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Thursday, September 23, 2010

5.(9-10) About Ears, Hearing, Balance

Physician's Notebooks 5 - http://physiciansnotebook.blogspot.com - See Homepage

  Update 30 Dec. 2017

Chapter 9: The Ears and Hearing
The below column of contents in order as each appears in text to guide your reading.

The importance of accurate hearing
                                                              The Structure and Function of the Human Ear
Look at your earlobe (the auricle) in mirror
Symptom of acute ear disease is earache
Blood in or from Ear Canal
Hearing & Loss
Earwax
Surgery to Restore Hearing
Prevention of  Hearing loss and Deafness
Abnormal ear sound – Tinnitus
Meniere's Syndrome
End Note 
The importance of accurate hearing has many obvious aspects but safety is first. Actually, survival depends on normal hearing. Think of walking the streets and not hearing well the sound of an approaching car's screeching brakes or the warning of someone that something much worse than raindrops is about to fall on your head or think about being inside the house and the ring of a telephone that wakes you in time to notice the start of a possibly fatal fire. And so on. Healthy longevity depends on good hearing. Now let us go into the descriptive anatomy of hearing. Note I am dealing with it from a medical specialist level but the explanations ought to be understood by everyone.
Sound consists of alternating compressions and rarefactions in a medium, the air, at a speed of 340 meters/second (c.1000 feet/s.).  In our normal lives, each of our ears must capture this mechanical energy, transmit it to the receptor organ, and change it into electrical signals suitable for our brains' understanding what it hears. These tasks are the functions of the external ear, the middle ear, and the inner ear, and, finally the hearing part of the brain's temporal lobe (Heschl's gyrus)
The structure and function of the human ear
The external ear, the auricle, focuses sound into the external auditory canal. Alternating increases and decreases in air pressure vibrate the eardrum (tympanum). These vibrations are transmitted across the air-filled middle ear by three tiny, linked bones: the malleus, the incus, and the stapes. The hammer vibration of the stapes through the oval window (Not labeled in the below figure) stimulates the cochlea, the hearing organ of the inner ear. The fact that the cochlea oval window area membrane is 35 times larger than the stapes hammer area magnifies the sound pressure going into the middle ear.



Comment:  The outer ears allow us to locate a sound and note whether it is above, below or at the same level as ourselves as well as the sides. Locating a sound to your left or right or front or back depends on the difference in the sound frequency received between the two ears. So, if it is important, you should turn your head around as you estimate the location of the sound source. Also, in estimating the elevation on the sound source, you are basing it on the difference between left and right ear in the loudness of the sound.
  The most common form of poor hearing verging on deafness is blockage of the external canal with wax, which is best prevented by not screwing your pinky finger into your ear canal every time it itches and by regular visits to your ear doctor for a cleaning (not by ear drops but a mechanical cleaning-out of the accumulating wax).
  A not uncommon cause of hearing loss in old age is a kind of aging arthritis of the 3 tiny ear bones causing them to lose their lever-and-hammer action. This can be reversed by an operation that replaces the bones with a tiny prosthesis.
  The inner ear is the snail-like tubular structure you see above. The whole structure is a hollow, multicompartmented tube that contains the nerves and apparatus for hearing. It is not directly open to external air pressure so pressure changes can mostly be effected by the ear bone stapes striking the membrane of the oval window.  The next figure, below, delves more into the inner ear structure.
A cross section of the cochlea shows the arrangement of the three liquid-filled duct-segments or scalae, each of which occupies separate length segments that make up the 33-mm-long cochlea. In the lower figure you see a cross section of a point segment of the cochlea tube. Essentially it consists of the 2 tube segments, the scalae vestibuli and tympani, which are more external and contain the extracellular perilymph, and are folded over the in-between 3rd tube, the scala media, which is more central and contains endolymph which is rich in potassium ion (K+) and very important for hearing because it bathes the sensitive neural hair tips that detect sound vibrations and translate them into nerve impulses that go to the brain and allow us to hear. The scala vestibuli first receives the sound vibrations from the stapes bone through the oval window in the middle ear; then the vibration travels up the scala vestibuli and at its apex communicates into the scala tympani which, at its reversed direction end has the round window membrane shutting it off from the middle ear. The communication of the scalae vestibuli and tympani at the apex of the cochlea is called the helicotrema and is shown in the below figure. At the base of the cochlea, each duct is closed by a sealed aperture-window: the scala vestibuli is closed by the oval window, against which the stapes strikes in response to sound; and the scala tympani is closed by the round window, a thin, flexible membrane. Between these two compartments lies the scala media, the endolymph-filled tube whose epithelial lining includes the 16,000 hair cells in the organ of Corti surmounting the basilar membrane and enclosed by the overlying Reissner's membrane..

