Structure and functions of the ear

There are external, middle and inner ear.

Outer ear

Consists of an auricle and an external auditory canal. The auricle is an elastic cartilage of complex shape, covered with skin. The human anuricle is elongated, the lower part is lobe, is devoid of cartilage and is filled with fat. The free curled edge of the auricle is called the curl ; on the concave surface, parallel to the curl is a counter- curvature , in front of it there is a depression-the ear shell, at the bottom of which there is an external auditory opening, in front it is bounded by a goat.

The external auditory meatus consists of bone and cartilage, its length in an adult is 2.5 cm, the lumen of a round or oval shape has a diameter of about 0.7 cm. In the course there is an S-shaped bend of the external auditory canal if the ear is pulled upward And back, the passage straightens. The skin of the cartilaginous part contains numerous sebaceous and sulfuric glands, as well as hair. Sulfur glands produce a viscous, yellowish secret - earwax. The skin of the bone is thin, devoid of hair and glands.

Middle ear

Includes a tympanum, a tympanic membrane, a system of airway cells of the mastoid process and an auditory tube.

The eardrum separates the outer ear from the middle ear. It is a plate consisting of two layers of fibers of connective tissue, in the outer layer of fiber located radially, in the inner layer - circularly. The surface facing the external auditory canal is covered with epithelium, the inner surface that faces the tympanum is covered with the mucosa of the middle ear. The diameter of the tympanic membrane is about 9 mm, the thickness is 0.1 mm. In the center there is an impression - the navel, in this place one of the auditory ossicles - a hammer - is attached to the membrane.

The tympanic cavity is a slit-like space of 0.75 cm3 in size, located in the temporal bone, and lined with a mucosa from the inside. There are three auditory ossicles in the cavity, the tendons of the muscles that stretch the tympanic membrane and the stirrup. Here also passes the drum string - the branch of the intermediate nerve (the sensitive part of the facial nerve). The drum cavity continues into the auditory tube, which opens in the nasopharynx with a pharyngeal opening of the auditory tube.

The drum cavity has six walls. The upper one separates the tympanum from the cranial cavity, the lower one separates the drum cavity from the jugular vein, the labyrinth separates the drum cavity from the bone labyrinth of the inner ear. In the labyrinth wall there are two windows: the window of the vestibule is closed by the base of the stapes, the window of the cochlea is closed by the secondary eardrum. The membranous wall is formed by the tympanic membrane. The front wall separates the tympanum from the canal of the internal carotid artery, in the region of the posterior wall there is an entrance to the mastoid cave.

Auditory ossicles - a stirrup, an anvil and a hammer, are named so owing to the form. These bones are the smallest in the human body, together they form a chain connecting the tympanic membrane with the window of the vestibule leading into the inner ear. Bones are designed to transmit sound vibrations. The handle of the malleus is fused to the eardrum. The head of the malleus and the body of the anvil are connected by a joint and strengthened by ligaments, the long process of the anvil is connected to the head of the stapes. The base of the stapes enters the window of the vestibule, connecting with its edge by means of a ring ligament. Bones covered with mucous membrane. The tendon of the muscle that strains the eardrum is attached to the handle of the malleus, the stirrup muscle to the stapes. These muscles regulate the motion of the bones.

Plate-shaped air-bearing cells are located in the thickness of the mastoid process of the temporal bone, through the mastoid cave communicate with the tympanic cavity.

The auditory tube (Eustachian) consists of bone (1/3 length) and cartilaginous (2/3 of length) parts, has a length of 3.5 cm, a lumen diameter of 1-2 mm. The auditory tube is lined with a mucous membrane. Normally, the walls of the cartilaginous part are in a collapsed state, the opening of this part of the tube occurs at the time of swallowing.

The auditory tube performs an important function - it helps to equalize the air pressure inside the tympanum with respect to the external environment.

The inner ear

Is located in the pyramid of the temporal bone. Functionally, the inner ear is divided into two parts: auditory (snail) and vestibular (vestibule and semicircular canals). In the bone labyrinth there is a membranous labyrinth, repeating the forms of the bone, between the labyrinths there is a gap filled with perilymph. The bone labyrinth is located between the tympanic cavity and the internal auditory meatus and consists of a vestibule, three semicircular canals and a cochlea. The bony vestibule is an oval cavity that communicates with semicircular canals; on the side wall there are two windows: the window of the vestibule and the window of the cochlea.

Three bone semicircular canals (anterior, posterior and lateral) lie in three mutually perpendicular planes. Each semicircular canal has two legs, one of which widens before the confluence into the vestibule, forming an ampoule. Bony snail forms 2.5 curls around a horizontally lying rod - a spindle, around which a spiral bone plate is wound like a spiral staircase. The plate divides the cavity of the channel of the cochlea into two ladders: a vestibule and a drum that communicate with each other in the dome area through the hole.

The wall of the membranous labyrinth is formed by a connective tissue, from the inside it is lined with epithelium and is filled with endolymph. The membranous labyrinth is represented on the eve of two small extensions (a sac and a queen). The membranous semicircular canals open in the dome. Sections of pouches containing sensitive cells are called spots , similar sections of ampullae of semicircular membranous canals are scallops . Sensitive stain cells contain hairs that are covered with a gelatinous membrane containing crystals of calcium carbonate (otoliths). Spots perceive a change in gravity and linear acceleration. Sensitive cells of the ampullar scallops also contain hairs covered with a gelatinous dome on top. Amplary scallops perceive a change in the angular acceleration (for example, when turning the head).

