The structure of the auditory section of the ear auditory analyzer. Structure and functions of the auditory analyzer
The auditory analyzer includes three main parts: the organ of hearing, auditory nerves, subcortical and cortical centers brain Not many people know how a hearing analyzer works, but today we will try to figure it out together.
A person recognizes the world around him and adapts to society thanks to his senses. One of the most important are the hearing organs, which pick up sound vibrations and provide a person with information about what is happening around him. The set of systems and organs that provide the sense of hearing is called the auditory analyzer. Let's look at the structure of the organ of hearing and balance.
The structure of the auditory analyzer
Functions auditory analyzer, as mentioned above, perceive sound and give information to a person, but despite all the simplicity at first glance, this is a rather complex procedure. In order to better understand how the sections of the auditory analyzer work in the human body, it is necessary to thoroughly understand what the internal anatomy of the auditory analyzer is.
The hearing organs in children and adults are identical; they include three types of hearing aid receptors:
- receptors that perceive vibrations of air waves;
- receptors that give a person an idea of the location of the body;
- receptor centers that allow you to perceive the speed of movement and its direction.
The hearing organ of each person consists of 3 parts; by examining each of them in more detail, you can understand how a person perceives sounds. So, the outer ear is the combination of the auricle and the auditory canal. The shell is a cavity made of elastic cartilage that is covered thin layer skin. represents a certain amplifier for conversion sound vibrations. The ears are located on both sides human head and they don’t play a role, because they’re just collecting sound waves. The ears are motionless, and even if they are absent outer part, then the structure of the human auditory analyzer will not receive much harm.
Considering the structure and, we can say that it is a small canal 2.5 cm long, which is lined with skin with small hairs. The canal contains apocrine glands that are capable of producing earwax, which, together with hairs, helps protect the following parts of the ear from dust, pollution and foreign particles. The outer part of the ear only helps to collect sounds and conduct them to the central part of the auditory analyzer.
Eardrum and middle ear
The eardrum has the shape of a small oval with a diameter of 10 mm; a sound wave passes through it, where it creates some vibrations in the liquid, which fills this section of the human auditory analyzer. To transmit air vibrations in the human ear there is a system auditory ossicles, it is their movements that activate the vibration of the liquid.
Between the outer part of the hearing organ and the inner part is the middle ear. This section of the ear looks like a small cavity, with a capacity of no more than 75 ml. This cavity is connected to the pharynx, cells and auditory tube, which is a kind of fuse that equalizes the pressure inside and outside the ear. I would like to note that the eardrum is always exposed to the same atmospheric pressure both outside and inside, this allows the organ of hearing to function normally. If there is a difference between the pressures inside and outside, then hearing acuity will be impaired. 
Structure of the inner ear
The most complex part of the auditory analyzer is the inner ear; it is also commonly called the “labyrinth”. The main receptor apparatus that picks up sounds are hair cells inner ear or, as they also say, “snails”.
Wiring department The auditory analyzer consists of 17,000 nerve fibers, which resemble the structure of a telephone cable with separately insulated wires, each of which transmits certain information to neurons. It is the hair cells that respond to vibrations of the fluid inside the ear and transmit nerve impulses in the form of acoustic information to peripheral section brain. And the peripheral part of the brain is responsible for the sensory organs.
The conductive pathways of the auditory analyzer ensure rapid transmission of nerve impulses. To put it simply, the pathways of the auditory analyzer connect the hearing organ with the human central nervous system. Excitement auditory nerve activate motor pathways, which are responsible, for example, for eye twitching due to strong sound. The cortical section of the auditory analyzer connects the peripheral receptors of both sides, and when capturing sound waves, this section compares sounds from both ears at once.
The mechanism of sound transmission at different ages
The anatomical characteristics of the auditory analyzer do not change at all with age, but I would like to note that there are certain age-related characteristics.
The hearing organs begin to form in the embryo at the 12th week of development. The ear begins to function immediately after birth, but initial stages Human auditory activity is more like reflexes. Sounds of different frequency and intensity cause different reflexes in children, this can be closing the eyes, shuddering, opening the mouth or rapid breathing. If a newborn reacts this way to distinct sounds, then it is clear that the auditory analyzer is developed normally. In the absence of these reflexes, additional research is required. Sometimes the child’s reaction is inhibited by the fact that initially the newborn’s middle ear is filled with a certain fluid that interferes with the movement of the auditory ossicles; over time, the specialized fluid dries out completely and air fills the middle ear instead.
The baby begins to differentiate different sounds from 3 months, and at the 6th month of life he begins to distinguish tones. At 9 months of life, a child can recognize the voices of his parents, the sound of a car, the singing of a bird and other sounds. Children begin to identify a familiar and alien voice, recognize it and begin to hoot, rejoice, or even look with their eyes for the source of their native sound if it is not nearby. The development of the auditory analyzer continues until the age of 6 years, after which the child’s hearing threshold decreases, but at the same time hearing acuity increases. This continues for up to 15 years, then works in the opposite direction.
In the period from 6 to 15 years, you can notice that the level of hearing development is different, some children catch sounds better and are able to repeat them without difficulty, they manage to sing well and copy sounds. Other children are less successful at this, but at the same time they hear perfectly well; such children are sometimes called “the bear is in their ear.” Communication between children and adults is of great importance; it shapes the child’s speech and musical perception.
