ear

Ear structure

The inner ear is embedded in the ear (or otic) capsule and has a common embryological development in all vertebrate groups. In comparing the inner ears of different vertebrates, the major structural differences are associated with the auditory part of the ear. With few exceptions, the vestibular portion of the inner ear is developmentally, structurally, and functionally nearly the same in all vertebrates.

The middle ear and external ear are not found in the fishes. All tetrapods (amphibians, reptiles, birds, and mammals) have a middle ear with a tympanic membrane. Reptiles, birds, and mammals also have an external auditory meatus (or canal) which extends from the tympanic membrane to the external surface of the head. Mammals generally have an external structure, the pinna, that helps “collect” and carry the sound to the ear canal and then to the tympanum. The major difference in the middle ear among tetrapods is that it has a single ear bone, or ossicle (often called the columella or stapes), in amphibians, reptiles, and birds, while mammals have three middle-ear bones (malleus, incus, and stapes).

The basic sensory unit in the inner ear is the sensory hair cell. These specialized cells are morphologically similar in all of the epithelial structures of the ear in all vertebrates (and in the lateral line of fishes and amphibians), but they may have either auditory or vestibular functions depending upon the associated superstructure. The superstructure serves to facilitate the transmission of vibrations from the environment to the hair cells. For the vestibular apparatus, the superstructure blocks external vibratory energy, but sensitizes the sensory hair cells to the pull of gravity and to acceleratory and deceleratory movements of the head. See Lateral line system

The sensory hair cell is a columnar, polarized structure from whose apex extend thin cilia that resemble hairs. Each hair cell has many such cilia, making up a ciliary bundle which bends in response to motional energy. The cilia in each bundle include many stereocilia and a single, eccentrically positioned kinocilium. The cilia extend into an extracellular fluid-filled space, with their tips embedded in a gelatinous membrane. See Cilia and flagella

The sensory hair cell is the detector of motion, either produced by compression and rarefaction of molecules due to sound waves, or imparted by movement of the head against gravity. This motion produces bending of the ciliary bundles, and this in turn results in a change in configuration of the membrane overlying the stereocilia and opening of channels in the membrane. It is generally thought that these channels admit calcium into the cell, and this in turn interacts with other components of the cell. Ultimately, the energy generated by these interactions causes release of neurotransmitter at the base of the cell, and results in stimulation of afferent neurons which contact the cell. See Neurobiology, Synaptic transmission

Fishes

In elasmobranchs and bony fishes the inner ear is located in the brain (cranial) cavity somewhat behind the eye. The inner ear has several regions, including three semicircular canals and otolith organs. Other than the very primitive jawless fishes which have one or two semicircular canals, all other vertebrates have three canals. All fishes, amphibians, reptiles, and birds have three otolith organs—the saccule, utricle, and lagena—while mammals do not have the lagena (Fig. 1).

Left-ear external view of the membranous labyrinths of (a) teleost, (b) frog, (c) bird, and (d) mammal

At one end of each endolymph-filled semicircular canal is a widened area, the ampulla, which has a sensory area called the crista (or crista ampullaris). The crista contains large numbers of sensory hair cells, as well as other cells which provide support for the hair cells. At the base of the hair cells are nerve endings from the vestibular branch of the eighth cranial nerve. Each of the otolith organs also has a sensory area, called a macula, that contains hair cells and supporting cells. The cilia of the otolithic organs are embedded in a thin gelatinous membrane that also contains very dense calcium carbonate crystals. In elasmobranchs, primitive fishes, and all tetrapods, these crystals are called otoconia. In most bony fishes the crystals are fused into a single mass in each otolith organ called the otolith.

Fishes are able to detect a wide range of sound using their inner ear. Tetrapods detect sounds that impinge on the tympanic membrane and then are carried by the middle-ear bones to the inner ear, where the sounds set the fluids of the ear into motion and thus stimulate the sensory hair cells. In fishes, however, this kind of pathway is not needed since sound is already traveling through water. Indeed, since the fish's body is the same density as that of water, sound would travel right through the fish were it not for the otoconia or otoliths. Since these structures are much denser than the fish's body and the water, they stay still while the fish's body and the attached sensory hair cells move with the sound field. Since the stereocilia are attached to both the top of the hair cells and to the otoconia or otolith, they are bent as their base moves with the macula and their tops stand still with the otoliths. This bending sends signals to the nerves and then to the brain, indicating the presence of a sound. Most fishes detect sounds from 30 to 800 or 1000 Hz, with best hearing from 200 to 500 Hz. However, some fishes, called hearing specialists, have evolved special mechanisms to enhance hearing to 3000 or 4000 Hz. The hearing specialists use a secondary structure, the swim bladder, to enhance hearing capabilities. The swim bladder is a bubble of gas found in the abdominal cavity of most bony fishes, and it is used primarily for buoyancy control, though it may also be used in sound production in some species. Since the swim bladder is filled with gas, its density is different from that of the rest of the fish, and in a sound field the walls of the swim bladder are set into vibration and act as a small sound source to send sounds to the ear. See Swim bladder

Tetrapods

Many structural and functional features of the fish inner ear are also found in the tetrapod ear. The inner ear of tetrapods is embedded in the otic bones of the skull, with the membranous labyrinth attached to the bony labyrinth by connective tissue but suspended in perilymphatic fluid. There are three semicircular canals, with cristae, and, except in mammals which do not have a lagena, the three otolithic organs (Fig. 1b, c, d). In their morphology and physiology the vestibular parts of fish and tetrapod ears are nearly the same. For the most part, the tetrapod otolithic organs function only as vestibular organs rather than playing an auditory role as they do in fishes.