  Comment: Sound units of hearing sensitivity (loudness) are decibels. (dB). 0 dB is the lower limit of loudness for normal human hearing and 120 dB is the maximum that can be tolerated.
You should note that 0 dB does not mean the absence of any sound. It simply means that the sound is at the lower limit of human hearing. Many animals can hear below 0 dB.
Also, human hearing is best at frequencies between 1-4 KHz. (1000-4000Hz or number of sound waves per second)
Motion of the basilar membrane. (As you read consult the below A, B, C D  with each:
Image not available.
A shows an uncoiled cochlea, with its base showing its relation to the scalae, and indicates the flow of stimulus energy. Sound vibrates the tympanum, which sets the three bones of the middle ear into motion. The piston-like action of the stapes, a bone inserted into the elastic oval window, produces oscillatory pressure differences that rapidly pass up the scala vestibuli and back down the scala tympani. Low-frequency pressure differences are shunted through the helicotrema, where the two ducts communicate. The oval and round windows do not actually lie at the extreme base of the cochlea, but occur at oblique angles.
B. The functional properties of the cochlea are conceptually simplified if the cochlea is viewed as a straightened-out structure with only two liquid-filled compartments scalae vestibuli and tympani separated by the elastic basilar membrane.
C. If the basilar membrane had uniform mechanical properties along its full length, a compression would drive the tympanum and the ear bones inward, increasing the pressure in the scala vestibuli and forcing the basilar membrane downward. Opposite movements would occur during a rarefaction. The pressure changes in the scala tympani are relieved by bowing of the round-window membrane. The movements of the tympanum, bones, and basilar membrane are greatly exaggerated in the Figure.
D. In fact, the basilar membrane's mechanical properties vary continuously along its length. The oscillatory stimulation of a sound causes a traveling wave on the basilar membrane, shown here within the envelope of maximal displacement over an entire cycle. The magnitude of movement is grossly exaggerated in the vertical direction; the loudest tolerable sounds move the basilar membrane by only ±150 nm, a scaled distance less than one-hundredth the width of the lines representing the basilar membrane in these figures.
E. An enlargement of the sound wave in the active region in D demonstrates the motion of the basilar membrane in response to stimulation with sound of a single frequency. The continuous curve depicts a traveling wave at one instant; the vertical scale of basilar-membrane deflection is exaggerated about one-millionfold. The dashed and dotted curves portray the traveling wave at successively later times as it progresses from the cochlear base (reader's left) toward the apex (right). As the wave approaches the characteristic place for the stimulus frequency, it slows and grows in amplitude. The stimulus energy is then transferred to hair cells at the position of the wave's peak.
F. Each frequency of stimulation excites maximal motion at a particular position along the basilar membrane. Low-frequency sounds produce basilar-membrane motion near the apex, where the membrane is relatively broad and flaccid. Mid-frequency sounds excite the membrane in its middle. The highest frequencies that we can hear excite the basilar membrane at its narrow, taut base.(the narrowness and broadness not shown in the diagram) The mapping of sound frequency onto the basilar membrane is approximately logarithmic which means that each third of the membranes length responds to vibrations at 10 times the frequency from apex to base of the membrane.
G. The basilar membrane actually is doing spectral analysis of complex sounds. In this example a sound with three prominent frequencies, such as the three formants of a vowel sound, excites basilar-membrane motion in three regions, each of which represents a logarithmically changing frequency range. Hair cells in the corresponding positions transduce the basilar-membrane oscillations into receptor potentials, which in turn excite the nerve fibers that innervate these particular regions.


         The cochlear basilar membrane extends from the base by the oval window to the distal apex. Normally it is coiled as you see in the initial figure, but if you uncoiled it (As in lower figures), you can see it is 33 mm in length. The membrane gets wider and thicker from base to apex. At the base it is like a taut violin string and at the apex, it is like a thick strap with gradual enlargement in between. Because of this, the part of the membrane closest to the apex responds to the lowest frequency (20Hz) of heard sound and that part at the base responds to the highest frequency (20KHz). It is a 1000 fold decrease in frequency response from apex to base. Also, the basilar membrane is not arithmetically proportional in its length vs frequency response; Rather it is logorithmically proportional like a slide rule which means that, if you divided the membrane in thirds from base to apex.
The basilar membrane acts the opposite of a piano. In a piano, complex sounds are struck on the piano keys and combined into the music you hear; oppositely, in the basilar membrane the sound struck on the stapes bone (like piano key) is decomposed by different parts of the membrane vibrating to the different frequencies of the sound.

