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The ear and the passage of sound through it. Ear and its function

Hearing organs

The process involves the perception, transmission and interpretation of sound. The ear captures and transforms auditory waves into nerve impulses, which are received and interpreted by the brain.

There is a lot in the ear that is not visible to the eye. What we observe is only part of the outer ear - a fleshy-cartilaginous outgrowth, in other words, the auricle. The outer ear consists of the concha and the ear canal, ending at the eardrum, which provides communication between the outer and middle ear, where the hearing mechanism is located.

The pinna directs sound waves into the ear canal, much like the ancient Eustachian trumpet directed sound into the pinna. The channel amplifies sound waves and directs them to the eardrum. Sound waves hitting the eardrum cause vibrations that are transmitted through three small auditory bones: the malleus, the incus and the stapes. They vibrate in turn, transmitting sound waves through the middle ear. The innermost of these bones, the stapes, is the smallest bone in the body.

The stapes vibrates and strikes a membrane called the oval window. Sound waves travel through it to the inner ear.

What happens in the inner ear?

There is a sensory part of the auditory process. The inner ear consists of two main parts: the labyrinth and the cochlea. The part, which starts at the oval window and curves like a real cochlea, acts as a translator, turning sound vibrations into electrical impulses that can be transmitted to the brain.

How does a snail work?

It is filled with liquid, in which the basilar (main) membrane is suspended, resembling a rubber band, attached at the ends to the walls. The membrane is covered with thousands of tiny hairs. At the base of these hairs are small nerve cells. When the vibrations of the stapes touch the oval window, the fluid and hairs begin to move. The movement of the hairs stimulates nerve cells, which send a message, in the form of an electrical impulse, to the brain through the auditory, or acoustic, nerve.

The labyrinth is a group of three interconnected semicircular canals that control the sense of balance. Each channel is filled with liquid and located at right angles to the other two. So, no matter how you move your head, one or more channels record that movement and transmit the information to the brain.

If you have ever had a cold in your ear or blown your nose too much, so that your ear “clicks”, then a guess appears - the ear is somehow connected to the throat and nose. And that's true. The Eustachian tube directly connects the middle ear to the oral cavity. Its role is to allow air into the middle ear, balancing the pressure on both sides of the eardrum.

Impairments and disorders in any part of the ear can impair hearing if they affect the passage and interpretation of sound vibrations.

Let's trace the path of the sound wave. It enters the ear through the pinna and is directed through the auditory canal. If the concha is deformed or the canal is blocked, the path of sound to the eardrum is hampered and hearing ability is reduced. If the sound wave successfully reaches the eardrum, but it is damaged, the sound may not reach the auditory ossicles. Any disorder that prevents the ossicles from vibrating will prevent sound from reaching the inner ear. In the inner ear, sound waves cause fluid to pulsate, moving tiny hairs in the cochlea. Damage to the hairs or the nerve cells to which they are connected will prevent the sound vibrations from being converted into electrical vibrations. But when the sound has successfully turned into an electrical impulse, it still has to reach the brain. It is clear that damage to the auditory nerve or brain will affect the ability to hear.

Why do such disorders and damage occur?

There are many reasons, we will discuss them later. But the most common culprits are foreign objects in the ear, infections, ear diseases, other diseases that cause complications in the ears, head injuries, ototoxic (i.e. poisonous to the ear) substances, changes in atmospheric pressure, noise, age-related degeneration. All of this causes two main types of hearing loss.

Hearing loss, causes, treatment, more details... http://www.medefect.ru/lor/#hear

How do we hear

So, we told you about the structure of the human speech organs. You learned how speech is filled with sound using the vocal cords, and you also became familiar with phonemic and diphonic speech patterns.

Humans (and animals) receive the greatest amount of information about the world around them through their eyes and ears. Having a pair of ears provides “stereophonic hearing,” with which a person can quickly determine the direction of a sound source.

The ears sense vibrations in the air and convert them into electrical signals that travel to the brain. As a result of processing using algorithms unknown to us, these signals turn into images. Creating such algorithms for computers is a scientific problem, the solution of which is necessary for the development of truly well-functioning speech recognition systems.

In the remainder of the first chapter, we will learn how the human hearing organs work to allow us to hear speech and various sounds. Studying the inner ear helps researchers understand the mechanisms by which humans are able to recognize speech, although it is not that simple. As we have already said, man discovers many inventions from nature. Such attempts are also being made by specialists in the field of speech synthesis and recognition.

We refer readers interested in anatomical details to. There you will find a complete description of the structure of the ear and all sorts of medical details that go far beyond the scope of our book.

Ear structure

To see the internal structure of the human ear, you need to turn to an anatomical atlas. In Fig. rice. 1-6 we showed a cross-section of the most important parts of the human ear.

Rice. 1-6. Internal structure of the ear

Medical students who have studied anatomy are well aware that the anatomical ear is divided into three parts:

· external ear;

· middle ear;

· inner ear.

Outer ear

You can examine the outer ear yourself using a mirror. It consists of the auricle and the external auditory canal.

Functionally, the outer ear is designed, firstly, to capture and focus sound waves (which is necessary to improve hearing), and, secondly, to protect the middle and inner ear from mechanical damage. As for the conversion of sound vibrations of air into electrical impulses, the outer ear has nothing to do with this process.

Middle ear

The internal structure of the middle ear is shown in Fig. 1-7. The middle ear is hermetically separated from the outer ear by the eardrum. Thus, when water gets into your ear, it may only flood the outer ear, but it will not go further.

The thickness of the eardrum is only 0.1 mm and is easily damaged. Therefore, take your doctors' advice seriously and never insert foreign objects into your ears.

Rice. 1-7. Middle ear

The inner region of the middle ear, called the tympanic cavity, is connected by the Eustachian tube to the nasopharynx. This allows the pressure inside the tympanic cavity to be maintained equal to the external atmospheric pressure.

Air enters the tympanic cavity through the Eustachian tube when a person swallows. When there is a sudden change in external pressure (for example, on an airplane), a pressing sensation appears in the ears. However, take a few sips and the problem will disappear, as the pressure is leveled through the Eustachian tube.