Concerning anatomical features, then in newborns the auditory tube is much shorter than in adults and wider, because of this, infection from respiratory tract so often affects their hearing organs.
Changes in hearing aid over the lifespan
Age characteristics The auditory analyzer changes slightly throughout a person’s life, for example, in old age, auditory perception changes its frequency. In childhood, the sensitivity threshold is much higher, it is 3200 Hz. From 14 to 40 years old we are at a frequency of 3000 Hz, and at 40-49 years old we are at 2000 Hz. After 50 years, only at 1000 Hz, it is from this age that the upper limit of audibility begins to decrease, which explains deafness in old age.
Older people often have blurred perception or intermittent speech, that is, they hear with some interference. They can hear part of the speech well, but miss a few words. In order for a person to hear normally, he needs both ears, one of which perceives sound, and the other maintains balance. As a person ages, their structure changes eardrum, it can become denser under the influence of certain factors, which will upset the balance. As for gender sensitivity to sounds, men lose hearing much faster than women.
I would like to note that with special training, even in old age, you can achieve an increase in the hearing threshold. Similarly, exposure to loud noise in a constant mode, which can negatively affect auditory system even at a young age. In order to avoid negative consequences from constant exposure to loud sound on the human body, you need to monitor. This is a set of measures aimed at creating normal conditions for functioning auditory organ. In people young The critical noise limit is 60 dB, and for school-aged children the critical threshold is 60 dB. It is enough to stay in a room with this noise level for an hour and Negative consequences will not keep you waiting.
Another age-related change in the hearing system is the fact that over time, earwax hardens, which prevents the normal vibration of air waves. If a person has a tendency to cardiovascular diseases. It is likely that blood will circulate faster in damaged vessels, and as a person ages, he will be able to hear extraneous noises in his ears.
Modern medicine has long figured out how the auditory analyzer works and is very successfully working on hearing aids, which allow people over 60 years of age and enable children with defects in the development of the auditory organ to live a full life.
The physiology and operation of the auditory analyzer is very complex, and it is very difficult for people without the appropriate skills to understand it, but in any case, every person should be theoretically familiar.
Now you know how the receptors and sections of the auditory analyzer work.
Topic 3. Physiology and hygiene of sensory systems
Purpose of the lecture– consideration of the essence and significance of the physiology and hygiene of sensory systems.
Keywords - physiology, sensory system, hygiene.
Main questions:
1 Physiology visual system
Perception as a complex systemic process of receiving and processing information is carried out on the basis of the functioning of special sensory systems or analyzers. These systems transform stimuli from the outside world into nerve signals and transmit them to the centers of the brain.
Analyzers as a unified system for analyzing information, consisting of three interconnected departments: peripheral, conductive and central.
The visual and auditory analyzers play special role in cognitive activity.
The age-related dynamics of sensory processes is determined by the gradual maturation of various parts of the analyzer. Receptor apparatuses mature in prenatal period and are more mature at the time of birth. The conductive system and the perceptive apparatus of the projection zone undergo significant changes, which leads to a change in the parameters of the reaction to an external stimulus. In the first months of a child’s life, there is an improvement in the mechanisms of information processing carried out in the projection zone of the cortex, as a result of which the ability to analyze and process a stimulus becomes more complicated. Further changes in the process of processing external signals are associated with the formation of complex nerve networks that determine the formation of the process of perception as a mental function.
1. Physiology of the visual system
The visual sensory system, like any other, consists of three sections:
1 Peripheral section – the eyeball, in particular the retina (receives light stimulation)
2 Conductor section - axons of ganglion cells - optic nerve - optic chiasm - optic tract - diencephalon(geniculate bodies) - midbrain(quadrigeminal) - thalamus
3 Central department- occipital lobe: area of the calcarine sulcus and adjacent gyri
Peripheral division of the visual sensory system.
Optical system of the eye, structure and physiology of the retina
The optical system of the eye includes: the cornea, aqueous humor, iris, pupil, lens and vitreous
The eyeball has spherical shape and is placed in the bone funnel - the orbit. In front it is protected by centuries. Eyelashes grow along the free edge of the eyelid, which protect the eye from dust particles entering it. At the upper outer edge of the orbit there is a lacrimal gland that secretes lacrimal fluid that washes the eye. The eyeball has several membranes, one of which is the outer one - the sclera, or tunica albuginea ( white). In front eyeball it passes into the transparent cornea (refracts light rays)
Under the tunica albuginea is the choroid, consisting of a large number of vessels. In the anterior part of the eyeball, the choroid passes into ciliary body and the iris (iris). It contains pigment that gives color to the eye. It has a round hole - the pupil. Here are the muscles that change the size of the pupil and, depending on this, more or less light enters the eye, i.e. the flow of light is regulated. Behind the iris in the eye is the lens, which is an elastic, transparent biconvex lens surrounded by the ciliary muscle. Its optical function is refraction and focusing of rays, in addition, it is responsible for the accommodation of the eye. The lens can change its shape - become more or less convex and, accordingly, refract light rays stronger or weaker. Thanks to this, a person is able to clearly see objects located at different distances. The cornea and lens have light refractive ability
Behind the lens, the eye cavity is filled with a transparent jelly-like mass - the vitreous body, which transmits light rays and is a light-refracting medium.
