For hearing instrument that the auditory perception enables belong except outer, middle and inner ear and the auditory nerve and the switching and processing stations in the central nervous system in mammals so some areas in the brain stem and midbrain , up to the auditory cortex .
Ears in general
The sense of hearing is to be distinguished from the sense of vibration . The latter picks up substrate noise, for example when the ground vibrates. Hearing, d. H. The perception of rhythmic pressure waves in air or water has evolved only in relatively few animal groups. Almost all terrestrial vertebrates (tetrapods), many fish and numerous insect species can therefore hear, as can some cephalopods . Most invertebrates , however, live in a silent world. In vertebrates , nature probably invented hearing two to three times independently. The first hearing organs arose in the Devonian around 380 million years ago. An essential step in acquiring good hearing was then the development of a middle and inner ear, including an eardrum. In the case of insects , hearing arose at least 20 times independently of one another.
The structure and placement of the hearing organs are very different in the different species. In grasshoppers , the ears are on the abdomen or legs, in cicadas on the legs and in mosquitoes and flies on the antennae. Some species of lizards and salamanders hear with their rib cage and lungs. Outer ears are present in most species of mammals and birds, with exceptions in some species of dolphins. Reptiles, amphibians, and fish do not have external ears. In reptiles and amphibians, the eardrum sits directly on the outside of the head.
The hearing range of the human ear ranges from around 16 Hertz to a maximum of 20,000 Hertz at a young age . Among other things, elephants can perceive even lower frequencies, the infrasound , while a number of animals, for example mice , dogs , dolphins and bats , can hear much higher frequencies, the ultrasound .
One of the tasks of hearing is orientation in space, i.e. to localize sound sources, i.e. to determine their direction and distance. Sound coming in from the side reaches the facing ear sooner than the one facing away and is louder there because the facing ear is shaded by the head. These time differences and level differences between the two ears are evaluated by the brain and used to determine the direction. In addition, the auricle generates specific changes in the frequency response depending on the direction , which are also evaluated and used to determine direction.
Many living beings, including humans, can localize existing sound sources, but orientation in space is mainly done with the help of a sense of balance and sight . Dolphins and bats have evolved the sense of hearing into a particularly sophisticated system of orientation. Both emit high-frequency signals in the ultrasonic range (up to 200 kHz) and orient themselves using the echo . This active method of orientation is called positioning . In bats, hearing has largely replaced the eyes , which are of no great use in the dark .
The anatomical structure and the exact function of the ear were still largely unknown in the Middle Ages. Modern knowledge about it gained among others Andreas Vesalius , Bartolomaeus Eustachius , Gabriel Falloppius and Johannes Philippus Ingrassia (1510-1580). In humans and other mammals, the ear is divided into three areas:
- The outer ear comprises the ear cartilage, the auricle , the earlobe and the external auditory canal or also the ear canal and the outside of the eardrum. It is not only used to capture the sound, but also to encode a certain direction of incidence of the sound through spectral minima and maxima (see localization ). The numerous elevations and depressions of the auricle form acoustic resonators , each of which is excited from a certain direction when sound is incident. This results in direction-dependent minima and maxima in the frequency spectrum of the ear signal, which are used by the hearing to determine the directions of incidence above, below, front or rear ( direction-determining bands ).
- The middle ear includes the eardrum and the ossicles, hammer , anvil and stapes . The round window connects the tympanic staircase of the inner ear with the middle ear. The Eustachian tube , also called the ear trumpet, connects the middle ear and nasopharynx . A mechanical impedance conversion takes place in the middle ear , which enables an optimal transmission of the signal from the outer ear to the inner ear. Since the acoustic impedance of water is around 3000 times that of air, only a small part of the sound energy that reaches the eardrum would be passed on to the inner ear without the lever system formed by the ossicles.
- The inner ear lies in a small cavity system (bony labyrinth, lat. Labyrinthus osseus ) within the petrous bone , part of the temporal bone. In this bony labyrinth is the membranous or membranous labyrinth (Latin: Labyrinthus membranaceus ), consisting of the cochlea (Latin: Labyrinthus cochlearis , short: cochlea ), in which sound is converted into nerve impulses, and the organ of equilibrium (Latin: Labyrinthus vestibularis ) . The organ of equilibrium consists of the semicircular canals and two vesicle-shaped parts, the utriculus and the saccule . It is used to recognize changes in movement and the direction of gravity. The cochlea and the organ of equilibrium are built similarly: Both are filled with two common parallel fluid systems ( perilymph and endolymph ) and have hair cells . The hair cells are cylindrical and get their name from the 30 to 150 hair-like appendages at the top of the cell ( stereocilia ). As the fluid moves, the hairs are bent and trigger nerve impulses. At the lower end there is a synapse with a sensory neuron . This already releases neurotransmitters in the resting state . If the hair extensions are now deflected by sound vibrations or changes in the movement of the head, the amount of neurotransmitters changes. In the organ of equilibrium, the hair extensions are coated with a kind of gelatinous layer on which small crystals of calcium carbonate are deposited, which intensify the effect of movements. The auditory nerve goes from the cochlea together with the nerve bundles of the equilibrium organ as the vestibulocochlear nerve towards the brain .
