Ear

 

The ear is the organ for hearing and balance. It has three parts: the outer ear, the middle ear, and the inner ear.

 

Hearing is one of our most important senses. Sounds reach our ear usually through vibrations of the air, reaching, via the ear tube, the eardrum or tympanum and making it vibrate..

 

The vibrations of the eardrum are transmitted by three small linked bones, the incus, malleus and stapes, to the spiral cochlea in the inner ear.

In the cochlea the vibrations are converted to nerve impulses, which are transmitted by the acoustic nerve to the brain’s center for hearing (in the cerebral cortex) through the auditory paths in the brain, causing us to hear the sounds. Through further complex brain connections we become aware or and interpret the sounds.

 

Sound vibrations also reach the inner ear directly, through the vibrations reaching the skull bones, and these can also be converted to nerve impulses just like the sounds that reach us via the vibration of the air.
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Outer ear

The parts of the external or outer ear – the auricle and the outer tube – are separated from the middle ear by the eardrum.


The external ear consists of a uniquely arched cartilage and the skin covering it. Its functions are collecting sound waves and transmitting them to the auditory canal.

 

The auditory canal is the direct continuation of the external ear. Its outer section is chondrous, while its inner section made up of bones. The boney part of the auditory canal extends to the petrous bone of the base of the skull.

The auditory canal transmits sound waves towards the eardrum. Small hairs and special sebaceous glands are also found in the auditory canal. These glands secrete the brown waxy substance called earwax or cerumen.

 

 

 Earwax is an acidic substance that plays a role in protecting the auditory canal and the middle ear. Because it is acidic, no alkaline soaps and lotions should be applied to it.


The quantity of earwax is sometimes too great, and its consistency can become thick, especially later in life.

It sometimes blocks the auditory canal, causing a feeling of deafness. Inappropriate attempts at cleaning the ear can push the earwax deeper into the ear, possibly as far as the eardrum. In such cases it can only be removed by a specialist.


The more we try to remove the earwax, the more earwax is secreted by the ear. If our ear is prone to blocking, 2-3 drops of paraffin oil once a week can help averting the blocking. Secretion of earwax is a normal process, not a symptom of an illness.
The skin of the auditory canal is grown to the perichondrium and periosteum beneath it. An inflammation of the auditory canal causes pain together with a partial loss of hearing.

 

The middle ear

 The middle ear consists of the tympanic cavity situated in the cerebral cortex, on the inside of the eardrum, and spongy mastoid cells communicating with it. These also communicate with the nasopharynx via the Eustachian tube.

 

The eardrum has three layers: keratinized pavement epithelium on the outside, a fibrous layer in the middle, and a mucous membrane on the inside. It is like a membrane of a speaker. Its color is silver grey under normal circumstances.

 


The tympanic cavity is a cavity of about 1/6 of a cube inch on the inside of the eardrum, in the petrous bone. In it there are three small bones (or auditory ossicles), connected to each other with joints. These ossicles are, from the outside in, the malleus (meaning “hammer” in Latin), the incus (“anvil” in Latin) and the stapes (“stirrups” in Latin).

The malleus looks like its “handle” is embedded in the eardrum, its “head” is the next bone, while it is connected with a joint to the base of the “anvil”. 


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The longer part of the “anvil” connects to the head of the “stirrup”..

 The base of the “stirrup” is on the opposite wall of the tympanic cavity from the eardrum in an oval window, connected to it with an elastic membrane. The “stirrup” can vibrate resiliently in the oval window..

The eardrum and the auditory ossicles work perfectly when the pressure outside the eardrum is the same as the pressure inside the tympanic cavity.

The inside and outside pressure are equalized by the Eustachian tube, a precondition of which is a good functioning of the nose. If breathing through the nose is blocked – due to an enlargement of the pharyngeal tonsils in children or septal deviation or nasal polyp in adults – the middle ear can be damaged.

 

 Sounds (vibrations between 20 and 20 thousand Hz transmitted in the air in the form of waves) pass through the auditory canal and strike the eardrum.

The eardrum starts to vibrate, and the vibration is transmitted to the stapes through the malleus and incus. 

The eardrum and the auditory ossicles increase the intensity of sounds by about 30 decibels (decibel [dB] is a unit of measurement of magnitude of sound), after which sounds pass into the inner ear.


The base of the stapes passes on the vibrations to the fluid of the inner ear through the oval window.
In the inner wall of the tympanic cavity there is another opening, the round window, which plays a role in equalizing the inner ear fluid.

 

The tympanic cavity is lined with mucosa. Normally, the tympanic cavity is filled with air of the same pressure as that outside the body.
Since the tympanic cavity communicates, through the Eustachian tube, with the nasopharynx, i.e. the upper part of the pharynx, it ventilates through it – which is very important in the correct functioning of hearing as well as in the prevention of illnesses.


The Eustachian tube permits the equalization of pressure on each side of the eardrum. If the outside pressure is great, it presses the eardrum into the tympanic cavity (this condition is known as tympanic retraction), and the free movement of the auditory ossicles is obstructed due to  pressure and a decrease of room in the middle ear.

 

When the Eustachian tube functions correctly, pressure equalizes when the canal opens at every swallowing. If it doesn’t, it feels like the ear is blocked.

This also happens in case of rhinitis, or when pharyngeal tonsils are enlarged in children, or when coming down from a mountain in a car or when the airplane loses altitude. If we swallow or blow air inside our head while blocking our nose, ears get unblocked.