Light-conducting and light-refracting media (cornea, aqueous humor, lens, vitreous body) also perform the function of filtering light, transmitting only light rays with a wavelength range from 400 to 760 microns. Wherein ultra-violet rays are retained by the cornea, and infrared - by aqueous humor.
Inner surface The eyes are lined with a thin, structurally complex and most functionally important membrane - the retina. It has two sections: posterior section or visual part and the anterior section - the blind part. The border separating them is called the jagged line. The blind part is adjacent from the inside to the ciliary body and to the iris and consists of two layers of cells:
Inner layer of cubic pigment cells
The outer layer is a layer of prismatic cells lacking the melanin pigment.
The retina (its visual part) contains not only the peripheral part of the analyzer - receptor cells, but also a significant part of its intermediate part. Photoreceptor cells (rods and cones), according to most researchers, are peculiarly modified nerve cells and therefore belong to the primary sensory or neurosensory receptors. The nerve fibers coming from these cells come together to form the optic nerve.
Photoreceptors are rods and cones located in the outer layer of the retina. Rods are more sensitive to color and provide twilight vision. Cones perceive color and color vision.
1.1 Age characteristics of the visual analyzer
In the process of postnatal development, the human visual organs undergo significant morphofunctional changes. For example, the length of the eyeball in a newborn is 16 mm, and its weight is 3.0 g; by the age of 20, these figures respectively increase to 23 mm and 8.0 g. During development, the color of the eyes also changes. In newborns in the first years of life, the iris contains little pigment and has a grayish-bluish tint. The final color of the iris is formed only by the age of 10-12 years.
The process of development and improvement of the visual analyzer, like other sense organs, proceeds from the periphery to the center. Myelination optic nerves ends by 3-4 months of postnatal ontogenesis. Moreover, the development of sensory and motor functions of vision occurs synchronously. In the first days after birth, eye movements are independent of each other. Coordination mechanisms and the ability to fix an object with one’s gaze, figuratively speaking, a “fine-tuning mechanism,” is formed between the ages of 5 days and 3-5 months. Functional maturation of the visual areas of the cerebral cortex, according to some data, occurs already before the birth of a child, according to others, somewhat later.
Accommodation in children is more pronounced than in adults; the elasticity of the lens decreases with age, and accommodation decreases accordingly. In preschoolers, due to more flat shape lens, farsightedness is very common. At 3 years of age, farsightedness is observed in 82% of children, and myopia in 2.5%. With age, this ratio changes and the number of myopic people increases significantly, reaching 11% by the age of 14-16. An important factor What contributes to the appearance of myopia is poor visual hygiene: reading while lying down, doing homework in a poorly lit room, increased eye strain, etc.
During development, a child’s color perceptions change significantly. In a newborn, only rods function in the retina; cones are still immature and their number is small. Elementary functions Newborns apparently have color perception, but the full involvement of cones in their work occurs only by the end of the 3rd year of life. However, at this age stage it is still incomplete. The sense of color reaches its maximum development by the age of 30 and then gradually decreases. Training is important for the formation of this ability. With age, visual acuity also increases and stereoscopic vision improves. Stereoscopic vision changes most intensively up to 9-10 years of age and reaches its optimal level by 17-22 years of age. From the age of 6, girls have higher stereoscopic visual acuity than boys. The eye level of girls and boys aged 7-8 years is significantly better than that of preschoolers, and has no gender differences, but is approximately 7 times worse than that of adults.
The field of view develops especially intensively in preschool age, and by 7 years it is approximately 80% of the size of an adult’s visual field. Sexual characteristics are observed in the development of the visual field. In subsequent years, the size of the visual field is compared, and from the age of 13-14 years, its size in girls is larger. The specified age and gender characteristics of the development of the visual field should be taken into account when organizing the education of children and adolescents, since the visual field determines the volume educational information perceived by the child, i.e. the bandwidth of the visual analyzer.
The auditory analyzer consists of three sections:
1. Peripheral section including the outer, middle and inner ear
2. Conducting section - axons of bipolar cells - cochlear nerve - nuclei medulla oblongata- internal geniculate body - auditory cortex area cerebral hemispheres
3. Central department – temporal lobe
Structure of the ear. Outer ear includes the auricle and external ear canal. Its function is to capture sound vibrations. Middle ear.
Rice. 1. Semi-schematic representation of the middle ear: 1 - external auditory canal", 2 - tympanic cavity; 3 - auditory tube; 4 - tympanic membrane; 5 - malleus; 6 - incus; 7 - stapes; 8 - window of the vestibule (oval); 9 - cochlear window (round); 10 - bone tissue.
The middle ear is separated from the outer ear by the eardrum, and from the inner ear by a bony septum with two holes. One of them is called the oval window or the window of the vestibule. The base of the stapes is attached to its edges with the help of an elastic annular ligament. The other hole, the round window, or cochlear window, is covered with a thin connective tissue membrane. Inside tympanic cavity There are three auditory ossicles - the malleus, the incus and the stapes, connected by joints.