In the tympanic cavity there is a system of so-called auditory ossicles, consisting of the malleus, incus and stirrup. These bones are interconnected into a single moving chain consisting of levers.

The function of the ossicular system is to transmit sound vibrations from the eardrum to the inner ear.

Inner ear

The inner ear is of greatest interest to speech recognition specialists because it is responsible for converting sound vibrations into electrical impulses.

The inner ear is filled with fluid. It consists of two parts: the vestibular apparatus and the cochlea. The snail got its name because of its shape - the snail is coiled, like the shell of an ordinary snail.

The mechanism of functioning of the inner ear is quite complex and is described in. It is important that inside the cochlea there are sensitive hairs that are “connected” to the brain via nerves (Fig. 1-8).

Rice. 1-8. Sensitive hairs inside the cochlea

The cochlea is divided by an elastic septum into two fluid-filled channels. The sensory hairs and nerves mentioned above are located in this septum.

Frequency range of sound vibrations

According to, the human ear perceives sound waves with a length of approximately 1.6 cm to 20 m, which corresponds to a frequency range of 16-20,000 Hz. Animals can hear sounds of lower or higher frequencies. For example, dolphins and bats can communicate using ultrasound, and whales can communicate using infrasound. Therefore, a person does not hear the entire frequency range of sounds produced by these and some other animals.

As for human speech, its frequency range is 300-4000 Hz. It should be noted that speech intelligibility will remain quite satisfactory when this range is limited to 300-2400 Hz. When we were involved in amateur radio communications, we added appropriate bandpass filters to the receivers to improve reception in conditions of interference. It must be said that the frequency range of ordinary telephone channels is also not very wide, but this does not noticeably affect speech intelligibility.

This means that to improve the quality of speech recognition, computer systems can exclude from analysis frequencies that lie outside the range of 300-4000 Hz or even outside the range of 300-2400 Hz.

HEALTHY HEARING IN HEALTHY SKIN.
“I heard a ringing, but I don’t know where it is...”

An audio signal of any nature can be described by a certain set of physical characteristics: frequency, intensity, duration, time structure, spectrum, etc. (Fig. 1). They correspond to certain subjective sensations that arise when the auditory system perceives sounds: volume, pitch, timbre, beats, consonance-dissonance, masking, localization-stereo effect, etc.

Auditory sensations are related to physical characteristics in an ambiguous and nonlinear way, for example, loudness depends on the intensity of the sound, its frequency, spectrum, etc.

Back in the last century, Fechner’s law was established, which confirmed that this relationship is nonlinear: “Sensations are proportional to the ratio of the logarithms of the stimulus.” For example, sensations of a change in volume are primarily associated with a change in the logarithm of intensity, height - with a change in the logarithm of frequency, etc.

He recognizes all the sound information that a person receives from the outside world (it makes up approximately 25% of the total) with the help of the auditory system and the work of the higher parts of the brain, translates it into the world of his sensations, and makes decisions on how to react to it.

Before we begin to study the problem of how the auditory system perceives pitch, let us briefly dwell on the mechanism of operation of the auditory system. Many new and very interesting results have now been obtained in this direction.

The auditory system is a kind of receiver of information and consists of the peripheral part and higher parts of the auditory system. The processes of transformation of sound signals in the peripheral part of the auditory analyzer have been most studied.

Peripheral part

This is an acoustic antenna that receives, localizes, focuses and amplifies the sound signal; - microphone; - frequency and time analyzer; - an analog-to-digital converter that converts an analog signal into binary nerve impulses - electrical discharges.

An overview of the peripheral auditory system is shown in Figure 2. Typically, the peripheral auditory system is divided into three parts: the outer, middle, and inner ear.

The outer ear consists of the pinna and the ear canal, which ends in a thin membrane called the eardrum. The outer ears and head are components of an external acoustic antenna that connects (matches) the eardrum to the external sound field. The main functions of the external ears are binaural (spatial) perception, sound source localization, and amplification of sound energy, especially in the mid- and high-frequency regions. The auditory canal is a curved cylindrical tube 22.5 mm long, which has a first resonant frequency of about 2.6 kHz, so in this frequency region it significantly amplifies the sound signal, and this is where the region of maximum hearing sensitivity is located. The eardrum is a thin film 74 microns thick, shaped like a cone with its tip facing the middle ear. At low frequencies it moves like a piston, at higher frequencies it forms a complex system of nodal lines, which is also important for amplifying the sound.

The middle ear is an air-filled cavity connected to the nasopharynx by the Eustachian tube to equalize atmospheric pressure. When atmospheric pressure changes, air can enter or leave the middle ear, so the eardrum does not respond to slow changes in static pressure - descent and ascent, etc. There are three small auditory ossicles in the middle ear: the malleus, the incus and the stapes. The malleus is attached to the eardrum at one end, and at the other end it is in contact with the incus, which is connected to the stapes with the help of a small ligament. The base of the stapes is connected to the oval window in the inner ear.

The middle ear performs the following functions: matching the impedance of the air environment with the liquid environment of the cochlea of ​​the inner ear; protection from loud sounds (acoustic reflex); amplification (lever mechanism), due to which the sound pressure transmitted to the inner ear is amplified by almost 38 dB compared to that which hits the eardrum.

The inner ear is located in a labyrinth of canals in the temporal bone, and includes the organ of balance (vestibular apparatus) and the cochlea.