So far, in the above we have dealt with sound in the ear in terms of the external, middle and cochlear basilar membrane response. The actual auditory nerve response will be dealt with in the chapter on the cranial nerves.
Look at your earlobe (the auricle) in mirror. Pinch it gently to feel consistency. Feel along outer rim, top curvature. Lumps in earlobe are either oily gland cyst or from high uric acid blood test >8 mg% (0.47 mM/L).
   Feel the hard edge of bone behind your ear, the mastoid. Pain there is infection that may spread to brain if not treated. Gently insert clean, nail-trimmed 5th finger into ear canal. It runs slightly upward to end at eardrum. If your fingertip comes away with wax on it, you have too much wax and should have it cleaned. Earwax piles up, and with habit of poking finger or cotton tip into ear it gets packed against eardrum, causing poor hearing. Worse than wax, is foreign body in ear canal, usually in child. Suspect it if the child complains of itching, pain, discharge, loss of hearing. 
Symptom of acute ear disease is earache or bleeding from ear. Usual earache is sharp, deep in ear and part of catching cold or rapid change in air pressure in taking off or landing in airplane. Best remedy is repeated swallowing. A more worrisome earache due to infection is worsened by pushing on ear lobe, or has discharge from ear or with mastoid pain.
Blood in or from Ear Canal
From trauma or infection, bleeding or clear fluid discharge from the canal after blow to head means basal skull fracture. Bleeding from infection with no recent head trauma, shows mixed blood and pus, at times with fever. Non-bloody clear discharge is from punctured eardrum or allergy.
Hearing & Loss
Everyone living long gets hearing loss. Overview of hearing shows sound vibration entering right and left ear canals, vibrating eardrums, which then resonate through middle ear bones to inner ear organ of hearing, where vibration results in mechanical movement of hairs on nerve cells that are stimulated to transmit electrical signals via each cell’s nerve fiber, the fibers summing to form auditory input through the Acoustic Nerve (a.k.a. auditory nerve, part of Cranial Nerve VIII). Left & right acoustic nerve each send representation of the sound to auditory cerebral cortex in both sides of the brain's temporal lobe. Destruction of one acoustic nerve will render its sided ear deaf but damage of the hearing area in one side of the brain's cerebral cortex as in a stroke will not much affect hearing because of the brain's mostly bilateral innervation of both ears.
   As with vision, the meaning of sound vibration depends on a wholly functioning brain and interconnections between visual, speech and memory centers. Furthermore, meaning depends on a tonotopic code. And auditory hallucination or illusion shows the importance of healthy cerebral cortex and connections.
   Hearing loss in old age may be from ear bones' aging change, or, nerve damage from lifetime toxins like the antibiotics streptomycin or kanamycin or degenerative effects from poor circulation. The hearing loss of old people is mainly high-pitch. So old persons have a problem hearing young women unless the women lower the pitch of their voices without lowering the volume.
Hearing loss, in younger persons, is mostly due to poor conduction in ear canal or middle ear. Its causes are earwax and Foreign Body blockage, a damaged ear drum, a blocked middle ear & tube from allergy or infection. The conduction hearing loss occurs more as one-sided, at younger-age, affecting low-pitch, and is the more treatable, usually by cleaning ear canal of wax or foreign body or in the case of middle ear, conduction deafness due to ear bone arthritis it can be cured by surgery that replaces the bones with the prosthesis. Finally, when we deal with the auditory nerve, there is also surgery that may help the distal end of the nerve and will be dealt with in the cranial nerves chapter.
Earwax
is the most common cause of poor hearing. It can creep up and become cause of poor hearing accident and loss. Two signs are finding wax at outlet of ear canal and a gradual difficulty in hearing in one or both ears. Clearing wax from ear is not simply ear drops or irrigation; it involves ear canal probing, scraping and suction down to eardrum and it takes special instrument and skill of ear specialist.
Surgery to Restore Hearing: If deafness is due to distal acoustic nerve failure, the recently developed cochlear implant may restore hearing. If due to middle ear bone changes, surgery also may help. But for most old persons just learning to live with poor hearing and adapt to it should suffice. (See below)
Prevention of Hearing Loss and Deafness
No poking in ear, no strepto- or kana-mycin antibiotics! No excessively loud audio entertainment into an ear! Once hearing loss is noted, cultivate a quiet, less distracting environment; also speak clearly so others will do the same for you, avoid chitchat, and concentrate on meaning via all sensory input (Body language, facial expression, and other visual cues). Keep in mind that old age hearing loss is mostly for high-pitch speech. So younger persons should use lower pitch without lowering volume when speaking to a hard-of-hearing oldster. Hearing aid should be delayed as long as tolerable. I have tolerated old age loss of best hearing without an aid for years. (My 84-year-old pal, Lou's hearing was not so bad but the day after he started using a hearing aid he got a bad case of vertigo from the irritation)
Abnormal ear sound – Tinnitus
is buzzing, clicking, pulsing, musical, usually temporary and mostly due to becoming excessively observant in quiet environment. Much tinnitus is from hearing one's own body sounds and being affected by worry. If it persists, see a neurologist and include MRI of head, an EEG brain-wave test, and neurology & otology exams. If all show normal, which is what usually happens, relax. Most tinnitus can be masked by avoiding a totally silent environment. So if it bothers at night, play sweet music at low volume for relief. Once the worry of serious brain disease is removed, the abnormal sound will fade away. The American Tinnitus Association website, http://www.ata.org  is useful to allay anxiety.
Meniere's Syndrome
is a combination of one or more of the 3 symptoms - tinnitus, vertigo attacks and loss of hearing - that occur over years and seem to be due to block of endolymph in inner ear canals from an virus infection. It should be tolerable but many patients become excessively neurotic and waste a life doting over it. If you get the symptoms, or the label, have one good checkup with MRI at an HMO to rule out tumor or other real illness and if all comes up negative, forget about your symptoms. Most doctors label patients with Meniere's as crocks of you-know-what.