Airborne sound waves entering the ear canal cause vibrations in the eardrum, which are transmitted through the system of auditory ossicles, as well as through the air in the middle ear, to the perilymph of the inner ear. The auditory ossicles articulated with each other can be considered as a lever of the first kind, the long arm of which is connected to the tympanic membrane, and the short arm is fixed in the oval window. When transferring movement from a long to a short arm, the range (amplitude) decreases due to an increase in the force developed. A significant increase in the strength of sound vibrations also occurs because the surface of the base of the stapes is many times smaller than the surface of the eardrum. In general, the strength of sound vibrations increases by at least 30-40 times.
With powerful sounds, due to contraction of the muscles of the tympanic cavity, the tension of the eardrum increases and the mobility of the base of the stapes decreases, which leads to a decrease in the force of transmitted vibrations.
Receptor (peripheral) section of the auditory analyzer, converting the energy of sound waves into energy nervous excitement, represented by receptor hair cells of the organ of Corti (Corti organ) located in the cochlea. Auditory receptors (phonoreceptors) belong to the mechanoreceptors, are secondary and are represented by inner and outer hair cells. Humans have approximately 3,500 inner and 20,000 outer hair cells, which are located on the basilar membrane inside the middle canal of the inner ear.
Rice. 2.6. Hearing organ
The inner ear (sound-receiving apparatus), as well as the middle ear (sound-transmitting apparatus) and the outer ear (sound-receiving apparatus) are combined into the concept organ of hearing (Fig. 2.6).
Outer ear Due to the auricle, it ensures the capture of sounds, their concentration in the direction of the external auditory canal and an increase in the intensity of sounds. In addition, the structures of the outer ear perform a protective function, protecting the eardrum from mechanical and temperature influences of the external environment.
Middle ear(sound-conducting section) is represented by the tympanic cavity, where three auditory ossicles are located: the malleus, the incus and the stapes. The middle ear is separated from the external auditory canal by the eardrum. The handle of the malleus is woven into the eardrum, its other end is articulated with the incus, which, in turn, is articulated with the stapes. The stirrup is adjacent to the membrane oval window. The middle ear has a special defense mechanism, represented by two muscles: the muscle that tightens the eardrum and the muscle that fixes the stapes. The degree of contraction of these muscles depends on the strength of sound vibrations. With strong sound vibrations, the muscles limit the amplitude of vibration of the eardrum and the movement of the stapes, thereby protecting the receptor apparatus in the inner ear from excessive stimulation and destruction. In case of instantaneous strong irritation (strike of a bell), this protective mechanism does not have time to operate. The contraction of both muscles of the tympanic cavity is carried out according to the mechanism unconditioned reflex, which closes at the level of the brain stem. The pressure in the tympanic cavity is equal to atmospheric pressure, which is very important for adequate perception of sounds. This function is performed by the Eustachian tube, which connects the middle ear cavity to the pharynx. When swallowing, the tube opens, ventilating the cavity of the middle ear and equalizing the pressure in it with atmospheric pressure. If external pressure changes quickly (rapid rise to altitude), but swallowing does not occur, then the pressure difference between atmospheric air and air in the tympanic cavity leads to tension of the eardrum and the appearance discomfort, decreased perception of sounds.
Inner ear represented by the cochlea - a spirally twisted bone canal with 2.5 turns, which is divided by the main membrane and the Reissner membrane into three narrow parts (staircases). The superior canal (scala vestibularis) starts from the oval window and connects to the inferior canal (scala tympani) through the helicotrema (hole in the apex) and ends with the round window. Both canals are a single whole and are filled with perilymph, similar in composition to cerebrospinal fluid. Between the upper and lower channels there is a middle one (middle staircase). It is isolated and filled with endolymph. Inside the middle channel on the main membrane there is the actual sound-receiving apparatus - the organ of Corti (organ of Corti) with receptor cells, representing the peripheral part of the auditory analyzer.
The main membrane near the oval window is 0.04 mm in width, then towards the apex it gradually expands, reaching 0.5 mm at the helicotrema.
Wiring department The auditory analyzer is represented by a peripheral bipolar neuron located in the spiral ganglion of the cochlea (the first neuron). Fibers of the auditory (or cochlear) nerve, formed by axons neurons of the spiral ganglion end on the cells of the nuclei of the cochlear complex of the medulla oblongata (second neuron). Then, after partial decussation, the fibers go to the medial geniculate body of the metathalamus, where switching occurs again (third neuron), from here the excitation enters the cortex (fourth neuron). In the medial (internal) geniculate bodies, as well as in the lower tuberosities of the quadrigeminal, there are centers of reflex motor reactions that occur when exposed to sound.
Central, or cortical, department auditory analyzer is located in the upper part of the temporal lobe big brain(superior temporal gyrus, areas 41 and 42 according to Brodmann). Important for the function of the auditory analyzer are transverse temporal gyri(Heschl's convolutions).
Auditory sensory system complemented by feedback mechanisms that provide regulation of the activity of all levels of the auditory analyzer with the participation of descending pathways. Such pathways begin from the cells of the auditory cortex, switching sequentially in the medial geniculate bodies of the metathalamus, the posterior (inferior) colliculus, and in the nuclei of the cochlear complex. As part of the auditory nerve, centrifugal fibers reach the hair cells of the organ of Corti and tune them to perceive certain sound signals.