The cochlea plays a major role in auditory perception. It is a tube of variable cross-section, coiled three times like a snake's tail. When unfolded, it is 3.5 cm long. Inside, the snail has an extremely complex structure. Along its entire length, it is divided by two membranes into three cavities: the scala vestibule, the median cavity and the scala tympani (Fig. 3). The middle cavity is closed from above by the Reissner membrane, from below by the basilar membrane. All cavities are filled with liquid. The upper and lower cavities are connected through an opening at the apex of the cochlea (helicotrema). In the upper cavity there is an oval window, through which the stapes transmits vibrations to the inner ear, in the lower cavity there is a round window that goes back into the middle ear. The basilar membrane consists of several thousand transverse fibers: length 32 mm, width at the stapes - 0.05 mm (this end is narrow, light and rigid), at the helicotrema - 0.5 mm wide (this end is thicker and softer). On the inner side of the basilar membrane is the organ of Corti, and in it there are specialized auditory receptors - hair cells. In the transverse direction, the organ of Corti consists of one row of inner hair cells and three rows of outer hair cells. A tunnel is formed between them. The auditory nerve fibers cross the tunnel and contact the hair cells.

The auditory nerve is a twisted trunk, the core of which consists of fibers extending from the apex of the cochlea, and the outer layers from its lower sections. Having entered the brainstem, neurons interact with cells at various levels, rising to the cortex and crossing along the way so that auditory information from the left ear comes mainly to the right hemisphere, where emotional information is mainly processed, and from the right ear to the left hemisphere, where semantic information is mainly processed. In the cortex, the main hearing zones are located in the temporal region, and there is constant interaction between both hemispheres.

The general mechanism of sound transmission can be simplified as follows: sound waves pass through the sound channel and excite vibrations of the eardrum. These vibrations are transmitted through the ossicular system of the middle ear to the oval window, which pushes fluid in the upper part of the cochlea (scala vestibule), a pressure impulse arises in it, which causes the fluid to flow from the upper half to the lower half through the scala tympani and helicotrema and puts pressure on the membrane of the round window , causing it to shift in the direction opposite to the movement of the stapes. The movement of fluid causes vibrations of the basilar membrane (traveling wave) (Fig. 4). The transformation of mechanical vibrations of the membrane into discrete electrical impulses of nerve fibers occurs in the organ of Corti. When the basilar membrane vibrates, the cilia on the hair cells bend, and this generates an electrical potential, which causes a flow of electrical nerve impulses that carry all the necessary information about the received sound signal to the brain for further processing and response.

The higher parts of the auditory system (including the auditory cortex) can be considered as a logical processor that identifies (decodes) useful sound signals against a background of noise, groups them according to certain characteristics, compares them with images in memory, determines their information value and makes decisions about response actions.

Dr. Howard Glicksman

Ear and hearing

The soothing sound of a babbling brook; the happy laughter of a laughing child; the growing sound of a troop of marching soldiers. All these and other sounds fill our lives every day and are the result of our ability to hear them. But what exactly is sound and how can we hear it? Read this article and you will get answers to these questions and, moreover, you will understand what logical conclusions can be drawn regarding the theory of macroevolution.

Sound! What are we talking about?

Sound is the sensation we experience when vibrating molecules in the environment (usually air) strike our eardrum. When these changes in air pressure, which are determined by measuring the pressure at the eardrum (middle ear) against time, are plotted against time, a waveform is produced. In general, the louder the sound, the more energy is required to produce it, and the more range changes in air pressure.

Loudness is measured in decibels, using as a starting point a hearing threshold level (that is, a loudness level that may sometimes be just barely audible to the human ear). The loudness scale is logarithmic, which means that any jump from one absolute number to the next, provided it is divisible by ten (and remember that a decibel is just one tenth of a bel), means an increase in order of magnitude by a factor of ten. For example, the hearing threshold level is designated as 0, and normal conversation occurs at approximately 50 decibels, so the loudness difference is 10 raised to the power of 50 and divided by 10, which is equal to 10 to the fifth power, or one hundred thousand times the loudness of the hearing threshold level. Or take, for example, a sound that gives you a strong sensation of pain in your ears and can actually damage your ear. This sound typically occurs at an amplitude of approximately 140 decibels; A sound such as an explosion or a jet plane means a fluctuation in sound intensity that is 100 trillion times the hearing threshold.

The smaller the distance between the waves, that is, the more waves fit in one second of time, the greater the height or the higher frequency audible sound. It is usually measured in cycles per second or hertz (Hz). The human ear is usually capable of hearing sounds whose frequency ranges from 20 Hz to 20,000 Hz. Normal human conversation includes sounds in the frequency range from 120 Hz for men, to about 250 Hz for women. A mid-volume C note played on a piano has a frequency of 256 Hz, while an A note played on an orchestral oboe has a frequency of 440 Hz. The human ear is most sensitive to sounds that have a frequency between 1,000-3,000 Hz.

Concert in three parts

The ear consists of three main sections called the outer, middle and inner ear. Each of these departments performs its own unique function and is necessary for us to hear sounds.

Figure 2.

  1. Outer part of the ear or the pinna of the outer ear acts like your own satellite antenna, which collects and directs sound waves into the external auditory meatus (the part of the ear canal). From here the sound waves travel further down the canal and reach the middle ear, or eardrum, which, by being pulled in and out in response to these changes in air pressure, forms a path for the vibration of the sound source.
  2. The three bones (auditory ossicles) of the middle ear are called hammer, which is directly connected to the eardrum, anvil And stirrup, which is connected to the oval window of the cochlea of ​​the inner ear. Together, these ossicles are involved in transmitting these vibrations to the inner ear. The middle ear is filled with air. By using eustachian tube, which is located just behind the nose and opens during swallowing to allow outside air into the middle ear chamber, it is able to maintain equal air pressure on both sides of the eardrum. Also, the ear has two skeletal muscles: the tensor tympani muscles and the stapedius muscles, which protect the ear from very loud sounds.
  3. In the inner ear, which consists of the cochlea, these transmitted vibrations pass through oval window, which leads to the formation of waves in internal structures snails Located inside the cochlea Organ of Corti, which is the main organ of the ear that is capable of converting these fluid vibrations into a nerve signal, which is then transmitted to the brain, where it is processed.

So that's a general overview. Now let's take a closer look at each of these departments.

What are you saying?

Obviously, the mechanism of hearing begins in the outer ear. If there weren't a hole in our skull that allows sound waves to travel further to the eardrum, we wouldn't be able to talk to each other. Maybe some would like it to be that way! How could this opening in the skull, called the external auditory canal, be the result of a random genetic mutation or random change? This question remains unanswered.