Amphibians

The tympanic membrane of frogs and toads is located on the lateral surface of the head. Attached to its inner aspect is a small rodlike bone, the stapes, or columella, which runs through the air space of the middle-ear cavity and plugs a small hole, the oval window, beyond which are the inner-ear fluids. The frog's tympanic membrane collects sound energy and transmits it through the columella to the inner-ear fluids. In the lagenar portion of the amphibian's membranous labyrinth are two areas of hair cells, the amphibian and basilar papillae, that are found in no other vertebrate group. The basilar papilla lies on the posterior wall of the saccule between the oval window and the round window, another membrane-covered opening between middle ear and inner ear. Vibratory energy enters the inner ear at the oval window, passes through the basilar and amphibian papillae causing them to vibrate, and then dissipates at the round window. See Amphibia

Birds and reptiles

In most reptiles and birds the tympanic membrane lies not on the surface of the head but internally, at the end of the tube called the external auditory meatus. A middle-ear cavity (with its eustachian tube to the mouth) lies medial to the tympanic membrane. A single ossicle, the columella, crosses this cavity from the tympanic membrane to the oval window at the inner ear. While both birds and reptiles have saccule, utricle, and lagena, as well as semicircular canals, they also have a newly evolved end organ, the basilar papilla, which is the part of the ear used for hearing in both groups of animals. (The avian and reptilian basilar papilla is thought to be a totally different structure, in terms of evolution and embryonic origin, than the basilar papilla found in amphibians.) The basilar papilla in birds and reptiles is often also called the cochlea, and there is some evidence to suggest that this end organ is directly related to the mammalian cochlea. The basilar papilla in reptiles is generally somewhat shorter than that found in birds, and there is considerable variation in the specific structure of this end organ in different species. The basilar papilla contains sensory hair cells. In birds, the basilar papilla sensory hair cells are differentiated into short and tall hair cells, which may have different functions in hearing. See Aves, Reptilia

Mammals

The mammalian ear consists of three parts: the external ear which receives the sound waves; the middle ear which transmits the vibrations by a series of three small bones; and the inner, or internal, ear, a complex bony chamber placed deep in the skull (Fig. 2). The external auditory meatus plus the newly evolved pinna, a cartilaginous structure projecting from the ear, compose the external ear. The shape and size of the pinna vary greatly. The auditory function of the pinna varies widely in different species. In some species the pinna is moved in the direction of a sound source and helps the animal focus sound to the external auditory meatus and then down the ear canal. In other species, such as humans, the pinna may have a lesser function, but even in humans the pinna helps to discriminate between sounds coming from the front and back of the head so that the person can better tell the direction of a sound source. See Mammalia

Schematic drawing of the human ear

As in other tetrapods, the first gill slit is modified as a middle-ear cavity, communicating with the pharynx by way of the eustachian tube. In other tetrapods this tube is permanently open, while in mammals it is usually closed. Instead of the single columella of other tetrapods, the mammalian middle ear has three bones, closely articulated with one another. The innermost is the stapes, which fits into the oval window of the inner ear and is homologous with the columella. Attached to the tympanic membrane is the malleus, and lying between the malleus and stapes is the incus. In spite of having additional bones, the mammalian middle ear functions basically as do those of amphibians, reptiles, and birds in transforming aerial vibrations into fluid vibrations within the inner ear.

In the mammalian inner ear the vestibular apparatus is much like that of other tetrapods. The auditory portion, however, is elongated and coiled into a snail shape. This structure is called the cochlea. The epithelium of the basilar papilla, called the organ of Corti, is more differentiated in mammals than in other tetrapods. The number of turns in the cochlea varies. At the base of the cochlea is the oval window, which carries sound energy into the inner ear, and the round window, where this energy is dissipated after traveling in the cochlea.

Running the length of the coiled cochlea are three channels; the uppermost, the scala vestibule, and the lowest, the scala tympani, are filled with perilymph. In the center is the scala media, or cochlear duct. The cochlear duct is filled with endolymph, and it is separated from the scala vestibule above by the thin Reissner's membrane and from the scala tympani below by the basilar membrane.