Human hearing is designed to pick up a wide range of sound waves and convert them into electrical impulses to be sent to the brain for analysis. Unlike those associated with the organ of hearing vestibular apparatus, working normally almost from birth, hearing takes a long time to develop. The formation of the auditory analyzer ends no earlier than at 12 years of age, and the greatest hearing acuity is achieved by the age of 14-19 years. the auditory analyzer has three sections: the peripheral or organ of hearing (ear); conductive, including nerve pathways; cortical, located in temporal lobe brain. Moreover, there are several auditory centers in the cerebral cortex. Some of them (the inferior temporal gyri) are designed to perceive simpler sounds - tones and noises, others are associated with the most complex sound sensations that arise when a person speaks, listens to speech or music.
The structure of the human ear The human auditory analyzer perceives sound waves with an oscillation frequency of 16 to 20 thousand per second (16-20000 hertz, Hz). The upper sound threshold for an adult is 20,000 Hz; lower threshold – ranging from 12 to 24 Hz. Children have higher upper limit hearing in the region of 22000 Hz; in older people, on the contrary, it is usually lower - about 15,000 Hz. The ear is most sensitive to sounds with frequencies ranging from 1000 to 4000 Hz. Below 1000 Hz and above 4000 Hz, the excitability of the hearing organ is greatly reduced. The ear is a complex vestibular-auditory organ. Like all our sense organs, the human hearing organ performs two functions. It perceives sound waves and is responsible for the position of the body in space and the ability to maintain balance. This paired organ, which is located in the temporal bones of the skull, limited externally by the auricles. The receptor apparatus of the auditory and vestibular systems are located in the inner ear. The structure of the vestibular system can be viewed separately, but now let’s move on to a description of the structure of the parts of the hearing organ.
The organ of hearing consists of 3 parts: the outer, middle and inner ear, with the outer and middle ear playing the role of a sound-conducting apparatus, and the inner ear - a sound-receiving apparatus. The process begins with sound - the oscillatory movement of air or vibration in which sound waves travel towards the listener, eventually reaching the eardrum. At the same time, our ear is extremely sensitive and can sense pressure changes of only 1-10 atmospheres.
Structure of the external ear The external ear consists of the auricle and the external auditory canal. First, sound reaches the ears, which act as receivers of sound waves. The auricle is formed by elastic cartilage, covered on the outside with skin. Determining the direction of sound in a person is associated with binaural hearing, that is, hearing with two ears. Any lateral sound reaches one ear before the other. The difference in time (several fractions of a millisecond) of arrival of sound waves perceived by the left and right ears makes it possible to determine the direction of the sound. In other words, our natural perception of sound is stereophonic.
The human auricle has its own unique relief of convexities, concavities and grooves. This is necessary for the finest acoustic analysis, also allowing you to recognize the direction and source of sound. The folds of the human auricle introduce small frequency distortions into the sound entering the ear canal, depending on the horizontal and vertical localization of the sound source. Thus, the brain receives Additional information to clarify the location of the sound source. This effect is sometimes used in acoustics, including to create a sense of surround sound when designing speakers and headphones. The auricle also amplifies sound waves, which then enter the external auditory canal - the space from the concha to the eardrum about 2.5 cm long and about 0.7 cm in diameter. The auditory canal has a weak resonance at a frequency of about 3000 Hz.
One more interesting characteristic external auditory canal is the presence of earwax, which is constantly secreted from the glands. Earwax- waxy secretion of 4000 sebaceous and sulfur glands of the auditory canal. Its function is to protect the skin of this passage from bacterial infection and foreign particles or, for example, insects that may get into the ear. U different people the amount of sulfur varies. If there is an excessive accumulation of sulfur, a sulfur plug may form. If the ear canal is completely blocked, there is a feeling of ear congestion and decreased hearing, including the resonance of one’s own voice in the blocked ear. These disorders develop suddenly, most often when water gets into the external auditory canal while swimming.
The outer and middle ears are separated by the eardrum, which is a thin connective tissue plate. The thickness of the eardrum is about 0.1 mm, and the diameter is about 9 millimeters. On the outside it is covered with epithelium, and on the inside with mucous membrane. The eardrum is located obliquely and begins to vibrate when sound waves hit it. The eardrum is extremely sensitive, but once vibration is detected and transmitted, the eardrum returns to its original position in just 0.005 seconds.
The structure of the middle ear In our ear, sound moves to the sensitive cells that perceive sound signals through a matching and amplifying device - the middle ear. The middle ear is a tympanic cavity, which has the shape of a small flat drum with a tightly stretched vibrating membrane and an auditory (Eustachian) tube. In the cavity of the middle ear there are auditory ossicles that articulate with each other - the hammer, incus and stapes. Tiny muscles help transmit sound by regulating the movement of these ossicles. When the sound reaches the eardrum, it vibrates. The handle of the hammer is woven into the eardrum and, by swaying, it sets the hammer in motion. The other end of the malleus is connected to the incus, and the latter is movably articulated with the stapes using a joint. Attached to the stapes is the stapedius muscle, which holds it against the membrane of the oval window (vestibulary window), which separates the middle ear from the inner ear, which is filled with fluid. As a result of the transmission of movement, the stapes, the base of which resembles a piston, is constantly pushed into the membrane of the oval window of the inner ear.