It has been revealed that the outer ear, or, if you please, the auricle, is an important part of sound localization. The underlying tissue that lines the surface of the outer ear and makes it so elastic is called cartilage and is very similar to the cartilage found in most of the ligaments in our body. If one supports a macroevolutionary model of hearing development, it is in order to explain how the cells that are capable of forming cartilage acquired this ability, not to mention how after all this, unfortunately for many young girls, they stretched out from each side head, something like a satisfactory explanation is required.

Those of you who have ever had a plug of wax in your ear can appreciate the fact that, despite the fact that they do not know what benefits this earwax brings to the ear canal, they are certainly glad that this natural substance does not have the consistency cement. Moreover, those who must communicate with these unfortunate people appreciate that they have the ability to raise the volume of their voice in order to produce sufficient sound wave energy to be heard.

Waxy product, commonly called earwax, is a mixture of secretions from various glands, and is contained in the external ear canal and consists of a material that includes cells that are constantly sloughed off. This material extends along the surface of the ear canal and forms a white, yellow or brown substance. Earwax serves to lubricate the external auditory canal and at the same time protects the eardrum from dust, dirt, insects, bacteria, fungi, and anything else that may enter the ear from the external environment.

It is very interesting that the ear has its own cleansing mechanism. The cells that line the external auditory canal are located closer to the center of the eardrum, then extend to the walls of the auditory canal and extend beyond the external auditory canal. Along the entire path of their location, these cells are covered with an ear waxy product, the amount of which decreases as it moves towards the external canal. It turns out that jaw movements enhance this process. In reality, this whole scheme is like one big conveyor belt, the function of which is to remove earwax from the ear canal.

Obviously, to fully understand the process of earwax formation, its consistency that allows us to hear well and which at the same time serves a sufficient protective function, and how the ear canal itself removes this earwax to prevent hearing loss, some logical explanation is required . How could simple gradual evolutionary developments, resulting from genetic mutation or random change, be the cause of all these factors and, despite this, ensure the correct functioning of this system throughout its entire existence?

The eardrum is made up of a special tissue whose consistency, shape, attachments, and precise placement allow it to be in a precise location and perform a precise function. All of these factors must be taken into account when explaining how the eardrum is able to resonate in response to incoming sound waves, thereby starting a chain reaction that results in an oscillatory wave within the cochlea. And just because other organisms have somewhat similar structural features that allow them to hear does not in itself explain how all these features appeared with the help of undirected natural forces. I am reminded here of a witty remark made by G. K. Chesterton, where he said: “It would be absurd for an evolutionist to complain and say that it is simply improbable for an admittedly inconceivable God to create 'everything' from 'nothing' and then claim that that 'nothing' itself has become 'everything' is more probable.” However, I have deviated from our topic.

Correct vibrations

The middle ear serves to transmit vibrations from the eardrum to the inner ear, where the organ of Corti is located. Just as the retina is the “organ of the eye,” the organ of Corti is the true “organ of the ear.” Therefore, the middle ear is actually a “mediator” that participates in the auditory process. As often happens in business, the intermediary always has something and thus reduces the financial efficiency of the transaction that is being concluded. Similarly, transmission of vibration from the eardrum through the middle ear results in little energy loss, resulting in only 60% of the energy being conducted through the ear. However, if it were not for the energy that is distributed to the larger tympanic membrane, which is mounted on the smaller oval window by the three auditory ossicles, together with their specific balancing action, this energy transfer would be much less and we would have a much more difficult time hear.

The outgrowth of part of the malleus (the first auditory ossicle), which is called lever, attached directly to the eardrum. The malleus itself is connected to the second auditory ossicle, the incus, which in turn is attached to the stapes. The stirrup has flat part, which is attached to the oval window of the cochlea. As we have already said, the balancing actions of these three interconnected bones allow vibrations to be transmitted to the cochlea of ​​the middle ear.

A review of my two previous sections, namely, “Hamlet Acquainted with Modern Medicine, Parts I and II,” may allow the reader to see what needs to be understood regarding bone formation itself. How these three perfectly formed and interconnected bones were placed in the exact position that ensures the correct transmission of the sound wave vibration requires another “same” explanation of macroevolution, which we must look at with a grain of salt.

It is interesting to note that inside the middle ear there are two skeletal muscles, the tensor tympani muscles and the stapedius muscles. The tensor tympani muscle is attached to the handle of the malleus and when contracted it pulls the eardrum back into the middle ear, thereby limiting its ability to resonate. The stapedius muscle ligament is attached to the flat part of the stapes and when it contracts it pulls away from the oval window, thus reducing the vibration that is transmitted through the cochlea.

Together, these two muscles reflexively try to protect the ear from sounds that are too loud, which can cause pain and even damage it. The time it takes for the neuromuscular system to respond to a loud sound is about 150 milliseconds, which is approximately 1/6 of a second. Therefore, the ear is not as protected from sudden loud sounds, such as artillery fire or explosions, compared to prolonged sounds or noisy environments.

Experience shows that sometimes sounds can cause pain, as can too bright light. The functional components of hearing, such as the eardrum, ossicles, and organ of Corti, perform their function by moving in response to sound wave energy. Moving too much can cause damage or pain, as can if you overuse your elbows or knees. Therefore, it seems that the ear has some kind of protection against self-damage that can occur with prolonged loud sounds.

A review of my three previous sections, namely “More than Just Sound, Parts I, II and III,” which deal with neuromuscular function at the bimolecular and electrophysiological levels, will enable the reader to better understand the specific complexity of the mechanism that is the natural defense against hearing loss. It remains only to understand how these ideally located muscles ended up in the middle ear and began to perform the function that they perform and do this reflexively. What genetic mutation or random change occurred once in time that led to such complex development within the temporal bone of the skull?

Those of you who have been on board an airplane and experienced a feeling of pressure on your ears during landing, which is accompanied by decreased hearing and the feeling that you are speaking into space, have actually become convinced of the importance of the Eustachian tube (auditory tube), which is located between the middle ear and the back of the nose.