The perception of acoustic signals is largely determined by how sound vibrations are transformed and processed on their way from the outer ear via the middle ear to the nerve cells of the inner ear. The human ear can only perceive acoustic events within a certain frequency and sound pressure level range . The hearing surface lies between the hearing threshold and the pain threshold .
The quietest perceptible sound pressure for people with normal hearing is around 20 micro- Pascals (20 µPa = 2 · 10 −5 Pa) with a tone of 2,000 Hz , which corresponds to L p = 0 dB SPL sound pressure level . These changes in sound pressure Δ p are transmitted via the eardrum and the middle ear ossicles into the inner ear, and the auditory impression is then created in the ear-brain system. Because the eardrum as a sensor with the ear system has the properties of a sound pressure receiver , the sound pressure level as a sound field variable best describes the strength of the hearing impression. The sound intensity J in W / m², on the other hand, is not suitable as a sound energy variable to describe the auditory impression; due to the complex impedance of the outer and middle ear with the same sound pressure level. The same applies analogously to the speed of sound .
The human ear can pick up an extremely low level of sound power . The quietest perceptible sound generates an output of less than 10 −17 W in the inner ear. Within a tenth of a second that the ear needs to convert this signal into nerve impulses, an energy of around 10 −18 joules creates a sensory impression. This shows how sensitive this sensory organ actually is.
The pain threshold is over 130 dB SPL , which is more than three million times the sound pressure of the smallest audible sound pressure (63.246: 0.00002 = 3.162.300). In particular, the inner ear and here the hair cells and their stereocilia are damaged by high sound pressure.
In directional hearing and in headphone stereophony, differences in transit time and level differences between the two ears and thus also the individual ear distance play a certain role, as do the spectral properties of the ear signals .
The techniques for examining hearing ability are summarized under the term audiometry . One result of a hearing test that examines hearing at different frequencies is called an audiogram . The hearing threshold can usually be read from this .
Outside the actual ear, however, are the nerve tracts that lead to the hearing center of the brain, as well as the hearing center itself. If these are impaired, sound perception can also be impaired in a functioning ear.
The path of sound: auricle → ear canal → tympanic membrane → ossicles → cochlea → auditory nerve
The human ear can become ill in various ways, each specific to the affected part of the ear.
- The outer ear is by its relatively thin skin in the ear canal in the pinna susceptible to infections with bacteria or fungi. These lead to the frequently observed ear infections ( otitis externa ). Due to a weakened immune system and inadequate treatment, the infection ( phlegmon , otitis externa diffusa, or ear canal furuncle , otitis externa circumscripta) can spread to the bone surrounding the ear canal and cause its suppuration (otitis externa maligna). If the auricle is affected, one speaks of auricular perichondritis .
- There are congenital and acquired auricular malformations . The most common congenital auricular malformations are protruding ears ; second- or third-degree auricular malformations such as microtia are less common . Acquired auricular malformations arise from external influences, such as B. Accidents or even animal bite injuries.
- The middle ear can also be affected by inflammation and suppuration. A distinction is made between acute otitis media acuta and chronic otitis media chronica. The ossicles can also be attacked and destroyed by the inflammation. The middle ear can still be damaged by high sound pressures such as those caused by explosions. Together with the other damage resulting from this, one speaks of explosion trauma. The otitis media can also be the starting point for mastoiditis .
- Frequent diseases of the inner ear occur in connection with permanent noise exposure and impact trauma . The hair cells are damaged here. The conversion of the mechanical stimuli into nerve impulses is then no longer possible and hearing loss is the result. In this context, tinnitus also often occurs. The inner ear is also the target of viral infections such as meningitis , measles and mumps . Various drugs (e.g. gentamicin ) can also damage the inner ear. The causes of the so-called sudden hearing loss , in which a sudden hearing loss, tinnitus and vertigo may occur are unknown. Similar symptoms can also occur as a result of semicircular canal dehiscence , a bone defect in the inner ear.
- The infectious disease mumps particularly affects the parotid gland in close proximity to the ear.
For diagnosing diseases of the ear, in particular ear, nose and throat medicine , hearing tests are available in addition to the commonly used methods of medicine such as x-ray examinations , serological and visual examinations .
The imprint of the ears can be used to identify a person. The earprint has a similarly high evidential value as a fingerprint . The criminalistics can be based on the earprints left, z. B. when eavesdropping on windows or front doors, can convict criminals . The advantage over fingerprints is that an earprint is usually not created by chance. Fingerprints can usually be found by many people at the crime scene.
The human outer ear continues to grow slowly after adolescence, averaging around 0.2 mm per year.
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