The function of the auditory ossicles is to provide an increase in the pressure of the sound wave when transmitted from the eardrum to the membrane of the oval window. This amplifier (about 30-40 times) helps weak sound waves incident on the eardrum overcome the resistance of the oval window membrane and transmit vibrations to the inner ear. When a sound wave passes from air to liquid, a significant part of the sound energy is lost and, therefore, a sound amplification mechanism is necessary. However, when loud sound the same mechanism reduces the sensitivity of the entire system so as not to damage it.
The air pressure inside the middle ear must be the same as the pressure outside the eardrum to ensure normal vibration conditions. To equalize pressure, the tympanic cavity is connected to the nasopharynx using the auditory (Eustachian) tube, 3.5 cm long and about 2 mm in diameter. When swallowing, yawning, and chewing, the Eustachian tube opens to let in outside air. When external pressure changes, the ears sometimes become blocked, which is usually resolved by yawning reflexively. Experience shows that ear congestion is solved even more effectively by swallowing movements. Malfunction of the tube leads to pain and even bleeding in the ear.
Structure of the inner ear. The mechanical movements of the bones in the inner ear are converted into electrical signals. Inner ear - hollow bone formation in the temporal bone, divided into bone canals and cavities containing the receptor apparatus of the auditory analyzer and the organ of balance. Because of its intricate shape, this section of the organ of hearing and balance is called the labyrinth. The bony labyrinth consists of the vestibule, cochlea and semicircular canals, but only the cochlea is directly related to hearing. The cochlea is a canal about 32 mm long, coiled and filled with lymphatic fluids. Having received vibration from the eardrum, the stapes, with its movement, presses on the membrane of the vestibule window and creates pressure fluctuations inside the cochlear fluid. This vibration travels through the fluid of the cochlea and reaches the organ of hearing itself, the spiral or organ of Corti. It turns the vibrations of the liquid into electrical signals that go through the nerves to the brain. In order for the stapes to transmit pressure through the fluid, in the central part of the labyrinth, the vestibule, there is a round window of the cochlea, covered with a flexible membrane. When the piston of the stapes enters the oval window of the vestibule, the membrane of the cochlear window bulges under the pressure of the cochlear fluid. Oscillations in a closed cavity are possible only in the presence of recoil. The role of such return is performed by the membrane of the round window.
The bony labyrinth of the cochlea is wrapped in the shape of a spiral with 2.5 turns and contains inside a membranous labyrinth of the same shape. In some places, the membranous labyrinth is attached to the periosteum of the bony labyrinth by connecting cords. Between the bony and membranous labyrinth there is a fluid - perilymph. The sound wave, amplified by 30-40 dB using the eardrum - auditory ossicles system, reaches the window of the vestibule, and its vibrations are transmitted to the perilymph. The sound wave first passes through the perilymph to the top of the spiral, where through the hole the vibrations propagate to the window of the cochlea. Inside, the membranous labyrinth is filled with another fluid - endolymph. The fluid inside the membranous labyrinth (cochlear duct) is separated from the perilymph above by a flexible covering plate, and below by an elastic main membrane, which together make up the membranous labyrinth. On the main membrane there is a sound-receiving apparatus, the organ of Corti. The main membrane consists of a large number (24,000) fibrous fibers of various lengths, stretched like strings. These fibers form an elastic network, which as a whole resonates in strictly graded vibrations.
Nerve cells The organ of Corti converts the oscillatory movements of the plates into electrical signals. They are called hair cells. Inner hair cells are arranged in one row, there are 3.5 thousand of them. Outer hair cells are arranged in three to four rows, there are 12–20 thousand of them. Each hair cell has an elongated shape, it has 60–70 tiny hairs (stereocilia) 4–5 µm long.
All sound energy is concentrated in the space limited by the wall of the bony cochlea and the main membrane (the only pliable place). The fibers of the main membrane have different lengths and, accordingly, different resonant frequencies. The shortest fibers are located near the oval window, their resonant frequency is about 20,000 Hz. The longest ones are at the top of the spiral and have a resonant frequency of about 16 Hz. It turns out that each hair cell, depending on its location on the main membrane, is tuned to a certain audio frequency, with cells tuned to low frequencies located in the upper part of the cochlea, and high frequencies are picked up by cells in the lower part of the cochlea. When hair cells die for some reason, a person loses the ability to perceive sounds of the corresponding frequencies.
The sound wave propagates through the perilymph from the window of the vestibule to the window of the cochlea almost instantly, in about 4 * 10-5 seconds. The hydrostatic pressure caused by this wave shifts the covering plate relative to the surface of the organ of Corti. As a result, the integumentary plate deforms the bundles of stereocilia of the hair cells, which leads to their excitation, which is transmitted to the endings of the primary sensory neurons.