The middle ear is a closed, air-filled chamber in which the air pressure on all sides of the eardrum must be equal in order to provide sufficient mobility, which is called distensibility of the eardrum. Distensibility determines how easily the eardrum moves when stimulated by sound waves. The higher the distensibility, the easier it is for the eardrum to resonate in response to sound, and accordingly the lower the distensibility, the more difficult it is to move back and forth and, therefore, the threshold at which a sound can be heard rises, that is, sounds must be louder in order to they could be heard.

Air in the middle ear is usually absorbed by the body, resulting in decreased air pressure in the middle ear and decreased distensibility of the eardrum. This occurs as a result of the fact that, instead of remaining in the correct position, the eardrum is pushed into the middle ear by external air pressure that acts on the external auditory canal. All this is a result of the external pressure being higher than the pressure in the middle ear.

The Eustachian tube connects the middle ear to the back of the nose and pharynx.

During swallowing, yawning or chewing, the Eustachian tube opens due to the action of the associated muscles, due to which outside air enters and passes into the middle ear and replaces the air that was absorbed by the body. In this way, the eardrum can maintain its optimal distensibility, which provides us with sufficient hearing.

Now let's get back to the plane. At 35,000 feet, the air pressure on both sides of the eardrum is equal, although the absolute volume is less than it would be at sea level. What is important here is not the air pressure itself, which acts on both sides of the eardrum, but that no matter how much air pressure acts on the eardrum, it is the same on both sides. As the plane begins to descend, the external air pressure in the cabin begins to rise and immediately acts on the eardrum through the external auditory canal. The only way to correct this air pressure imbalance across the eardrum is to be able to open the Eustachian tube to allow in new external air pressure. This usually occurs when chewing gum or sucking on candy and swallowing, which is when force is applied to the pipe.

The speed at which the plane descends and the rapidly changing increases in air pressure cause some people to feel fullness in their ears. Additionally, if the passenger has a cold or has recently had a cold, if they have a sore throat or runny nose, their Eustachian tube may not function during these pressure changes and they may experience severe pain, prolonged congestion and, occasionally, severe bleeding in the middle ear!

But the dysfunction of the Eustachian tube does not end there. If any of the passengers have a chronic illness, over time the vacuum effect in the middle ear can draw fluid out of the capillaries, which can lead (if not sought medical attention) to a condition called exudative otitis media. This disease can be prevented and treated with myringotomy and tube insertion. The otolaryngologist-surgeon makes a small hole in the eardrum and inserts tubes so that the fluid that is in the middle ear can flow out. These tubes replace the Eustachian tube until the cause of this condition is eliminated. Thus, this procedure preserves adequate hearing and prevents damage to the internal structures of the middle ear.

It is great that modern medicine can solve some of these problems with Eustachian tube dysfunction. But the question immediately arises: how did this tube originally appear, what parts of the middle ear formed first, and how did these parts function without all the other necessary parts? Thinking about this, is it possible to think about multi-stage development based on hitherto unknown genetic mutations or random changes?

A careful consideration of the constituent parts of the middle ear and their absolute necessity for the production of sufficient hearing so necessary for survival shows that we have before us a system of irreducible complexity. But nothing we have considered so far can give us the ability to hear. There is one major component to this entire puzzle that needs to be considered, which itself is an example of irreducible complexity. This remarkable mechanism takes vibrations from the middle ear and converts them into a nerve signal that travels to the brain, where it is then processed. This main component is the sound itself.

Sound conduction system

The nerve cells that are responsible for transmitting signals to the brain for hearing are located in the “Organ of Corti,” which is located in the cochlea. The cochlea consists of three interconnected tubular channels, which are rolled approximately two and a half times into a coil.

(see figure 3). The upper and lower canals of the cochlea are surrounded by bone and are called scala vestibule (superior canal) and accordingly drum ladder(lower channel). Both of these channels contain a fluid called perilymph. The composition of sodium (Na+) and potassium (K+) ions in this fluid is very similar to that of other extracellular fluids (outside cells), that is, they have a high concentration of Na+ ions and a low concentration of K+ ions, unlike intracellular fluids (inside cells).


Figure 3.

The canals communicate with each other at the top of the cochlea through a small opening called helicotrema.

The middle channel that enters the membrane tissue is called middle staircase and consists of a liquid called endolymph. This fluid has a unique property, as it is the only extracellular fluid of the body with a high concentration of K+ ions and a low concentration of Na+ ions. The scala media is not directly connected to the other canals and is separated from the scala vestibuli by an elastic tissue called Reissner's membrane and from the scala tympani by an elastic basilar membrane (see Figure 4).

The organ of Corti is suspended, like the Golden Gate Bridge, on the basilar membrane, which is located between the scala tympani and the scala media. Nerve cells that are involved in the production of hearing, called hair cells(due to their hair-like projections) are located on the basilar membrane, which allows the lower part of the cells to come into contact with the perilymph of the scala tympani (see Figure 4). Hair-like projections of hair cells known as stereocilium, are located at the top of the hair cells and thus come into contact with the scala media and the endolymph that is contained within it. The importance of this structure will be better understood when we discuss the electrophysiological mechanism that underlies stimulation of the auditory nerve.

Figure 4.

The organ of Corti consists of approximately 20,000 such hair cells, which are located on a basilar membrane covering the entire coiled cochlea, and is 34 mm long. Moreover, the thickness of the basilar membrane varies from 0.1 mm at the beginning (base) to approximately 0.5 mm at the end (apex) of the cochlea. We will understand how important this feature is when we talk about pitch or frequency of sound.

Let's remember: sound waves enter the external auditory canal, where they cause the eardrum to resonate at an amplitude and frequency that is characteristic of the sound itself. The internal and external movement of the eardrum allows vibrational energy to be transmitted to the malleus, which is connected to the incus, which in turn is connected to the stapes. Under ideal circumstances, the air pressure on either side of the eardrum is the same. Thanks to this, and the ability of the Eustachian tube to pass outside air into the middle ear from the back of the nose and throat during yawning, chewing and swallowing, the eardrum has a high distensibility, which is so necessary for movement. The vibration is then transmitted through the stapes to the cochlea, passing through the oval window. And only after this the auditory mechanism starts.