End Note: Again, note that explanation of hearing at the level the cerebral cortex is found in the Notebooks 9 chapter of Cranial Nerves under Cranial Nerve VIII hearing portion.
10. The Vestibular Organs of Balance
This is meant to give the reader a useful knowledge of how the human body's balance - the vestibular system - works. It aims higher than the understanding of most medical doctors. Motivation to learn is the key. Also the reader should not be frightened away by the idea to "learn it all at one reading. "So stop reading if you are getting weary of it, and then, later, when you are again motivated you pick up the reading. You should read without trying to memorize and always with the happy feeling of acquiring as much knowledge as you may for the moment. And reread as much as you need for complete understanding. Also use Google Wikipedia for supplementary information.
  The descending list of contents is in order as it appears and gives you an idea of the topics dealt with and locates a topic by search & find or scroll down. Best do a first read of the entire chapter in a relaxed state of mind then at leisure reread parts as needed for better understanding.
Introduction
The Vestibular Organs of Balance Are a Rapid-Response System
The Structures of the Inner Ear 
endolymph fluid 
Gravity, Tilting and Linear Horizontal Movements 
The special hair cells, stereocilia  
The AMPULLA of a semicircular canal
The Utricle - the organ that detects linear tilting 
The Semicircular Canals Sense Head Rotation
Vestibular inputs signalling body posture and motion can be ambiguous.
The Saccule detects straight gravity
The Vestibular Organs Affect on the Eyes and Vision 
Vestibular nystagmus.
 The Vestibular Nerve Carries Information on Head Velocity
 Symptoms from Vestibular Dysfunction, Illness or Positional Affects 
Vertigo
                       Unilateral Vestibular Hypofunction Causes Pathological Nystagmus & Vertigo 
Bilateral Vestibular Hypofunction Interferes with Normal Vision - an example
Acute Alcohol Intake and Positional Vertigo
End Summary
End Note
Introduction
Airplanes and submarines navigate in the 3 dimensions, the x, y, z axes. Humans use a guidance system, the vestibular system in the inner ear. Acceleration of the head deflects hair bundles in the organ of balance's semicircular canals and the 2 other organs of balance, the utricle and saccula; and this makes signals that tell us our position in space and allow us to be aware of our movements even in the dark.

The Vestibular Organs of Balance Are a Rapid-Response System that keeps the eye's gaze from moving when the head moves, that helps to maintain upright posture, and influences how we perceive our own movement and the space around us by providing a measure of, and giving feeling for the gravitational field in which we live.
Several systems detect and maintain our posture and position in space. These include the vestibular, the proprioceptive (muscle position sense), the visual and the somatic-sensory (touch feel) systems. But only the vestibular system has rapid enough response for our real world of accidents and violence where rapid reaction is the key to survival. But if we depend only on the vestibular system we may get into situations where we cannot tell if we are moving or not (sitting in a stationary train looking at a beside-us train moving or not?) or for an astronaut in orbit whether or not he is being affected by gravity or centrifugal force like on a marry-go-round? In order to maintain stable balance and erect posture, we must make use mostly of the vestibular plus visual system because the vestibular system is fastest but it is also easily fooled in the dark. Persons who lose the touch-and-feel position sense, due to aging or toxicity, suffer many falls due to mistaking the affect of gravity. So, ideally, for healthy longevity strive to make use of the systems to help you maintain good balance (good visual, head upright, body well nourished so that proprioceptive and somatosensory systems function maximally to age 100)

The Structures of the Inner Ear -  (the 3 rotational Semicircular Canals, and the 2 of linear-movements-sensing; namely, the Utricle & Saccule)

Image not available. 