Differences in the ionic composition of endolymph and perilymph create a potential difference. And between the endolymph and the intracellular environment of the receptor cells, the potential difference reaches approximately 0.16 volts. Such a significant potential difference contributes to the excitation of hair cells even under the influence of weak sound signals, causing slight vibrations of the main membrane. When the stereocilia of hair cells are deformed, a receptor potential arises in them, which leads to the release of a regulator that acts on the endings of the auditory nerve fibers and thereby excites them.
Hair cells are connected to the endings of nerve fibers that, upon exiting the organ of Corti, form the auditory nerve (cochlear branch of the vestibulocochlear nerve). Sound waves converted into electrical impulses are transmitted along the auditory nerve to temporal zone cerebral cortex.
The auditory nerve consists of thousands of tiny nerve fibers. Each of them starts from a certain part of the cochlea and, thereby, transmits a certain sound frequency. Each fiber of the auditory nerve is associated with several hair cells, so that about 10,000 fibers enter the central nervous system. Impulses from low-frequency sounds are transmitted through fibers emanating from the top of the cochlea, and from high-frequency sounds - through fibers connected to its base. Thus, the function of the inner ear is to convert mechanical vibrations into electrical ones, since the brain can only perceive electrical signals.
The organ of hearing is the apparatus through which we receive sound information. But we hear the way our brain perceives, processes and remembers. Sound ideas or images are created in the brain. And, if music sounds in our head or someone’s voice is remembered, then due to the fact that the brain has input filters, a storage device and a sound card, it can be both a boring speaker and a convenient music center for us.
PHYSIOLOGY OF THE HEARING ANALYZER
(Auditory sensory system)
Lecture questions:
1. Structural and functional characteristics of the auditory analyzer:
a. Outer ear
b. Middle ear
c. Inner ear
2. Divisions of the auditory analyzer: peripheral, conductive, cortical.
3. Perception of height, sound intensity and sound source location:
a. Basic electrical phenomena in the cochlea
b. Perception of sounds of different pitches
c. Perception of sounds varying intensity
d. Determining the sound source ( binaural hearing)
e. Auditory adaptation
1. The auditory sensory system is the second most important distant human analyzer, plays important role specifically in humans in connection with the emergence of articulate speech.
Hearing analyzer function: transformation sound waves into the energy of nervous excitation and auditory sensation.
Like any analyzer, the auditory analyzer consists of a peripheral, conductive and cortical section.
PERIPHERAL DEPARTMENT
Converts the energy of sound waves into energy nervous excitation – receptor potential (RP). This department includes:
· inner ear (sound-receiving apparatus);
· middle ear (sound-conducting apparatus);
· outer ear (sound-collecting apparatus).
The components of this department are combined into the concept organ of hearing.
Functions of the organs of hearing
Outer ear:
a) collecting sound (auricle) and directing the sound wave into the external auditory canal;
b) conducting a sound wave through the ear canal to the eardrum;
c) mechanical and temperature protection environment all other parts of the hearing organ.
Middle ear(sound-conducting section) is the tympanic cavity with 3 auditory ossicles: the malleus, the incus and the stapes.
The eardrum separates the external auditory canal from the tympanic cavity. The handle of the malleus is woven into the eardrum, its other end is articulated with the incus, which, in turn, is articulated with the stapes. The stapes is adjacent to the membrane of the oval window. The pressure in the tympanic cavity is equal to atmospheric pressure, which is very important for adequate perception of sounds. This function is performed by the Eustachian tube, which connects the middle ear cavity to the pharynx. When swallowing, the tube opens, resulting in ventilation of the tympanic cavity and equalization of the pressure in it with atmospheric pressure. If external pressure changes rapidly (rapid rise to altitude), and swallowing does not occur, then the pressure difference between atmospheric air and air in the tympanic cavity leads to tension of the eardrum and the occurrence of unpleasant sensations (“stuck ears”), and a decrease in the perception of sounds.
The area of the eardrum (70 mm2) is significantly more area oval window (3.2 mm 2), due to which it occurs gain the pressure of sound waves on the membrane of the oval window is 25 times. Lever mechanism of bones reduces the amplitude of sound waves is 2 times, so the same amplification of sound waves occurs at the oval window of the tympanic cavity. Consequently, the middle ear amplifies sound by about 60-70 times, and if we take into account the amplifying effect of the outer ear, then this value increases by 180-200 times. In this regard, in case of strong sound vibrations, to prevent destructive action sound on the receptor apparatus of the inner ear, the middle ear reflexively turns on the “protective mechanism”. It consists of the following: in the middle ear there are 2 muscles, one of them stretches the eardrum, the other fixes the stapes. Under strong sound impacts, these muscles, when contracting, limit the amplitude of vibration of the eardrum and fix the stapes. This “quenches” the sound wave and protects overexcitement and destruction of phonoreceptors of the organ of Corti.
Inner ear: represented by the cochlea - a spirally twisted bone canal (2.5 turns in humans). This channel is divided along its entire length into three narrow parts (ladders) with two membranes: the main membrane and the vestibular membrane (Reisner).
Located on the main membrane spiral organ– the organ of Corti (organ of Corti) is the actual sound-receiving apparatus with receptor cells – this is the peripheral section of the auditory analyzer.
The helicotrema (orifice) connects the superior and inferior canals at the apex of the cochlea. The middle channel is separate.