The basilar membrane is suspended on both sides by ligaments or bone. The basilar membrane varies regularly in width, being narrow at the base (where it is most responsive to high frequencies) and wide at the apex (where it is most responsive to low frequencies). Resting upon the basilar membrane is the organ of Corti. The organ of Corti contains several cell types in addition to the auditory hair cells. The hair cells lying on the internal side of the pillar cells are called the inner hair cells, and those lying on the external side are called the outer hair cells. There may be up to 20,000 sensory hair cells in a cochlea of a normal young human, although the number of hair cells declines with age as a result of normal cell death, damage due to some medications, and trauma caused by loud sounds. A healthy teenager may hear sounds from below 20 Hz to upward of 20,000 Hz, while an adult 40 or 50 years old may hear sounds only to 14,000 Hz (or even less). This loss of hearing is associated with death of sensory hair cells.

Sounds entering the mammalian inner ear at the oval window travel along the basilar membrane from basal to apical ends, causing vibrations of the membrane. Different frequencies maximally excite different regions of the basilar membrane based on differences in the stiffness of the membrane itself. The response of the different regions of the organ of Corti to specific frequencies is also thought to be enhanced by the sensory hair cells themselves. Whereas early investigations suggested that both inner and outer hair cells were involved in detection of sound per se, recent evidence suggests that the inner hair cells have the major role in hearing, while the outer hair cells modify the function of the ear and help to enhance the sensitivity of the inner hair cells. See Hearing (human)

the organ of hearing and balance in vertebrates, including man; the peripheral part of the auditory system. During the course of evolution the ear developed in the anamniote ancestors of vertebrates from specialized sensory skin organs. There is an inner, middle, and external ear.

The inner ear, found in all classes of vertebrates, has a complex membranous labyrinth that is lined with sensory epithelium containing receptor hair cells and otoliths and that is filled with a fluid, the endolymph. The membranous labyrinth lies within a cartilaginous or bony labyrinth. The narrow area between the membranous and bony labyrinths is filled with perilymph; in terrestrial vertebrates this area is connected to the lymph sinuses of the head. The function of the inner ear is to perceive acoustic oscillations as well as changes in the body’s position in space; this is achieved by means of the cochlea and vestibule.

The middle ear occurs only in terrestrial vertebrates. It transmits acoustic oscillations to the inner ear; consequently, the elements of the latter become altered in shape and convert acoustic pressure into nerve impulses. The middle ear consists of an air-filled space, the tympanic cavity, containing the ossicles and the eustachian tube, which is connected to the pharynx. The outer wall of the tympanic cavity turns into a thin, elastic tympanic membrane. The sound waves that reach the membrane are transmitted to the inner ear. In amphibians, reptiles, and birds this occurs by means of an auditory ossicle, the columella, which oscillates in order to transmit sound waves efficiently.

In mammals the sound-conducting system consists of three ossicles. The inner ossicle, or stapes, is adjacent to the oval window of the auditory cavity and corresponds to the columella in amphibians, reptiles, and birds. The outer ossicle, or malleus, is connected to the tympanic membrane, and the middle ossicle, or incus, is joined to the malleus and stapes and fixed to the wall of the auditory cavity. The middle ear of many vertebrates has undergone extensive structural change because of ecological changes and increased specialization in the vertebrates’ way of life. A secondary reduction of the auditory cavity was a common result of the transition to an aquatic mode of life; it is observed, for example, in dolphins. The same change has occurred in burrowing mammals and in some lizards and snakes. The structure of the middle ear and its components is particularly complex in birds and mammals whose hearing is exceptionally sensitive.

The external ear occurs in rudimentary form in reptiles and birds and is well developed in mammals in the form of the pinna. The external ear first appeared in crocodiles because a tympanic membrane was located under the skin and because an external auditory canal with prominences on its periphery developed to form the rudiment of the pinna. In mammals the external ear is generally an elongated auditory canal.

The ear is subject to congenital defects, injuries, and diseases. Congenital defects include atresia (imperforation) of the external auditory canal, often combined with an underdeveloped pinna (microtia) or complete absence of the pinna (anotia). Anomalies of the middle ear are generally associated with impaired development of the external and inner ear. Among these anomalies are the presence of bone tissue in the tympanic cavity and the absence of auditory ossicles or their fusion. Congenital defects of the inner ear include absence or underdevelopment of the organ of Corti.

The external ear is the part of the ear most susceptible to injury. Injuries of the pinna, particularly those that occur during sports, often result in a hematoma, or accumulation of blood under the skin or perichondrium of the pinna. The middle and inner ear are generally injured during injuries to the skull. Localized injury to the inner ear results from exposure to very loud sounds, from prolonged exposure to loud noise, and from sudden drops in air pressure.

The most common noninflammatory disease of the external ear is the excessive secretion of ear wax, which sometimes completely obstructs the external auditory canal. Fungus infections are also common in the external auditory canal. Progressive loss of hearing may result from a proliferation of bone tissue in the region of the oval window, which connects the middle and inner ear. Noninflammatory diseases of the inner ear occur in poisonings and in disorders of the circulatory and autonomic nervous systems and of the endocrine glands. Such diseases, called labyrinthopathies, are sometimes manifested by recurrent attacks of dizziness and progressive loss of hearing.