When the gravity, the position of the head and the body change, the otolith membrane and the gelatinous dome are shifted at accelerations, this leads to tension of the hairs and a change in the activity of the enzymes of the sensory cells. The nerve impulse is transmitted along the anterior part of the VIII cranial nerve to the bridge of the brain, and then to the cerebellum and Cortex of parietal and temporal lobes (cortical center of equilibrium).

The cochlear labyrinth has the appearance of blind protrusion of the vestibule, which is in the cochlea and blindly terminates at its apex, it is called the cochlear duct . The cochlear duct is filled with endolymph and divides the bone spiral canal into three parts, occupying the middle part. The upper part is the staircase of the vestibule, the lower one is the drum ladder, they are filled with perilymph. In the area of ​​the dome of the cochlea, both staircases communicate with each other through the opening of the cochlea. The drum ladder extends to the base of the cochlea, where it ends at the round window of the cochlea closed by the secondary tympanic membrane, the staircase of the vestibule communicates with the perilymphatic space of the vestibule.

The cochlear duct on the cross-section is triangular in shape. Its lower wall, facing the tympanum, is a basilar membrane on which a spiral (corti) organ is located - it realizes the perception of sounds. Perceptive cells are located on a membrane having a different width at the beginning and at the apex of the cochlea. It is believed that as a result of this, the membrane resonates with different parts in response to sounds of different heights. Its receptor cells have microscopic hairs, which, when vibrating the membrane, touch another plate hanging over them in the form of a canopy (cover membrane). This is the stimulus to the formation of nerve impulses.

The irritant for Corti's organ is sound, the human ear is capable of perceiving sound vibrations with a frequency of 16 to 20,000 Hz. To sounds that have a frequency of 1,000-3,000 Hz (vibrations per second), the ear is most sensitive, in this frequency range is human speech.

The auricle is a kind of horn that collects and directs sound waves into the external auditory canal. In addition, the auricle is important in the ototope, i.e. In determining where the source of sound is. The external auditory meatus serves to carry out sound vibrations in the middle ear, then the sound vibrations reach the eardrum and are transmitted along the chain of auditory ossicles to the oval window. The auditory ossicles are connected to each other in the manner of a lever whose long shoulder is the handle of the malleus entwined in the tympanic membrane, and the short long leg of the anvil is short. The mechanism of the lever device is aimed at reducing the swings of oscillations and at strengthening the corresponding shocks on the window of the vestibule, so that sound waves propagating from the air of the outer and middle ear into the liquid medium of the inner ear are greatly enhanced. This amplification is particularly needed by low sounds.

A certain role in sound conduction is played by the auditory tube. Normally, the usual atmospheric pressure in the tympanum cavity is provided by the ventilation function of the auditory tube, When swallowing and yawning, the duct of the auditory tube opens, and air through it penetrates into the tympanum. When the patency of the auditory tube is impaired, the air present in the tympanic cavity is absorbed, and the new one does not act, this leads to a decrease in the mobility of the auditory ossicles and a decrease in hearing.

From the window of the vestibule, oscillatory movements are transmitted to the liquids of the labyrinth and its webbed formations. In this case, any deflection of the stirrup in the window of the vestibule corresponds to the bending of the secondary tympanic membrane in the cochlear window, with a secondary eardrum defect, the sound wave reaches both windows with equal force, the perilymph movement becomes minimal and the hearing decreases.

Sound waves coming from the middle ear to the inner one cause complex movements of membranous formations and liquids of the cochlea, which leads to an active state of the corti's organ. In his sensitive cells, mechanical energy is converted into a process of nervous excitation.

Nerve impulse VIII cranial nerve - is transmitted to the bridge of the brain, and through its centers and centers of the intermediate brain - in the temporal lobe of the hemisphere, where the cortical center of hearing is located. This is the so-called air way of sound transmission.

In addition to air, there is also a bone mechanism for sound transmission. In this case, the sound source is directly in contact with the surface of the head or other dense structures of the body. Under the influence of low sounds, the skull oscillates as a whole and due to the inertia of the chain of the auditory ossicles, the stapes move relative to the window of the vestibule. Under the influence of high sounds, the skull oscillates in separate segments, periodic labyrinth capsule compression and perilymph pressure on the labyrinthine windows occur.

Irritants for vestibular receptors are gravity, linear and angular acceleration. The value of the vestibular apparatus in a person's life is great, the pulses constantly signal from the receptors, signaling the position of the head in space and regulating muscle tone, ensuring the correct correlation of individual parts of the body (normal posture). In animals, changing the position of the head immediately affects the tone of the muscles and a number of motor reflexes are created, aimed at returning the head and trunk to a normal symmetrical position. In humans, otolith reflexes are so inhibited that they can occupy any posture that is required for their work, can sleep both on their sides and on their backs, i.e. In such conditions, when the otolith apparatus is subjected to unusual irritation. But this happens until the load on the otolith apparatus is not large. With increased load, i.e. With a lot of irritation and accumulation of irritation (driving in a car, train, flight by air, at sea), many people show signs of so-called "seasickness". There is a clear failure of the body in the form of severe dizziness, nausea, vomiting.