Above the organ of Corti is a tectorial membrane, one end of which is fixed and the other remains free. The hairs of the outer and inner hair cells of the organ of Corti come into contact with the tectorial membrane, which is accompanied by their excitation, i.e. the energy of sound vibrations is transformed into the energy of the excitation process.
Structure of the organ of Corti
The transformation process begins with sound waves entering the outer ear; they move the eardrum. Vibrations of the tympanic membrane through the system of auditory ossicles of the middle ear are transmitted to the membrane of the oval window, which causes vibrations of the perilymph of the scala vestibularis. These vibrations are transmitted through the helicotrema to the perilymph of the scala tympani and reach the round window, protruding it towards the middle ear (this prevents the sound wave from dying out when passing through the vestibular and tympanic canal of the cochlea). Vibrations of the perilymph are transmitted to the endolymph, which causes vibrations of the main membrane. The fibers of the basilar membrane begin to vibrate together with the receptor cells (outer and inner hair cells) of the organ of Corti. In this case, the phonoreceptor hairs come into contact with the tectorial membrane. The cilia of the hair cells are deformed, this causes the formation of a receptor potential, and on its basis - an action potential ( nerve impulse), which is carried along the auditory nerve and transmitted to the next section of the auditory analyzer.
CONDUCTING DEPARTMENT OF THE HEARING ANALYZER
The conductive section of the hearing analyzer is presented auditory nerve. It is formed by the axons of neurons of the spiral ganglion (1st neuron of the pathway). The dendrites of these neurons innervate the hair cells of the organ of Corti (afferent link), the axons form the fibers of the auditory nerve. The auditory nerve fibers end on the neurons of the nuclei of the cochlear body (VIII pair of h.m.n.) (second neuron). Then, after partial decussation, the fibers of the auditory pathway go to the medial geniculate body of the thalamus, where switching occurs again (third neuron). From here, excitation enters the cortex (temporal lobe, superior temporal gyrus, transverse gyri of Heschl) - this is the projection auditory zone of the cortex.
CORTICAL DIVISION OF THE AUDITORY ANALYZER
Presented in the temporal lobe of the cerebral cortex - superior temporal gyrus, transverse temporal gyri of Heschl. Cortical gnostic auditory zones are associated with this projection zone of the cortex - Wernicke's sensory speech area and praxial zone – Broca's speech motor center(inferior frontal gyrus). The cooperative activity of the three cortical zones ensures the development and function of speech.
The auditory sensory system has feedback connections that provide regulation of the activity of all levels of the auditory analyzer with the participation of descending pathways that start from the neurons of the “auditory” cortex and sequentially switch in the medial geniculate body of the thalamus, the inferior colliculus of the midbrain with the formation of tectospinal descending pathways and on the nuclei cochlear body of the medulla oblongata with the formation of vestibulospinal tracts. This ensures, in response to the action of a sound stimulus, the formation of a motor reaction: turning the head and eyes (and in animals, the ears) towards the stimulus, as well as increasing the tone of the flexor muscles (flexion of the limbs in the joints, i.e. readiness to jump or run ).
Auditory cortex
PHYSICAL CHARACTERISTICS OF SOUND WAVES THAT ARE PERCEIVED BY THE HEARING ORGAN
1. The first characteristic of sound waves is their frequency and amplitude.
The frequency of sound waves determines the pitch of the sound!
A person distinguishes sound waves with a frequency from 16 to 20,000 Hz (this corresponds to 10-11 octaves). Sounds whose frequency is below 20 Hz (infrasounds) and above 20,000 Hz (ultrasounds) by humans not felt!
Sound that consists of sinusoidal or harmonic vibrations is called tone(high frequency - high tone, low frequency - low tone). A sound consisting of unrelated frequencies is called noise.
2. The second characteristic of sound that the auditory sensory system distinguishes is its strength or intensity.
The strength of sound (its intensity) together with the frequency (tone of sound) is perceived as volume. The unit of loudness measurement is bel = lg I/I 0, but in practice it is more often used decibel (dB)(0.1 bel). A decibel is 0.1 decimal logarithm of the ratio of sound intensity to its threshold intensity: dB = 0.1 log I/I 0. Maximum volume level when sound causes painful sensations, equal to 130-140 dB.
The sensitivity of the auditory analyzer is determined by the minimum sound intensity that causes auditory sensations.
In the range of sound vibrations from 1000 to 3000 Hz, which corresponds to human speech, the ear has the greatest sensitivity. This set of frequencies is called speech zone(1000-3000 Hz). Absolute sound sensitivity in this range is 1*10 -12 W/m2. For sounds above 20,000 Hz and below 20 Hz, absolute hearing sensitivity decreases sharply - 1*10 -3 W/m2. In the speech range, sounds are perceived that have a pressure of less than 1/1000 of a bar (a bar is equal to 1/1,000,000 of the normal atmospheric pressure). Based on this, in transmitting devices, in order to ensure adequate understanding of speech, information must be transmitted in the speech frequency range.
MECHANISM OF PERCEPTION OF HEIGHT (FREQUENCY), INTENSITY (STRENGTH) AND LOCALIZATION OF SOUND SOURCE (BINAURAL HEARING)
Perception of sound wave frequency