The transfer of vibrational energy into the cochlea leads to the formation of a wave of fluid, which must be transmitted through the perilymph into the scala vestibule of the cochlea. However, due to the fact that the scala vestibuli is protected by bone and is separated from the scala medialis, not by a dense wall, but by an elastic membrane, this oscillatory wave is also transmitted through the Reisner membrane to the endolymph of the scala medialis. As a result, the fluid wave of the scala media also causes the elastic basilar membrane to oscillate in waves. These waves quickly reach their maximum and then also quickly decrease in the region of the basilar membrane in direct proportion to the frequency of the sound that we hear. Higher frequency sounds cause more movement at the base or thicker part of the basilar membrane, and lower frequency sounds cause more movement at the top or thinner part of the basilar membrane, the helictorema. As a result, the wave enters the scala tympani through the helictorema and is dissipated through the round window.

That is, it is immediately clear that if the basilar membrane sways in the “breeze” of endolymphatic movement inside the scala media, then the suspended organ of Corti, with its hair cells, will jump like on a trampoline in response to the energy of this wave movement. So, in order to appreciate the complexity and understand what actually happens for hearing to occur, the reader must become familiar with the function of neurons. If you don't already know how neurons function, I encourage you to check out my article, “More than Just Conducting Sound, Parts I and II,” which goes into more detail about the function of neurons.

At rest, hair cells have a membrane potential of approximately 60 mV. From neuronal physiology we know that the resting membrane potential exists because when the cell is not excited, K+ ions leave the cell through K+ ion channels, and Na+ ions do not enter through Na+ ion channels. However, this property is based on the fact that the cell membrane is in contact with extracellular fluid, which is usually low in K+ ions and rich in Na+ ions, similar to the perilymph with which the base of the hair cells is in contact.

When the action of the wave causes the movement of the stereocilia, that is, the hair-like outgrowths of the hair cells, they begin to bend. The movement of stereocilia leads to the fact that certain channels, intended for signal transduction, and which transmit K+ ions very well, begin to open. Therefore, when the organ of Corti experiences a step-like action of a wave that occurs as a result of vibrations during the resonance of the eardrum through the three auditory ossicles, K+ ions enter the hair cell, as a result of which it is depolarized, that is, its membrane potential becomes less negative.

“But wait,” you would say. “You just told me all about neurons, and my understanding is that when the transduction channels open, K+ ions must leave the cell and cause hyperpolarization, not depolarization.” And you would be absolutely right, because under normal circumstances, when certain ion channels open in order to increase the passage of that particular ion across the membrane, Na+ ions enter the cell and K+ ions exit. This occurs due to gradients in the relative concentrations of Na+ ions and K+ ions across the membrane.

But we must remember that our circumstances here are somewhat different. The upper part of the hair cell is in contact with the endolymph of the scala tympani and not with the perilymph of the scala tympani. The perilymph, in turn, comes into contact with the lower part of the hair cell. A little earlier in this article, we emphasized that endolymph has a unique feature in that it is the only fluid that is found outside the cell and has a high concentration of K+ ions. This concentration is so high that when the transduction channels that carry K+ ions open in response to the flexion motion of the stereocilium, the K+ ions enter the cell and thus cause its depolarization.

Depolarization of the hair cell leads to the fact that in its lower part, voltage-gated calcium ion channels (Ca++) begin to open and allow Ca++ ions to pass into the cell. As a result, a hair cell neurotransmitter (that is, a chemical transmitter of impulses between cells) is released and stimulates a nearby cochlear neuron, which ultimately sends a signal to the brain.

The frequency of sound at which a wave is generated in a liquid determines where along the basilar membrane the wave will be highest. As we said, this depends on the thickness of the basilar membrane, in which higher pitched sounds cause more activity in the thinner base of the membrane, and lower frequency sounds cause more activity in the thicker top part.

It can be easily seen that hair cells that are located closer to the base of the membrane will respond maximally to very high sounds at the upper limit of human hearing (20,000 Hz), and hair cells that are located at the opposite very top of the membrane will respond maximally to sounds at the lower end of the membrane. limits of human hearing (20 Hz).

Nerve fibers of the cochlea illustrate tonotopic map(that is, groupings of neurons with similar frequency characteristics) is that they are more sensitive to certain frequencies that are eventually decoded in the brain. This means that certain neurons in the cochlea are connected to certain hair cells, and their nerve signals are subsequently transmitted to the brain, which then determines the pitch of the sound depending on which hair cells were stimulated. Moreover, it has been shown that the nerve fibers of the cochlea have spontaneous activity, so that when they are stimulated by a sound of a certain pitch with a certain amplitude, this leads to a modulation of their activity, which is ultimately analyzed by the brain and decoded as a specific sound.

In conclusion, it is worth noting that hair cells that are located at a specific location on the basilar membrane will bend maximally in response to a specific sound wave height, causing that location on the basilar membrane to receive the crest of the wave. The resulting depolarization of this hair cell causes it to release a neurotransmitter, which in turn irritates a nearby cochlear neuron. The neuron then sends the signal to the brain (where it is decoded) as a sound that is heard at a specific amplitude and frequency depending on which neuron in the cochlea sent the signal.

Scientists have compiled many diagrams of the pathways of activity of these auditory neurons. There are many more neurons that are in the connective regions that receive these signals and then transmit them to other neurons. As a result, the signals are sent to the auditory cortex of the brain for final analysis. But it is still not known how the brain converts huge amounts of these neurochemical signals into what we know as hearing.

The obstacles to solving this problem can be as mysterious and mysterious as life itself!

This brief overview of the structure and functioning of the cochlea can help the reader prepare for questions that are often asked by admirers of the theory that all life on earth arose as a result of the action of random forces of nature without any reasonable intervention. But there are leading factors, the development of which must have some plausible explanation, especially if we take into account the absolute necessity of these factors for the function of hearing in humans.

