Physiology of hearing. Vestibular analyzer.
Hearing is the process by which the ear transforms sound vibrations in the external environment into nerve impulses that are conveyed to the brain, where they are interpreted as sounds. Sounds are produced when vibrating objects, such as the plucked string of a guitar, produce pressure pulses of vibrating air molecules, better known as sound waves. The ear can distinguish different subjective aspects of a sound, such as its loudness and pitch, by detecting and analyzing different physical characteristics of the waves. Pitch is the perception of the frequency of sound waves—i.e., the number of wavelengths that pass a fixed point in a unit of time. Frequency is usually measured in cycles per second, or hertz. The human ear is most sensitive to and most easily detects frequencies of 1,000 to 4,000 hertz, but at least for normal young ears the entire audible range of sounds extends from about 20 to 20,000 hertz. Sound waves of still higher frequency are referred to as ultrasonic, although they can be heard by other mammals. Loudness is the perception of the intensity of sound—i.e., the pressure exerted by sound waves on the tympanic membrane. The greater their amplitude or strength, the greater is the pressure or intensity, and consequently the loudness, of the sound. The intensity of sound is measured and reported in decibels (dB), a unit that expresses the relative magnitude of a sound on a logarithmic scale. Stated in another way, the decibel is a unit for comparing the intensity of any given sound with a standard sound that is just perceptible to the normal human ear at a frequency in the range to which the ear is most sensitive. On the decibel scale, the range of human hearing extends from 0 dB, which represents a level that is all but inaudible, to about 130 dB, the level at which sound becomes painful. (For a more in-depth discussion, see sound.)
External ear consists of auricle and external auditory meatus. Function of external ear is collection of sound waves.
External auditory meatus conducts sound waves from auricle to tympanic membrane.
External ear also helps in protection of middle and internal ear. It provides constant temperature and humidity near tympanic membrane and mechanical defense also.
Role of middle ear in sound perception
Middle ear occupies tympanic cavity. Tympanic membrane forms lateral wall of tympanic cavity. Handle of malleus attaches to point of maximal concavity of tympanic membrane on its inner surface. Other end of malleus is bound to incus by ligaments. Opposite end of incus articulates with stem of stapes.
Faceplate of stapes lies against membranous labyrinths in oval window, where sound waves are conducted into cochlear. Auditory ossicles increase pressure exerted by sound waves on fluid of cochlear. Thus provides impedance matching between sound waves in air and sound vibrations in fluid of cochlear.
Tympanic cavity is filled with air. Besides auditory ossicles tympanic cavity also contains tensor tympani muscle and stapedius muscle. There are two windows in medial wall of tympanic cavity, round window and oval window.
Attenuation reflex and adaptation to sound
It is a reflex that occurs when loud sounds low frequency sounds are transmitted through ossicular system into central nervous system. It occurs after latent period of 40-80 ms.
Tensor tympani muscle pulls handle of malleus inwards. Stapedius muscle pulls stapes out of oval window. These two forces are opposite to each other. It causes entire ossicular system to become highly rigid.
This mechanism reduces ossicular conduction of loud or low frequency sounds. As a result intensity of sound, which comes into inner ear reduces to 30-40 decibels.
Attenuation reflex importance
Attenuation reflex protects basilar membrane of cochlear from damaging vibrations of loud sounds. It masks also low frequency sounds in loud environment. This phenomenon a person to concentrate on high frequency sounds.
Attenuation reflex is actual also for decrease person’s hearing sensitivity to his own speech. Prolonged hearing of a loud sound also lead to transient loss of sensitivity of central auditory neurons because of inhibitory process. It is called central adaptation to sound.
Mechanism of conduction of sound waves
Sound waves strike tympanic membrane. Ossicular system conducts this sound. Faceplate of stapes moves inward into scala media at oval window. So fluid moves inward into scala media. It causes vibration of basilar membrane. When basilar membrane bends upward to scala vestibuli, hair cells depolarize and generate action potential to nerve fibers of cochlear nerve.