Note that the above figures show the organ of balance as a combined structure with the inner ear, which has been detailed in the previous chapter. Note that both hollow structures communicate - the beginning part of the Cochlear duct with the Saccule compartment of the vestibular organ via the Ductus reuniens.
 Look at the 3 semicircular canals shown best above in A: the Horizontal canal (in red), the Anterior vertical ("superior") canal (in lavender) and the Posterior vertical ("inferior") canal (in orange-yellow). The canals detect rotatory movements as when you twirl around or bend in a curving motion. With your fingertip, trace out each of the 3 canals from inflow origins at the light blue sac labelled "utricle" and note that each canal widens into a bulge (called "ampulla"). The 3 canals are continuous with each other and the utricle and filled with a freely flowing fluid endolymph, and each canal is at right angles to the other 2 canals and together they encompass the 3 dimensions in our real world - width, height and length (the xyz planes of space).
 The utricle and the saccule (blue and light purple in top figure) within which are the organs that detect linear, or straight line movement; the utricle best detects horizontal tilts or movements, and the saccule best detects up-down, or straight gravitational effects.
The bilateral symmetry of the semicircular canals and the utricle & saccule.
The horizontal canals on both sides lie in approximately the same plane, which would be a horizontal bread-knife slice-cut at a level centered at the openings of left and right ear canals. Concerning the anterior-vertical and posterior-vertical canals, the anterior canal on the left side and the posterior canal on the right side lie in one oblique plane and the posterior canal on the left side and the anterior canal on the right side lie in the other oblique plane (each canal plane 90 degrees apart) and therefore these opposites are at right angles to each other and together with the horizontal pair constitute the 3 functional right angle plane-pairs.

Image not available.
 Legend Visualize the planes of the 3 functional pairs of semicircular canals: right & left horizontal canals for the horizontal plane, left anterior-vertical & right posterior-vertical canals for the left anterior diagonal plane and right anterior-vertical & left posterior-vertical canals for the right anterior diagonal plane. As the above diagram shows by the dotted lines, the 2 diagonal planes are oblique (45 degrees) to the mid vertical plane (midsagittal line). Also the utricle & saccule are bilateral, one each symmetrically on left and right sides at the base of the canals.

(Text continues) The semicircular canals and the utricle & saccule sacs are filled with endolymph fluid that is free to flow back and forth and is affected by inertia in response to rotatory movements of head and body and by gravity through tilts and in the straight up vertical. For each canal-pair, this flow is most for rotatory movements in the plane of the canals, and the direction of head turning shows the side that is excited. (Turn to your left and you excite your left horizontal canal vestibular nerve). The left and right horizontal canals are sensitive to rotation in the horizontal plane for example when spinning around in a freely revolving chair or an ice skater performing a rapid spin and also rotation of the head around the axis of the neck, for example turning your head to look left or right before crossing a road. The vertical canal pairs are sensitive to angular rotation around a horizontal plane, for example, a leftward and downward head motion, such as tilting the head toward the front of the left shoulder, excites the left anterior canal and inhibits the right posterior canal. 
   Gravity, Tilting and Linear Horizontal Movements: The utricle sensitivity is greatest for linear horizontal movements such as tilting the head from side to side on the shoulder or movement sideways on a roller platform. The saccule is sensitive to up-down movements and is why you may get seasick or airsick because your head bobs up and down in a rough sea or air turbulence.

The special hair cells, stereocilia grow in bunches of hairs in the ampulla end of each semicircular canal and on special structures inside the utricle & saccule, and these hairs, by back and forth movements, convert the mechanical stimuli of movements into neural signals that tell us our position and movements in space and help our eyes to focus on the written page.
 In the below figure, a single hair bundle, a stereocilium, is shown. You should note the obliquely increasing length of each hair from the bundle's shortest hair to its longest hair, called kinocilium, toward your right.
Note at the apex of each cell the hair bundle stereocilium whose single hairs increase in length toward the single kinocilium. The excitability of the receptor cell depends on the direction in which the hair bundle is bent. Deflection toward the kinocilium increases the nerve's rate of firing and stimulates the vestibular nerve fiber to transmit the signal of motion in a particular direction at a particular strength. Bending away from the kinocilium decreases (inhibits) the firing rate and stops the signal.