Is it possible that these factors were formed in stages through processes of genetic mutation or random change? Or maybe each of these parts performed some hitherto unknown function in other numerous ancestors, which later united and allowed man to hear?

And assuming that one of these explanations is correct, what exactly were these changes, and how did they allow the formation of such a complex system that converts air waves into something that the human brain perceives as sound?

  1. Development of three tubular canals called the vestibule, scala media and scala tympani, which together form the cochlea.
  2. The presence of an oval window, through which the vibration from the stapes is received, and a round window, which allow the wave action to dissipate.
  3. The presence of a Reissner membrane, thanks to which the oscillatory wave is transmitted to the middle staircase.
  4. The basilar membrane, with its variable thickness and ideal location between the scala media and scala tympani, plays a role in hearing function.
  5. The organ of Corti has a structure and position on the basilar membrane that allows it to experience a spring effect, which plays a very important role in human hearing.
  6. The presence of hair cells inside the organ of Corti, the stereocilium of which is also very important for human hearing and without which it simply would not exist.
  7. The presence of perilymph in the upper and lower scala and endolymph in the middle scala.
  8. The presence of nerve fibers of the cochlea, which are located close to the hair cells located in the organ of Corti.

Final word

Before I started writing this article, I looked at that medical physiology textbook that I used back in medical school, 30 years ago. In that textbook, the authors noted the unique structure of endolymph compared to all other extracellular fluids of our body. At that time, scientists did not yet “know” the exact cause of these unusual circumstances, and the authors freely admitted that although it is known that the action potential that was generated by the auditory nerve was associated with the movement of hair cells, how exactly this happened could not be explained. could. So how can we better understand from all this how this system works? And it's very simple:

Would anyone, while listening to their favorite piece of music, think that the sounds that sound in a certain order were the result of the random action of natural forces?

Of course not! We understand that this beautiful music was written by the composer so that listeners could enjoy what he created and understand what feelings and emotions he experienced at that moment. To do this, he signs the author's manuscripts of his work so that the whole world knows who exactly wrote it. If anyone thinks differently, he will simply be exposed to ridicule.

Likewise, when you listen to a cadenza played on violins, does it occur to anyone that the sounds of music produced by a Stradivarius violin were simply the result of random forces of nature? No! Our intuition tells us that we have before us a talented virtuoso who plays certain notes in order to create sounds that his listener should hear and enjoy. And his desire is so great that his name is put on the packaging of CDs so that customers who know this musician will buy them and enjoy their favorite music.

But how can we even hear the music that is being performed? Did this ability of ours arise with the help of undirected forces of nature, as evolutionary biologists believe? Or maybe one day, one intelligent Creator decided to reveal Himself, and if so, how can we discover Him? Did He sign His creation and leave His names in nature that can help draw our attention to Him?

There are many examples of intelligent design inside the human body that I have described in articles over the past year. But when I began to understand that the movement of the hair cell causes the K+ ion transport channels to open, causing K+ ions to flow into the hair cell and depolarize it, I was literally stunned. I suddenly realized that this is the “signature” that the Creator left us. Before us is an example of how an intelligent Creator reveals Himself to people. And when humanity thinks that it knows all the secrets of life and how everything came to be, it should stop and think about whether this is really so.

Remember that the almost universal mechanism of neuronal depolarization occurs as a result of the entry of Na+ ions from the extracellular fluid into the neuron through Na+ ion channels after they have been sufficiently stimulated. Biologists who adhere to evolutionary theory still cannot explain the development of this system. However, the entire system depends on the existence and stimulation of Na+ ion channels, coupled with the fact that the concentration of Na+ ions is higher outside the cell than inside. This is how the neurons of our body work.

Now we must understand that there are other neurons in our body that work exactly the opposite. They require that not Na+ ions, but K+ ions enter the cell for depolarization. At first glance it may seem that this is simply impossible. After all, everyone knows that all the extracellular fluids of our body contain a small amount of K+ ions compared to the internal environment of the neuron, and therefore it would be physiologically impossible for K+ ions to enter the neuron in order to cause depolarization in the way that Na+ ions do.

What was once considered “unknown” has now become completely clear and understandable. It is now clear why endolymph should have such a unique property, being the only extracellular fluid of the body with a high content of K+ ions and a low content of Na+ ions. Moreover, it is located exactly where it should be, so that when the channel through which K+ ions pass opens into the membrane of the hair cells, they depolarize. Evolutionary-minded biologists must be able to explain how these seemingly contradictory conditions could arise, and how they could appear in a specific place in our body, exactly where they are needed. It's just like a composer arranges the notes correctly, and then the musician plays a piece of those notes correctly on the violin. For me, this is an intelligent Creator who tells us: “Do you see the beauty that I have endowed with My creation?”

Undoubtedly, for a person who views life and its functioning through the prism of materialism and naturalism, the idea of ​​​​the existence of an intelligent designer is something impossible. The fact that all the questions I have asked about macroevolution in this and my other articles are unlikely to have plausible answers in the future does not seem to frighten or even bother defenders of the theory that all life evolved through natural selection , which influenced random changes.

As William Dembski so artfully noted in his work The Design Revolution:“Darwinists use their misunderstanding in writing about the 'undetected' designer, not as a correctable fallacy or as evidence that the designer's abilities are far superior to ours, but as evidence that there is no 'unidentified' designer.”.

Next time we'll talk about how our body coordinates its muscular activity so that we can sit, stand, and remain mobile: this will be the last episode that focuses on neuromuscular function.

The auricle, external auditory canal, tympanic membrane, auditory ossicles, annular ligament of the oval window, membrane of the round window (secondary tympanic membrane), labyrinthine fluid (perilymph), and the main membrane take part in the conduction of sound vibrations.