Place principle determines sound frequency by determining the position along basilar membrane that it most stimulated. Low frequency sounds cause maximal stimulation of basilar membrane near apex of cochlear. High frequency sounds cause maximal stimulation of basilar membrane near base of cochlear. Intermediate frequency sounds cause maximal stimulation of basilar membrane at middle of cochlear.
Determination of loudness of sounds
There are three ways for determination of loudness of sounds during its perception. As sound becomes louder, amplitude of vibrations of basilar membrane increases. so hair cells produce impulses at rapid rate.
Increase of sound amplitude cause rise of quantity of excited hair cells. Loud sound stimulates also special high-threshold hair cells, which appraise the central nervous system, that sound is loud.
Central division of auditory analyzer
Hair cells are secondary sensitive cells, which give receptor potential to neurons of spiral ganglion of Corti. Then impulse puss to vestibulocochlear nerve – dorsal and ventral cochlear nuclei in upper medulla – trapezoid body – superior olivary nucleus – lateral lemniscus. Then fibers divide into three parts, which go to:
nucleus of lateral lemniscus;
– higher centers;
– inferior colliculi – medial geniculate nucleus – auditory cortex through auditory radiation.
Auditory cortex lies in superior gyrus of temporal lobe and performs final processing of auditory information.
Binaural hearing
Binaural hearing helps in determination of direction to sound origin. Binaural hearing provides detection of time-lag between entry of sound into one ear and into opposite ear. Medial superior olivary nucleus detects this information.
Difference between intensities of sound in two ears also is important for determination of direction to sound origin. Lateral superior olivary nucleus detects it.
ENDOLYMPH AND PERILYMPH
The perilymph, which fills the space within the bony labyrinth surrounding the membranous labyrinth, is similar, but not identical, in composition to other extracellular fluids of the body, such as cerebrospinal fluid. The concentration of sodium ions in the perilymph is high (about 150 milliequivalents per litre), and that of potassium ions is low (about 5 milliequivalents per litre), as is true of other extracellular fluids. Like these fluids, the perilymph is apparently formed locally from the blood plasma by transport mechanisms that selectively allow substances to cross the walls of the capillaries. Although it is anatomically possible for cerebrospinal fluid to enter the cochlea by way of the perilymphatic duct, experimental studies have made it appear unlikely that the cerebrospinal fluid is involved in the normal production of perilymph.
The membranous labyrinth is filled with endolymph, which is unique among extracellular fluids of the body, including the perilymph, in that its potassium ion concentration is higher (about 140 milliequivalents per litre) than its sodium ion concentration (about 15 milliequivalents per litre).
The process of formation of the endolymph and the maintenance of the difference in ionic composition between it and perilymph are not yet completely understood. Reissner’s membrane forms a selective barrier between the two fluids. Blood-endolymph and blood-perilymph barriers, which control the passage of substances such as drugs from the blood to the inner ear, appear to exist as well. Evidence indicates that the endolymph is produced from perilymph as a result of selective ion transport through the epithelial cells of Reissner’s membrane and not directly from the blood. The secretory tissue called the stria vascularis, in the lateral wall of the cochlear duct, is thought to play an important role in maintaining the high ratio of potassium ions to sodium ions in the endolymph. Other tissues of the cochlea, as well as the dark cells of the vestibular organs, which must produce their own endolymph, are also thought to be involved in maintaining the ionic composition of the endolymph. Because themembranous labyrinth is a closed system, the questions of flow and removal of the endolymph are also important. The endolymph is thought to be reabsorbed from the endolymphatic sac, although this appears to be only part of the story. Other cochlear and vestibular tissues may also have important roles in regulating the volume and maintaining the composition of the inner-ear fluids.