The system of stimulus-sensitive hairs is used by the semicircular canals to detect rotation around an axis and translation by body movement (rolling side to side movement). It also signals movement like tilting and gravity effects in the utricle & saccule. But the mechanism in the canals differs from that in the utricle & saccule. The below diagrams explain the differences and also give more detail about the utricle & saccule. 
The AMPULLA of a semicircular canal is the canal's rounded out bulge at the end opposite the inflow of endolymph through the utricle sac.
A figure (Right enlargement). A thickened zone of epithelium, the ampullary crista, contains the hair cell roots. The hairs extend as bundles, each bundle made up of short, longer, longest hair; the longest hair is called the kinocilium, and these are enmeshed in a gelatinous material that forms a diaphragm across the ampulla, the cupula, that stretches from the crista to the roof of the ampulla. Note that the cupula obstructs but does not completely block the free flow of endolymph at its end of each semicircular canal.
B. The cupula is displaced by the flow of endolymph when the head moves in the plane of the canal. As a result, the hair bundles are also displaced. When the displacement is toward the kinocilium, (In the direction of the rotation) the hair excites its attached nerve fiber; and, when; opposite, it inhibits; For the horizontal canals endolymph flow towards the cupula is excitatory. On the other hand, flow away from the cupula is excitatory for the anterior and posterior vertical canals. Practically, this works out to excitation of the cupulas in the direction of head rotation for particular planar pairs of canals.



Hair cells in the epithelium of the utricle have apical hair bundles that project into the otolithic membrane, a gelatinous material that is covered by millions of calcium carbonate particles (otoconia). The hair bundles are polarized but are oriented in different directions (see Figure). Thus when the head is tilted, the gravitational force on the otoconia bends each hair bundle in a particular direction. When the head is tilted in the direction of a hair cell's axis of polarity, that excites the fiber. When the head is tilted in the opposite direction, the same cell inhibits the fiber.

Image not available.
 
(Legend for the above diagram continues): Note that the deflection of the hair tips in the utricle (and in the saccule too) is due to gravity weighing down on the overlying otolithic membrane from the tiny calcium stones (the otoconia). This is in contrast to what happens in the semicircular canal where the excitable hair tips are deflected by the current of endolymph set up by the rotational motions.

The Semicircular Canals Sense Head Rotation

An object undergoes angular acceleration (rotation about a center) when its rate of rotation about an axis (its speed, or velocity) changes as when your head starts or stops to turn or tilt, when your body starts or stops to rotate, and during increasing or decreasing active or passive locomotion. The three semicircular canals of each vestibular labyrinth detect these angular accelerations due to the effect of the flow of endolymph on the cupula hair bunches and signal their magnitudes and orientations to the brain. Thus your consciousness tells you, you are moving and also the direction and approx. speed of the movement. And if something goes wrong with one side of the vestibular organs you will get symptoms of vertigo (dizziness).

Vestibular inputs signalling body posture and motion can be ambiguous.
The postural system cannot distinguish between tilt and linear acceleration of the body based on vestibular inputs alone. The same force acting on vestibular hair cells can result from tilting of the head (below left), which exposes the hair cells to a portion of the acceleration (a) owing to gravity (Fg), or from horizontal linear acceleration of the body (below right). You vision prevents that confusion. That is why you become more unstable in the dark or with eyes closed.



Image not available.
 
The Saccule detects straight gravity
The saccule which is a vertically oriented body is most sensitive to the affect of straight gravity meaning it makes us aware we are standing erect and it may make us sea sick or air sick when we bob up and down on a boat in rough waves or in an aircraft with turbulence. This contrasts to the utricle in which the hairs are oriented on a horizontal platform so they are affected more by tilt than by straight up-down motion. A useful bit from this is that immediate relief from airsick and seasick symptoms is lying the head flat on its side (Which side you find out by trying right and then left to see which most relieves the sickness) where gravity has minimal affect on the saccule's sensitive hairs

The Vestibular Organ; its Affect on the Eyes and Vision - the Vestibular Ocular Reflex (VOR) and Nystagmus
Now do a simple test: Take your iPhone (or any cell phone that receives text) and read one of its messages. Shake your head back and forth (The No sign) at about 1 a second while trying to read. You will see that you can still read it despite the back and forth head movement. Next try reading by keeping your head steady but moving the screen at the same horizontal back and forth motion speed. You will find it very hard to read the text when the screen moves as compared to when your head moves even though the basic motion and the rate of motion is the same. The reason you can keep your vision when your head alone moves is the vestibular system's vestibular-ocular reflex and it will also introduce nystagmus, an important condition and symptom.
And consider a remarkable fact: Experiments have shown, even when you turn out all lights, the eyes in a head that starts rotating in the horizontal plane will follow the stationary object of a gaze exactly as if it is being seen. This remarkable "flying blind" is due to the living computer in your vestibular nerve nuclei based solely on movements of the endolymph in the horizontal semicircular canals.



