoustic meatus. The tragus protrudes and partially obscures the ear canal. The ear canal stretches for a distance of about 1 inch, and consists of an inner portion surrounded by bone, and an outer portion surrounded by cartilage. The skin surrounding the external acoustic meatus contains glands that produce ear wax (cerumen). The ear canal ends at the external surface of the ear drum (tympanic membrane)
Two sets of muscles are associated with the outer ear; the intrinsic and extrinsic muscles. In some mammals these muscles can adjust the direction of the pinna. In humans these muscle have very little action if any at all. These muscles are supplied by the facial nerve, which also supplies sensation to the skin of the ear itself, as well as the ear cavity. The vagus nerve, auriculotemporal nerve of the mandibular nerve, lesser occipital branch of C2, and the greater occipital nerve branch of C3 all supply sensation to portions of the outer ear and surrounding skin.
The auricle consists of a single piece of fibrocartilage with a complicated relief on the anterior, concave side and a fairly smooth configuration on the posterior, convex side. The Darwinian tubercle, which is present in some people, lies in the descending part of the helix and corresponds to the true ear tip of the long-eared mammals. The lobule merely contains subcutaneous tissue. In some animals with mobile pinnae (like the horse), each pinna can be aimed independently to better receive the sound. For these animals, the pinnae help localize the direction of the sound source. Human beings localize sound within the central nervous system, by comparing arrival-time differences and loudness from each ear, in brain circuits that are connected to both ears. This process is commonly referred to as EPS, or Echo Positioning System.
The middle ear is an air-filled cavity behind the tympanic membrane, includes three bones (ossicles): the malleus (or hammer), incus (or anvil), and stapes (or stirrup). The middle ear also connects to the upper throat via the Eustachian tube.
The three ossicles transmit sound from the tympanic membrane to the ventricles of the ear. The malleus is connected to the tympanic membrane, and transmits vibrations of the membrane produced by sound waves. The malleus has a long process (the manubrium, or handle) that is attached to the mobile portion of the eardrum. The incus is the bridge between the malleus and stapes. The stapes connects to the oval window, and is the smallest named bone in the human body. The three bones are arranged so that movement of the tympanic membrane causes movement of the malleus, which causes movement of the incus, which causes movement of the stapes. When the stapes footplate pushes on the oval window, it causes movement of fluid within the cochlea (a portion of the inner ear). The ossicles help in amplification of sound waves by nearly thirty times.
In humans and other land animals the middle ear (like the ear canal) is normally filled with air. Unlike the open ear canal, however, the air of the middle ear is not in direct contact with the atmosphere outside the body. The Eustachian tube connects from the chamber of the middle ear to the back of the nasopharynx. The middle ear is very much like a specialized paranasal sinus, called the tympanic cavity; it, like the paranasal sinuses, is a hollow mucosa-lined cavity in the skull that is ventilated through the nose. The mastoid portion of the human temporal bone, which can be felt as a bump in the skull behind the pinna, also contains air, which is ventilated through the middle ear.
Normally, the Eustachian tube is collapsed, but it gapes open both with swallowing and with positive pressure. When taking off in an airplane, the surrounding air pressure goes from higher (on the ground) to lower (in the sky). The air in the middle ear expands as the plane gains altitude, and pushes its way into the back of the nose and mouth. On the way down, the volume of air in the middle ear shrinks, and a slight vacuum is produced. Active opening of the Eustachian tube is required to equalize the pressure between the middle ear and the surrounding atmosphere as the plane descends. The diver also experiences this change in pressure, but with greater rates of pressure change; active opening of the Eustachian tube is required more frequently as the diver goes deeper into higher pressure.
Abnormalities such as impacted ear wax (occlusion of the external ear canal), fixed or missing ossicles, or holes in the tympanic membrane generally produce conductive hearing loss. Conductive hearing loss may also result from middle ear inflammation causing fluid build-up in the normally air-filled space. Tympanoplasty is the general name of the operation to repair the middle ear's tympanic membrane and ossicles. Grafts from muscle fascia are ordinarily used to rebuild an intact ear drum. Sometimes artificial ear bones are placed to substitute for damaged ones, or a disrupted ossicular chain is rebuilt in order to conduct sound effectively.
The inner ear is split anatomically into bony and membranous labyrinths. This contains the sensory organs for balance and motion, namely the vestibules of the ear (utricle and saccule), and the semicircular canals. This also contains the sensory organ for hearing, the cochlea.
The bony labyrinth refers to a bone matrix which opens externally into the oval window, which connects with the incus, which transmits vibrations into a fluid called endolymph, which fills the membranous labyrinth. The endolymph is situated in two vestibules, the utricle and saccule, and eventually transmits to the cochlea, a spiral-shaped structure. The cochlea consists of three fluid-filled spaces: the scala tympani, the scala vestibuli and the scala media, which together are responsibly for hearing.
Sound waves travel through the outer ear, are modulated by the middle ear, and are transmitted to a nerve in the inner ear, the vestibulocochlear nerve. This nerve transmits information to the temporal lobe of the brain, where it is registered as sound.
Sound that travels through the outer ear impacts on the tympanic membrane (ear drum), and causes it to vibrate. The three ossicles transmit this sound to a second window, the oval window, which protects the fluid-filled inner ear. In detail, the pinna of the outer ear helps to focus a sound, which impacts on the tympanic membrane. The malleus rests on the membrane, and receives the vibration. This vibration is transmitted along the incus and stapes to the oval window. Two small muscles, the tensor tympani and stapedius, also help modulate noise. The tensor tympani dampens noise, and the stapedius decreases the receptivity to high-frequency noise. Vibration of the oval window causes vibration of the endolymph within the ventricles and cochlea.
The hollow channels of the inner ear are filled with liquid, and contain a sensory epithelium that is studded with hair cells. The microscopic "hairs" of these cells are structural protein filaments that project out into the fluid. The hair cells are mechanoreceptors that release a chemical neurotransmitter when stimulated. Sound waves moving through fluid flows against the receptor cells of the Organ of Corti. The fluid pushes the filaments of individual cells; movement of the filaments causes receptor cells to become open to the potassium-rich endolymph. This causes the cell to depolarise, and creates an action potential that is transmitted along the spiral ganglion, which sends information through the auditory portion of the vestibulocochlear nerve to the temporal lobe of the brain.
The human ear can generally hear sounds with frequencies between 20 Hz and 20 kHz (the audio range). Although hearing requires an intact and functioning auditory portion of the central nervous system as well as a working ear, human deafness (extreme insensitivity to sound) most commonly occurs because of abnormalities of the inner ear, rather than in the nerves or tracts of the central auditory system. Sound below 20 Hz is considered infrasound, which the ear cannot process.