Middle ear

The middle ear (Latin auris media ) is part of the ear of the people, but also of the other land-living vertebrates.

It consists of a cave, which is located at the two sides of the skull. This tympanic cavity ( cavum tympani ) arises from the embryonic first pharyngeal pouch. It is filled with air and lined with a mucous membrane that firmly to the periosteum ( periosteum ) is connected. About the Eustachian tube ( "ear trumpet "), the middle ear is connected to the pharynx. About the Eustachian tube, a pressure equalization to the outside world takes place.

Ossicles

In the middle ear are the tiny ossicles, which connect the outer ear to the inner ear. With the eardrum ( tympanic membrane ), the handle of the hammer ( malleus ) is grown. The hammer is pivotally connected to the anvil ( incus ), which in turn communicates with the stirrup ( stapes ) in contact. The stapes footplate is fitted with its movable in the oval window ( fenestra ovalis or vestibular ) of the inner ear (inner ear ).

The ossicles are movably suspended by fine ligaments and muscles. Forming a lever system, which transmits the mechanical vibrations of the tympanic membrane to the inner ear. As the vibrations of an air-filled space on a liquid-filled chamber ( cochlea) to be transferred, an amplification of the signal must take place. This signal amplification is achieved firstly by the leverage of the ossicles, and secondly by the fact that the eardrum has a larger area than the oval window. Thus, a sound pressure increase takes place. In the range 1-3 kHz, this coupling is optimally about 60 % of the total noise power is transmitted from the eardrum to the inner ear. At lower frequencies, but also in the high frequency range, this mechanism works significantly less.

Middle ear muscles

With the help of two small muscles of the properties of sound transmission can be changed: The tensor tympani muscle inserts on the hammer and tightens the eardrum. The stapedius muscle inserts on the stapes and tilted the stapes in the oval window. This coupling of the eardrum to the inner ear is impaired, it is not transmitted by the global sound power to the inner ear, but a part is reflected at the tympanic membrane or discharged into the surrounding bone. This hearing can protect themselves to a certain extent from damage due to excessive sound pressure levels.

On the protective mechanism against high sound only the stapedius muscle is involved, he contracted as a result of the stapedius reflex, which is triggered by loud sound. The acoustic reflex is at noise levels of 70-95 dB ( stapedius reflex threshold ) and is about 50 ms after application of the sound effect. The acoustic reflex acts on both ears even if only one ear is sonicated with a high noise level. By an impedance measurement of the external auditory canal can be observed the use of stapedius reflex and the use for diagnostic purposes.

Phylogeny of the middle ear

The temporomandibular joint ( temporo- mandibular articulation Latin ) is the movable connection between the mandible and the rest of the skull. In vertebrates, more specifically from the gnathostomes ( gnathostomata ) ( evolution of mammals ), with the exception of mammals the TMJ articular by a connection between Os and Os formed quadratum (primary jaw joint ).

These two bones are evolved in mammals ( therapsids, evolution of mammals ] to the ossicles and forms with them the joint between the hammer and anvil. Temporomandibular joint of mammals is therefore a secondary jaw joint and is formed by the condyle of the mandible and the shed portion of the temporal bone and in the cavum tympani shifted.

Thus, the jaw bone ( articular, a small bone at the posterior end of the mandible and quadrate, a small bone at the posterior end of the maxilla ) hiked the amniotes in mammals into the middle ear. This allows a better transmission of the acoustic signal can be achieved. The earliest amniotes had a jaw joint, which was composed of the articular and quadrate the. All non- mammal -like amniotes ( lizards, crocodiles, dinosaurs and their descendants, the birds, and therapsids ) are characterized by this TMJ. However, the mammals use a different jaw joint system which is composed only of the dentary bone deck ( the tooth-bearing lower jaw bone) and squamosal ( small skull bone ). In mammals the quadrate and articular were for the anvil ( incus ) and the hammer ( malleus ) of the middle ear.