In the above figure you are looking down on the head of a sitting person rotating in a clockwise direction, or rotating toward his right shoulder. In the figure's upper green area, the larger curved black arrow shows the automatic counterclockwise rotation of his eye globes, opposite to the clockwise rotation of the head, in order to keep the in-focus object in his vision despite the rightward rotatory movement of the head. If the eye globes did not oppositely rotate in socket, the in-focus object of his vision would move to his left and out of his field of view which as he turns his head is constantly moving rightward and would cause the text to blur with movement and then disappear off to his left. To keep the original object of his gaze fixed in focus as his visual field shifts rightward, he rotates both his eyes leftward. And the remarkable thing is that he would do this in the dark without the assistance of his vision because it is being computed and effected solely by his vestibular nuclei based on the movement of endolymph in both his left and right horizontal semicircular canals caused by the sudden movement of his head in the rightward rotatory motion.
  And how does that rightward head motion affect the horizontal canals and the endolymph in it? 
   In turning the head rightward, at the start, the solid parts of the head turn right but the liquid endolymph in the left & right horizontal semicircular canals, due to inertia, seems to move leftwards. (Test this out by looking at a surface bubble in a cup of coffee; while you twirl the cup clockwise, to your right the surface bubble will seem to move oppositely to your left due to the inertia effect of the sudden movement on fluid) This leftward movement of the fluid causes the right semicircular canal cupula to bend in a way that excites the right vestibular nerve fibers; meanwhile in the left canal the cupula bends oppositely and inhibits the left sided nerve fibers. These signals flash to the eye control center and cause a tracking movement of the eye globes left to keep the rightward moving image on the retina in view, un-moving and in focus. But in the process of full head rotation there is a limit to a single eye globe sideways rotation because of the limit of the eye-movement muscles. Thus to continue tracking the retinal image leftwards, the globes need to quickly reset their original starting position by a fast movement to right and then start another slower rotation further to the left. This slow tracking eye movement in the opposite direction of the head rotation followed by the quick twitch in the direction of the head rotation, resets the eyes for the next slow movement in a continuing set of same movements that is called vestibular nystagmus. What you see if you look at such an eye is first a slow movement to the left, then at the extremity of that slow movement a fast twitch rightward resetting movement and then the cycle of slow movement and fast twitch continues until the head movement stops.
See the below  

Vestibular nystagmus.


Note the "living nystagmus" in the below video; the slow movement is to the person's left (the reader's right) and the fast twitch movement to the person's right (reader's left). In this case the nystagmus is stimulated by starting to rotate the head clockwise, to one's right.

Optokinetic nystagmus.gif



The Vestibular Nerve Carries Information on Head Velocity to the Vestibular Nuclei in Midbrain

When the head is at rest there is a spontaneous tonic (at a continuous rate of  discharges) discharge in the vestibular nerves equal on both sides. That there is no imbalance in the firing rates indicates to the brain that the head is not moving. When the head rotates, the horizontal canal toward which the head is turning is excited whereas the opposite canal is inhibited, resulting in phasic increases and decreases in the vestibular signal (Figure).
Because of inertia, rotation of the head, for example in a counterclockwise (to left) direction, causes endolymph to move clockwise (to right) with respect to the canals. This deflects the stereocilia in the left canal in the excitatory direction, thereby exciting the afferent fibers on that side. In the right canal the afferent fibers are hyperpolarized so that firing decreases.

Image not available.


Symptoms from Vestibular Dysfunction, Illness or Positional Affects
Vertigo is unsteadiness and instability of one’s surrounding, an “I’ve got a feeling I’m falling.” feeling. It is due, in illness, to irritation or inflammation, which affects the sensitive hairs of the inner ear organs of balance on one side or the other; or else, as explained and diagrammed above, it is due to unusual head movements or eye movements.  Frequent in jet setter is Positional Vertigo (aka acute labyrinthitis) due to air-pressure-change. In usual case, a jet setter who has just checked into hotel is floored by the vertigo. It is evoked, or made worse, by lying flat with head turned to one side (Usually this brings it on within 10 to 15 seconds and it will be relieved by turning head from the side that causes the vertigo to the opposite side). So the sufferer lies on floor or bed, eyes closed and, if he has read this, his head will be turned to the side that relieves his vertigo. A doctor may mistake it for food poisoning or intestinal flu or even cerebellar brain stroke because of vomiting.
Self-diagnosis is easy: Vertigo is the key symptom; recent air travel the stimulus; and its being worse lying down with head to one side; and accompanied by "jerky" eye motions (nystagmus) distance it from the brain stroke of cerebellum. The vertigo of labyrinthitis may have loss of hearing and tinnitus while cerebellar vertigo may show the other signs of brain stroke (One sided weakness, ataxia or uneasy gait). In acute vestibular vertigo the worst will be gone in 24 hours but may flare again after viral infection or jet trip.
   Because mild vertigo may linger for months after the acute illness, it has caused expensive medical consultations. If you have lingering vertigo, living with it is better than visiting specialist in pursuit of elusive brain tumor, multiple sclerosis, or psychiatric diagnosis. Connected with this problem is Meniere's Disease or Syndrome (See previous chapter), which is the combination of vertigo, tinnitus and degrees of hearing loss that may during its course involve one or the other symptoms. Most Meniere's is believed to be due to swelling in the labyrinth of the ear of one side from an endolymph blockage. My advice is, right at the start, if you have vertigo or tinnitus that is not going away, get to a University HMO. Meniere's Disease often gradually fades into the background of one's life. If an endolymph cyst of the labyrinth can be demonstrated by MRI, an ear surgeon of the HMO may get good results. Anyone who develops vertigo that cannot be explained by labyrinthitis should first look at medication he may be getting. Sleep pill, tranquilizer, anti-depressive, and lithium cause vertigo based on dose and age. Progressive vertigo may be symptom of brain tumor, stroke, vitamin B12 deficiency or syphilis. An MRI of brain and blood tests will quickly spot the cause or re-assure by negative test. 

