The auditory system consists of three parts: receptor system (in the inner ear), the auditory nerve and the central part that is located in the tempo

Ear structure (outer, middle, inner)

The auditory system consists of three parts: receptor system (in the inner ear), the auditory nerve and the central part that is located in the temporal cortex (figure 178).

The organ of hearing — ear — has three sections: the outer, middle and inner. Sound vibrations, before reaching the auditory receptors, pass through the system of sound-conducting and sound-amplifying structures.

Outer ear

The outer ear (figure 179) consists of the pinna — the cartilage plate covered with skin. In the area of ​​the ear lobe the cartilage is absent. The structure of the ear is bound with its function to capture sound vibrations and precisely guide them to the external auditory canal. Outer ear also includes an external ear canal 2-3 cm long and 1 cm in diameter, that comes out of the pinna to the middle ear, from which it is separated by the eardrum (tympanic membrane). The auricle acts as a sound amplifier. It collects sound waves by routing them to the eardrum, which area is ​​2.2-5 times less than the sound perception area of ​​the ear. If a person does not hear well, he can enhance the sound, putting his hand to his ear.

Eardrum (tympanic membrane)

The eardrum separates the outer ear canal from the middle ear. It is 0.1 mm thick and is woven by connective fibers which branch out in various directions. The tympanic membrane resembles a watering can, which center is directed into the cavity of the middle ear. Sound vibrations are forcing it to oscillate with the same frequency. With age, the tympanic membrane becomes thicker and rougher, which is one of the reasons why older people hear badly.

Middle ear

The middle ear (diagram 179) consists of the tympanum and the auditory ossicles (diagram 180). On the border of the middle and inner ears are two windows, closed with membranes: oval and round. Stapes is adjacent to the membrane of the oval window.

Tympanum

On the inner side of the eardrum is a tympanum cavity that has auditory ossicles inside.

Auditory ossicles

Auditory ossicles are three tiny ear bones, which are interconnected sequentially semi-mobile: malleus, incus and stapes. They transmit the oscillation of the eardrum to the inner ear. One end of malleus is woven into the eardrum, and the other — connected with an incus, which is connected to a stapes that transmits the sound vibrations to the inner ear.

Auditory ossicles are not only transmitters of sound vibrations but also their converters and amplifiers. Due to a special structure of the connections between the bones, which form a system of levers, sound waves passing through them, reduces in amplitude, but increases in strength for 20-60 times. Therefore, even weak sound waves, which effect on the eardrum, cause vibrations of sound-perceiving inner ear structures. With age, the bones merge with each other, become inactive, and this leads to hearing loss in the elderly.

Diagram 178. Schematic structure of the auditory system: 1 — hearing centers (the central part of the system); 2 — the outer ear; 3 — inner ear; 4 — auditory nerve
Diagram 179. The structure of the external, middle and inner ear: 1 — pinna; 2 — the external auditory meatus; 3 — the eardrum; 4 — auditory ossicles; 5 — semicircular canals; 6 — auditory and vestibular nerves; 7 — a cochlea; 8 — tympanic cavity (tympanum); 9 — Eustachian tube
Diagram 180. The auditory ossicles: 1 — malleus; 2 — incus; 3 — stapes

Eustachian tube

The tympanum is not closed: the Eustachian tube (figure 179) 3.5-4.5 cm in length connects it through the nasal passages with the atmosphere. Due to this air pressure on both sides of the eardrum is equal. This creates better conditions for its fluctuations. Taken from http://worldofschool.org

The Eustachian tube also protects the eardrum from being destroyed if the atmospheric pressure is suddenly increased or decreased (on the plane, while diving into the water).

Inner ear

The inner ear (diagram 179) placed in the cavity of the temporal bone pyramid and consists of a system of canals that formed by the cochlea and the semicircular canals with two sacks (vestibular system).

Cochlea

Cochlea is a bone helical duct which forms 2.5 turns and gradually expands from 0.04 to 0.5 mm. The cochlea chamber is divided inside into three ducts (upper, middle and lower) by two membranes: the dense and supple — the main membrane and thin — vestibular. Moreover, the upper and lower ducts are interconnected at the end of the cochlea.

Cochlea chambers

Cochlea chambers are not empty; they are filled with a liquid that conducts sound waves. Middle cochlear duct (scala media), on the main membrane, has a hearing aid — the organ of Corti, named after the Italian anatomist A. Korte, who described it in the middle of the XIX century. Organ of Corti’s auditory receptors — are 24-30 thousand special hair cells that touch the top of cover membrane.

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Questions:
  • Explain the relationship of structure and functions of the tympanic membrane.

  • Determine the function of the auditory ossicles.

  • What is the biological significance of the Eustachian tube?

  • Identify the relationship of the structure and functions of the inner ear, specifically the cochlea.

  • Explain the relationship of the structure and functions of the organ of Corti.