Vestibular analyzer
The vestibular system is the sensory apparatus of the inner ear that helps the body maintain its postural equilibrium. The information furnished by the vestibular system is also essential for coordinating the position of the head and the movement of the eyes. There are two sets of end organs in the inner ear, or labyrinth: the semicircular canals, which respond to rotational movements (angular acceleration); and the utricle and saccule within the vestibule, which respond to changes in the position of the head with respect to gravity (linear acceleration). The information these organs deliver is proprioceptive in character, dealing with events within the body itself, rather than exteroceptive, dealing with events outside the body, as in the case of the responses of the cochlea to sound. Functionally these organs are closely related to the cerebellum and to the reflex centres of the spinal cord and brain stem that govern the movements of the eyes, neck, and limbs. For anatomical descriptions of the vestibular apparatus see Anatomy of the human ear: Inner ear: Vestibular system.
lthough the vestibular organs and the cochlea are derived embryologically from the same formation, the otic vesicle, their association in the inner ear seems to be a matter more of convenience than of necessity. From both the developmental and the structural point of view, the kinship of the vestibular organs with the lateral line system of the fish is readily apparent. The lateral line system is made up of a series of small sense organs located in the skin of the head and along the sides of the body of fishes. Each organ contains a crista, sensory hair cells, and a cupula, as found in the ampullae of the semicircular ducts. The cristae respond to waterborne vibrations and to pressure changes.
The anatomists of the 17th and 18th centuries assumed that the entire inner ear, including the vestibular apparatus, is devoted to hearing. They were impressed by the orientation of the semicircular canals, which lie in three planes more or less perpendicular to one another, and believed that the canals must be designed for localizing a source of sound in space. The first investigator to present evidence that the vestibular labyrinth is the organ of equilibrium was a French experimental neurologist, Marie-Jean-Pierre Flourens, who in 1824 reported a series of experiments in which he had observed abnormal head movements in pigeons after he had cut each of the semicircular canals in turn. The plane of the movements was always the same as that of the injured canal. Hearing was not affected when he cut the nerve fibres to these organs, but it was abolished when he cut those to the basilar papilla (the bird’s uncoiled cochlea). It was not until almost half a century later that the significance of his findings was appreciated and the semicircular canals were recognized as sense organs specifically concerned with the movements and position of the head.
Because the three semicircular canals—superior, posterior, and horizontal—are positioned at right angles to one another, they are able to detect movements in three-dimensional space (seeAnatomy of the human ear: Inner ear: Semicircular canals). When the head begins to rotate in any direction, the inertia of the endolymph causes it to lag behind, exerting pressure that deflects the cupula in the opposite direction. This deflection stimulates the hair cells by bending their stereocilia in the opposite direction. The German physiologist Friedrich Goltz formulated the “hydrostatic concept” in 1870 to explain the working of the semicircular canals. He postulated that the canals are stimulated by the weight of the fluid they contain, the pressure it exerts varying with the head position. In 1873 the Austrian scientists Ernst Mach and Josef Breuer and the Scottish chemistCrum Brown, working independently, proposed the “hydrodynamic concept,” which held that head movements cause a flow of endolymph in the canals and that the canals are then stimulated by the fluid movements or pressure changes. The German physiologist J.R. Ewald showed that the compression of the horizontal canal in a pigeon by a small pneumatic hammer causes endolymph movement toward the crista and turning of the head and eyes toward the opposite side. Decompression reverses both the direction of endolymph movement and the turning of the head and eyes. The hydrodynamic concept was proved correct by later investigators who followed the path of a droplet of oil that was injected into the semicircular canal of a live fish. At the start of rotation in the plane of the canal the cupula was deflected in the direction opposite to that of the movement and then returned slowly to its resting position. At the end of rotation it was deflected again, this time in the same direction as the rotation, and then returned once more to its upright stationary position. These deflections resulted from the inertia of the endolymph, which lags behind at the start of rotation and continues its motion after the head has ceased to rotate. The slow return is a function of the elasticity of the cupula itself.
These opposing deflections of the cupula affect the vestibular nerve in different ways, which have been demonstrated in experiments involving the labyrinth removed from a cartilaginous fish. The labyrinth, which remained active for some time after its removal from the animal, was used to record vestibular nerve impulses arising from one of the ampullar cristae. When the labyrinth was at rest there was a slow, continuous, spontaneous discharge of nerve impulses, which was increased by rotation in one direction and decreased by rotation in the other. In other words, the level of excitation rose or fell depending on the direction of rotation.