Unilateral Vestibular Hypofunction Causes Pathological Nystagmus: As we have seen, rotation excites hair cells in the semicircular canal whose hair bundles are oriented in the direction of motion and inhibits those on the side whose hair bundles are oriented away from the motion. This imbalance in vestibular signals is responsible for the compensatory eye movements and the sensation of rotation that accompanies head movement. It can also originate from disease of one labyrinth or vestibular nerve, which results in a pattern of vestibular signaling like that stemming from rotation away from the side of the lesion, that is, more discharge from the intact side. There is accordingly a strong feeling of spinning.

Bilateral Vestibular Hypofunction Interferes with Normal Vision: Vestibular function is sometimes lost on both sides, for example from toxicity owing to aminoglycoside antibiotics such as gentamicin, kanamycin or streptomycin. The symptoms are different from those of unilateral loss. First, there is no vertigo because there is no imbalance in vestibular signals; input is reduced equally from both sides. For the same reason there is no spontaneous nystagmus. In fact, these patients may have no symptoms when they are at rest and the head is still. Nevertheless, the loss of vestibular reflexes is devastating. A physician who lost his vestibular hair cells because of a toxic reaction to streptomycin wrote a dramatic account of this loss. Immediately after the onset of the toxicity he could not read in bed without steadying his head to keep it motionless. Even after partial recovery he could not read street signs or recognize friends; while walking in the street he had to stop to see clearly. Some patients may even "see" their heartbeat if the vestibulo-ocular reflex fails to compensate for the very slight head movements that accompany each arterial pulse.

Acute Alcohol Intake and Positional Vertigo: Too much alcohol drink on the inner ear causes density differences between the endolymph and cupula and the cupula becomes lighter than the endolymph. The changing difference in density explains the vertigo of drunkenness and the police use it for an on-the-spot test for DUI, i.e., nystagmus in a driver usually signifies an alcoholic drunken state. So before you get in the car after a party, check yourself (or have a friend check you) for nystagmus.  But, better yet, do not drive after partying.

End Summary:
The vestibular system evolved to answer 2 of the questions basic to human life: "Which way is up?" and "Where am I going?" The system provides the brain with a rapid estimate of head motion. Although this estimate could be derived from vision and neck muscle senses, those sensory mechanisms are slow and cumbersome. In contrast, the vestibular system senses head acceleration speedily, and this responsiveness allows those reflexes that require information about head motion to act quickly.
Projections from the vestibular nuclei to the oculomotor system allow the eye muscles to compensate for head movement by moving in such a way as to hold the image of the external world motionless on the retina. Sustained rotation results in a pattern of alternating slow and fast eye movements called nystagmus. The slow eye movement is equal and opposite to the head movement, whereas the fast eye movement represents a resetting movement in the opposite direction. Nystagmus in the absence of sustained head rotation is a sign of disease of the vestibular apparatus or its central connections. Vestibular signals habituate during sustained rotation and are relatively insensitive to very slow head movements.


Head movement evokes motion of the entire visual image on the retina as the moving eyes sweep across a stable visual field. Thus, visual signals supplement the vestibular signal in the brain and compensate for the tendency of the vestibular signal to adapt and fade during prolonged rotation. This so-called optokinetic system provides the visual input to the central vestibular system.


The vestibulo-ocular reflex is adaptable. If a process such as muscle weakness or visual distortion alters the relationship between the visual input and the motor output, the brain compensates and resets for that change. 

End Note: Note that explanation of hearing at the level the cerebral cortex is found in the Notebooks 9 chapter of Cranial Nerves under Cranial Nerve VIII vestibular portion.


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