 


When the Eustachian tube malfunctions for longer periods, watery fluid builds up in the Eustachian tube due to the continuing vacuum effect. If the situation continues, the goblet cells of the Eustachian tube start producing mucus. The watery fluid thickens and jellifies.


This makes the movement of auditory ossicles in the fluid even more difficult, and hearing is negatively affected. This condition is called chronic otitis.

It does not cause pain or fever, its only symptom might be the decrease of hearing, so screening has a great role at this stage. If at this stage of chronic otitis bacteria proliferate in the watery, serous or thick and mucous liquid, pus may form, developing the condition into acute middle ear catarrh.

In the back part of the tympanic cavity there is an opening which leads to the central cell of the spongy mastoid cavity system, through which inflammations spread to the mastoid cells in the bumpy part of the skull bone behind the ear.

 

The upper wall of the tympanic cavity and the mastoid portion of the ear is a thin bone plate with a cranial fossa above it. A neglected case of otitis can spread through here to the meninx and find its way to the brain. The facial nerve runs in the tympanic cavity through a thin walled bone canal and can be easily affected to the damage of inflammations. A further complication can be when pus develops around one of the veins of the brain (sinus sigmoideus).

 

Inner ear

The inner ear (labyrinth) is the organ with the center of both hearing and balance. The inner ear is encased in the petrous bone in a system of hollows with hard bone walls.

 
The system of hollows consists of the vestibule and the snail shell shaped cochlea (the center of hearing) and three semi-circular canals, the saccule and the utricle (the center of balance). 

 The bony labyrinth is filled with fluid (perilymph), which communicates with the cerebrospinal fluid. The cochlea is divided into two canals along its entire length by the membraneous cochlear partition. The upper and the lower fluid paths communicate with each other through the tip of the spiral of the cochlea. 

 The auditory ossicle, the stapes, vibrates due to the sounds waves.
Its base is situated in the oval window of the inner ear such that it pushes the upper fluid chamber of the cochlea.
The lower wall of this triangle is the basilar membrane, while the upper one is the Reissner’s membrane

 

 

 The basilar membrane widens towards its base and has strings in it which lengthen as the membrane widens.


On the basilar membrane hair cells are found: as their name suggests, these are topped with hair-like structures. Here there is another cellular layer, the Organ of Corti, the organ of hearing, where sound vibrations are turned into neural electrical signaling.

 

 
The discovery and description of its functioning was the life work of the Hungarian Nobel-prize laureate György Békésy.


Sound waves make the upper fluid chamber of the cochlea (the scala vestibuli) vibrate, which transmits them, turns them around at the tip, and transmits them along the lower fluid chamber (scala tympani) to the round window.

 

This makes the basilar membrane vibrate: the strings in it vibrate in accordance with the pitch of the sound waves.

 

Sounds of high pitch vibrate most strongly the strings at the base of the cochlea, while sounds of low pitch those at the tip.

Where the amplitude of the vibration is great, the hair cells touch the membrane above them and the neural electrical signaling is passed from them to the auditory nerve.


The stronger the sound, the greater the amplitude and the more electrical signaling is created.


So, the cochlea is actually capable of analyzing sound: it can differentiate between sounds by pitch and amplitude. Signals travel in the auditory nerve path to the temporal lobe of the cerebral cortex, which is where the sense of sound is detected.

 

The other part of the inner ear is the organ of the sense of balance.

The parts of the inner ear that play a role in it are the utricle and saccule in the bony vestibule as well as the three semi-circular canals which are situated in three different planes orthogonally (at right angles) from each other. All of these parts of the inner ear are filled with fluid.

The utricle and saccule also have hair cells. The hair cells are covered with a gelatinous envelopment with very small lime chrystals in it that register horizontal and vertical changes in the acceleration of head movement. When this gelatinous layer is tilted due to acceleration or deacceleration, the hair cells generate signals.

 

The wider parts of three semi-circular canals have similar hair cells, also enveloped in a gelatinous layer. This canal system indicates rotational movements in any direction.


The fluid in the canal system can move. When we spin or stop spinning, the fluid moves in relation to the canal walls, and the hair cells can detect that movement.


Hungarian Nobel prize laureate Róbert Bárány had a significant role in describing the working of the inner ear.


The brain receives information about the positioning of our body from the inner ear as well as via vision and palpation (i.e. the flexing of muscles).

All the signals together are processed by the nervous system. If there is something wrong with the balance system (due to excitation phenomena or intermission of functioning), we experience a feeling of dizziness.

 


Real dizziness occurs when there is a discrepancy between the real vs. our sensed space relations: we detect spinning, dropping or lifting off even though no such movement occurs. In everyday usage people often refer to any feeling of weakness and indisposition, incorrectly, as dizziness.

 

The system of detecting and processing balance is highly complicated. The balance system – or vestibular system – has an extensive nerve track, and various illnesses of the nervous system of the body are manifested as dizziness.

 Its complexity explains why there is a whole separate branch of medicine, otoneurology, dealing with dizziness.

A phenomenon which often occurs in the illnesses of the balance system is nystagmus, or rapid involuntary movements of the eye, generated by the nervous path between the nuclei of the vestibular system and the oculomotor system in the brain stem.

The phenomenon can also occur in a healthy person, for instance, if the person is spinned around. In such a case both eyes start twitching horizontally – similarly to when looking out of the train window. This phenomenon is pathological and a sign of the illness of the balance system if it occurs spontaneously, when the body is at rest, and occurs together with a feeling of spinning dizziness and often with vomiting and ear complaints.

 

 

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