The deflection of the cupula excites the hair cells by bending the cilia atop them: deflection in one direction depolarizes the cells; deflection in the other direction hyperpolarizes them. Electron-microscopic studies have shown how this polarization occurs. The hair bundles in the cristae are oriented along the axis of each canal. For example, each hair cell of the horizontal canals has itskinocilium facing toward the utricle, whereas each hair cell of the superior canals has its kinocilium facing away from the utricle. In the horizontal canals deflection of the cupula toward the utricle—i.e., bending of the stereocilia toward the kinocilium—depolarizes the hair cells and increases the rate of discharge. Deflection away from the utricle causes hyperpolarization and decreases the rate of discharge. In superior canals these effects are reversed.
Detection of linear acceleration: static equilibrium
The gravity receptors that respond to linear acceleration of the head are the maculae of the utricle andsaccule (see Anatomy of the human ear: Inner ear: Vestibule). The left and right utricular maculae are in the same, approximately horizontal, plane and because of this position are more useful in providing information about the position of the head and its side-to-side tilts when a person is in an upright position. The saccular maculae are in parallel vertical planes and probably respond more to forward and backward tilts of the head.
Both pairs of maculae are stimulated by shearing forces between the otolithic membrane and the cilia of the hair cells beneath it. The otolithic membrane is covered with a mass of minute crystals of calcite (otoconia), which add to the membrane’s weight and increase the shearing forces set up in response to a slight displacement when the head is tilted. The hair bundles of the macular hair cells are arranged in a particular pattern—facing toward (in the utricle) or away from (in the saccule) a curving midline—that allows detection of all possible head positions. These sensory organs, particularly the utricle, have an important role in the righting reflexes and in reflex control of the muscles of the legs, trunk, and neck that keep the body in an upright position. The role of the saccule is less completely understood. Some investigators have suggested that it is responsive to vibration as well as to linear acceleration of the head in the sagittal (fore and aft) plane. Of the two receptors, the utricle appears to be the dominant partner. There is evidence that the mammalian saccule may even retain traces of its sensitivity to sound inherited from the fishes, in which it is the organ of hearing.
Disturbances of the vestibular system
The relation between the vestibular apparatus of the two ears is reciprocal. When the head is turned to the left, the discharge from the left horizontal canal is decreased, and vice versa. Normal posture is the result of their acting in cooperation and in opposition. When the vestibular system of one ear is damaged, the unrestrained activity of the other causes a continuous false sense of turning (vertigo) and rhythmical, jerky movements of the eyes (nystagmus), both toward the uninjured side. When the vestibular hair cells of both inner ears are injured or destroyed, as can occur during treatment with theantibiotics gentamicin or streptomycin, there may be a serious disturbance of posture and gait (ataxia) as well as severe vertigo and disorientation. In younger persons the disturbance tends to subside as reliance is placed on vision and on proprioceptive impulses from the muscles and joints as well as on cutaneous impulses from the soles of the feet to compensate for the loss of information from the semicircular canals. Recovery of some injured hair cells may occur.
Routine tests of vestibular function traditionally have involved stimulation of the semicircular canals to elicit nystagmus and other vestibular ocular reflexes. Rotation, which can cause vertigo and nystagmus, as well as temporary disorientation and a tendency to fall, stimulates the vestibular apparatus of both ears simultaneously. Because otoneurologists are usually more interested in examining the right and left ears separately, they usually employ temperature change as a stimulant. Syringing the ear canal with warm water at
The vestibular system may react to unaccustomed stimulation from the motion of an aircraft, ship, or land vehicle to produce a sense of unsteadiness, abdominal discomfort, nausea, and vomiting. Effects not unlike motion sickness, with vertigo and nystagmus, can be observed in the later stages of acute alcoholic intoxication. Vertigo accompanied by hearing loss is a prominent feature of the periodic attacks experienced by patients with Ménière’s disease, which, until the late 19th century, was confused with epilepsy. It was referred to as apoplectiform cerebral congestion and was treated by purging and bleeding. Other forms of vertigo may present the otoneurologist with more difficult diagnostic problems.