In humans, the role of the auricle is relatively small. In animals that have the ability to move their ears, the pinnae help determine the direction of the source of sound. In humans, the auricle, like a megaphone, only collects sound waves. However, in this respect its role is insignificant. Therefore, when a person listens to quiet sounds, he puts his palm to his ear, due to which the surface of the auricle significantly increases.

Sound waves, having penetrated the auditory canal, set the eardrum into friendly vibration, which transmits sound vibrations through the chain of auditory ossicles to the oval window and further to the perilymph of the inner ear.

The eardrum responds not only to those sounds whose number of vibrations coincides with its own tone (800-1000 Hz), but also to any sound. This resonance is called universal, in contrast to acute resonance, when a secondary sounding body (for example, a piano string) responds to only one specific tone.

The eardrum and auditory ossicles do not simply transmit sound vibrations entering the external auditory canal, but transform them, that is, they transform air vibrations with large amplitude and low pressure into vibrations of the labyrinth fluid with low amplitude and high pressure.

This transformation is achieved due to the following conditions: 1) the surface of the tympanic membrane is 15-20 times larger than the area of ​​the oval window; 2) the malleus and incus form an unequal lever, so that the excursions made by the foot plate of the stapes are approximately one and a half times less than the excursions of the malleus handle.

The overall effect of the transformative effect of the eardrum and the lever system of the auditory ossicles is expressed in an increase in sound intensity by 25-30 dB.

Disruption of this mechanism in case of damage to the eardrum and diseases of the middle ear leads to a corresponding decrease in hearing, i.e., by 25-30 dB.

For the normal functioning of the eardrum and the chain of auditory ossicles, it is necessary that the air pressure on both sides of the eardrum, i.e. in the external auditory canal and in the tympanic cavity, be the same.

This pressure equalization occurs due to the ventilation function of the auditory tube, which connects the tympanic cavity to the nasopharynx. With each swallowing movement, air from the nasopharynx enters the tympanic cavity, and thus the air pressure in the tympanic cavity is always maintained at atmospheric level, i.e. at the same level as in the external auditory canal.

The sound-conducting apparatus also includes the muscles of the middle ear, which perform the following functions: 1) maintaining the normal tone of the eardrum and the chain of auditory ossicles; 2) protection of the inner ear from excessive sound stimulation; 3) accommodation, i.e. adaptation of the sound-conducting apparatus to sounds of varying strength and height.

When the muscle that stretches the tympanic membrane contracts, auditory sensitivity increases, which gives reason to consider this muscle “alert.” The stapedius muscle plays the opposite role - when it contracts, it limits the movements of the stirrup and thereby, as it were, muffles sounds that are too strong.

The outer ear includes the pinna, the ear canal, and the eardrum, which covers the inner end of the ear canal. The ear canal has an irregularly curved shape. In an adult, its length is about 2.5 cm and its diameter is about 8 mm. The surface of the ear canal is covered with hairs and contains glands that secrete earwax, which is necessary to maintain moisture in the skin. The ear canal also provides constant temperature and humidity to the eardrum.

  • Middle ear

The middle ear is an air-filled cavity behind the eardrum. This cavity connects to the nasopharynx through the Eustachian tube, a narrow cartilaginous canal that is usually closed. Swallowing movements open the Eustachian tube, which allows air to enter the cavity and equalize pressure on both sides of the eardrum for optimal mobility. In the middle ear cavity there are three miniature auditory ossicles: the malleus, the incus and the stapes. One end of the malleus is connected to the eardrum, the other end is connected to the incus, which in turn is connected to the stirrup, and the stirrup to the cochlea of ​​the inner ear. The eardrum constantly vibrates under the influence of sounds picked up by the ear, and the auditory ossicles transmit its vibrations to the inner ear.

  • Inner ear

The inner ear contains several structures, but only the cochlea, which gets its name because of its spiral shape, is related to hearing. The cochlea is divided into three channels filled with lymphatic fluids. The liquid in the middle channel has a different composition from the liquid in the other two channels. The organ directly responsible for hearing (the organ of Corti) is located in the middle canal. The organ of Corti contains about 30,000 hair cells that detect fluid vibrations in the canal caused by the movement of the stapes and generate electrical impulses that are transmitted along the auditory nerve to the auditory cortex. Each hair cell responds to a specific sound frequency, with high frequencies tuned to cells in the lower part of the cochlea and cells tuned to low frequencies located in the upper part of the cochlea. If hair cells die for any reason, a person stops perceiving sounds of the corresponding frequencies.

  • Auditory pathways

The auditory pathways are a collection of nerve fibers that conduct nerve impulses from the cochlea to the auditory centers of the cerebral cortex, resulting in auditory sensation. The auditory centers are located in the temporal lobes of the brain. The time it takes for the auditory signal to travel from the outer ear to the auditory centers of the brain is about 10 milliseconds.

How the human ear works (drawing courtesy of Siemens)

Sound perception

The ear sequentially converts sounds into mechanical vibrations of the eardrum and auditory ossicles, then into vibrations of the fluid in the cochlea, and finally into electrical impulses, which are transmitted along the pathways of the central auditory system to the temporal lobes of the brain for recognition and processing.
The brain and the intermediate nodes of the auditory pathways extract not only information about the pitch and volume of the sound, but also other characteristics of the sound, for example, the time interval between the moments when the right and left ear picks up the sound - this is the basis of a person’s ability to determine the direction in which the sound is coming. In this case, the brain evaluates both the information received from each ear separately and combines all the information received into a single sensation.

Our brain stores “patterns” of the sounds around us - familiar voices, music, dangerous sounds, etc. This helps the brain, when processing information about sound, quickly distinguish familiar sounds from unfamiliar ones. With hearing loss, the brain begins to receive distorted information (sounds become quieter), which leads to errors in the interpretation of sounds. On the other hand, changes in brain function due to aging, head injury, or neurological diseases and disorders may be accompanied by symptoms similar to those of hearing loss, such as inattention, detachment from the environment, and inappropriate reactions. In order to correctly hear and understand sounds, coordinated work of the auditory analyzer and the brain is necessary. Thus, without exaggeration, we can say that a person hears not with his ears, but with his brain!