Since the advent of space exploration, interest in experimental and clinical studies of the vestibular system has greatly increased. Investigators are concerned particularly about its performance when persons are exposed to the microgravity of spaceflight, as compared with the Earth’s gravitational field for which it evolved. Investigations include the growing use of centrifuges large enough to rotate human subjects, as well as ingeniously automated tests of postural equilibrium for evaluating the vestibulospinal reflexes. Some astronauts have experienced relatively minor vestibular symptoms on returning from spaceflight. Some of these disturbances have lasted for several days, but none have become permanent.( http://www.britannica.com/EBchecked/topic/175622/human-ear/65062/The-physiology-of-balance-vestibular-function)
Functions of vestibular analyzer
It is analyzer system, which detects sensation concerned with equilibrium and determines normal orientation and body movement with respect to direction of gravity power or acceleratory forces.
Information from vestibular analyzer helps in proper autonomic and metabolic control of skeletal muscles, which support proper locomotion and body posture under the influence of Earth Gravity power.
Peripheral division of vestibular analyzer
It is located in bony tubes and chambers in petrose portion of temporal bone. Membranous labyrinth is a system of soft tissue tubes and chambers filling within bony labyrinth.
There are three semicircular canals ant two chambers, i.e. utricle and sacule, which are filled with endolymph
Utricle and saccule function
Macula is sensory organ of utricle and saccule. It is covered with gelatinous layer. Cilia of hair cells lay there. Hair cells synapse with sensory axons of vestibular nerve. Macula of utricle determines normal orientation of willi respect to direction of gravitational or acceleratory forces.
Macula of sacule detects certain tips of sounds and detects equilibrium when head is not in vertical position. In the beginning or end of linear motion of the head or whole the body, otolith membrane moves because of its inertion. So, irritation of sensory hair cells occurs.
Semicircular canal function
Ampula is enlargement at one end of semicircular canal. It has a small crest on top of which is a gelatinous mass known as cupula. Hair cells have two kinds of cilia – kinocilium and stereocilia.
Kinocilium is large cilium located at one end of hair cell. Stereocilia are small. When stereocilia are bent towards kinocilium, hair cell is depolarized, i.e. stimulated.
Conductive and central division of vestibular analyzer
Hair sensory cells give impuls to vestibulocochlear nerve, vestibular nuclei and uvula and flocculonodular lobe. Superior and medial vestibular nuclei cause corrective movements of eyes and appropriate movements of head and neck.
Lateral vestibular nucleus controls body movements. Inferior vestibular nucleus sends signals to cerebellum and reticular formation.
Vestibulosomatic reactions
These are reactions of skeletal musculature as a result of irritation of vestibular sensory organs. Vestibulosomatic reactions are presented by distribution of muscle tone through the body, general motor reactions or eyeball movements.
In case of assuasive or unusual vestibular stimulation rapid uneven movements of both eyeballs occur. This is nystagmus.
If head is rotated to left, eyes move towards right in order to prevent image from moving off fovea. When eyes have rotated as far as they can, they are rapidly returned to center of socket. If rotation of head continues, eyes move in direction opposite to head rotation.
Vestibuloautonomic reactions
Diverging of impulses to autonomic subcortical nuclei is necessary for proper distribution of blood supply and metabolic activity between visceral organs and contracting muscles.
Inadequate stimulation of vestibular receptors lead to improper autonomic stimulation, which may result in changing of heart beat rate, arterial pressure, motoric of digestive tract so on.
Age peculiarities of vestibular sensory system
Until the moment of birth human has vestibular analyzer system developed well. All the conductive pathways are mielinised.
Vestibular reactivity has genetic basis and could not be changed considerably through the life.
