Hearing Inner ear: Cochlea:

Transcription

1 Hearing Inner ear: The inner ear (labyrinth) is made up of two parts; one within the other, the (bony labyrinth) is a series of channels in the petrous portion of the temporal bone. Inside these channels, surrounded by a fluid called (perilymph) is the (membrane labyrinth). This membrane structure more or less duplicates the shape of the bony channels. It is filled with a fluid called (endolymph), and there is no communication between the space filled with endolymph and those filled with perilymph. Cochlea: The cochlear portion of the labyrinth is a coiled tube which in human is 3.5 mm long and makes 2¼ turns. Throughout its length, the basilar membrane and Reissner`s membrane divided it into three chamber s (scalae). The upper scala vestibuli and the lower scala tympani contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicortrema. At the base of the cochlea, the cochlea, the scala vestibuli ends at the oval window, which is closed by the footplate of the stapes. The scala tympani end at the round window, a foramen on the medial wall of the middle ear that is closed by the flexible secondary tympanic membrane. The scala media, the middle cochlear chamber, is continuous with the membranous labyrinth and does not communicate with the other two scala. It contains endolymph. The scala vestibuli and scala media are separated from each other by Reissner`s membrane (also called the vestibular membrane); the scala tympani and scala media are separated from each other by the basilar membrane. On the surface of the basilar membrane lies the organ of Corti, which contains a series of electromechanically sensitive cells, the hair cells. The Reissner`s membrane is so thin and so easily 1

2 moved that it does not obstruct the passage of sound vibrations through fluid from the scala vestibuli into scala media. The basilar membrane contains 20,000 to 30,000 basilar fibers that project from the bony center of the cochlea, the modiolus, toward the outer wall. These fibers are stiff, elastic, reed like structure that are fixed at their basal ends in the central bony structure of the (the modiolus) but not fixed at their distal ends, except that the distal ends are embedded in the loose basilar membrane. Because the fibers are stiff and.الر شة الت تتحرك عند نفخ الهواء ف الهارمونكةharmonica free at one end, they can vibrate like the reeds of a The basilar membrane characterized by: 1. The lengths of the basilar fibers increase progressively beginning at the oval window and going from the base of the cochlea to the apex, increasing from a length of about 0.04 millimeter near the oval and round windows to 0.5 millimeter at the tip of the cochlea (the helicotrema ), a 12-fold increase in length 2. The diameters of the fibers, however, decrease from the oval window to the helicotrema, so their overall stiffness decreases more than 100-fold. As a result, the stiff, short fibers near the oval window of the cochlea vibrate best at a very high frequency, whereas the long, limber fibers near the tip of the cochlea vibrate best at a low frequency. 3. Another feature of the traveling wave is that it travels fast along the initial portion of the basilar membrane but becomes progressively slower as it goes farther into the cochlea. The cause of this difference is the high coefficient of elasticity of the basilar fibers near the oval window and a progressively decreasing coefficient farther along the membrane. Low-frequency resonance occurs near the helicotrema, mainly because of the less stiff fibers and also because of increased loading with extra masses of fluid that must vibrate along the cochlear tubules. The high-frequency resonance of the basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval window. Transmission of sound waves in the cochlea (the traveling wave): When the foot of the stapes moves inward against the oval window and round window must bulge outward because the cochlea is bounded on all sides by bony walls. Therefore, the initial effect of a sound wave entering at the oval window is to cause the basilar membrane at the base of the cochlea to bend in the direction of the round window. However, the elastic tension that is built up in the basilar fibers as they bend toward the round window initiates a fluid wave that travels along the basilar membrane toward the helicortrema. 2

3 Each wave is relatively weak at the outset but becomes strong when it reaches that portion of the basilar membrane that has a natural resonant frequency equal to the respective sound frequency. At this point, the basilar membrane can vibrate back and forth with such ease that energy in the wave is dissipated. Consequently, the wave dies out at this point and fails to travel the remaining distance along the basilar membrane. Thus, a high-frequency sound wave travels only a short distance along the basilar membrane before it reaches its resonant point and dies out; a medium-frequency sound wave travels about halfway and then dies out; and finally, a very low frequency sound wave travels the entire distance along the membrane. This rapid initial transmission of the wave allows the high-frequency sounds to travel far enough into the cochlea to spread out and separate from one another on the basilar membrane. Without this rapid initial transmission, all the high-frequency waves would be bunched together within the first millimeter or so of the basilar membrane, and their frequencies could not be discriminated. Organ of Corti: Located on the basilar membrane is the organ of Corti, the structure that contains the hair cells which the auditory receptors. This organ extended from the apex to the base of the cochlea and consequently has a spiral shape. The processes of the hair cells pierce the tough, membrane-like reticular lamina that is supported by the rods of Corti. Outer hair cells 1. Three rows. 2. lateral to the tunnel formed by the rods of Corti 3. 20,000 outer hair cells. 4. Measuring 8 µm. 5. stereocilia, are embedded in the tectorial membrane to 10 % synapse with a network of cochlear nerve ending Inner hair cells 1. One row 2. medial to the tunnel formed by the rods of Corti inner hair cells. 4. measuring about 12 µm 5. stereocilia, are not embedded in the tectorial membrane ( just touch) to 95 % synapse with a network of cochlear nerve ending emphasizing their special importance for the detection of sound. 3

4 The nerve fibers stimulated by the hair cells lead to the spiral ganglion of Corti, which lies in the modiolus (center) of the cochlea. The spiral ganglion neuronal cells send axons (a total of about 30,000) into the cochlear nerve and then into the central nervous system at the level of the upper medulla. The outer ends of the hair cells are fixed tightly in a rigid structure composed of a flat plate, called the reticular lamina, supported by triangular rods of Corti, which are attached tightly to the basilar fibers. The basilar fibers, the rods of Corti, and the reticular lamina move as a rigid unit. The tips of the stereocilia on the hair cells are embedded in the tectorial membrane, and the bodies of hair cells rest on the basilar membrane 4

5 Depolarization of hair cell: Each hair cell has about 100 stereocilia on its apical border. 5

6 These stereocilia become progressively longer on the side of the hair cell away from the modiolus When the hair bundle is displaced in the direction of the tallest stereocilium depolarization occurs When the hair bundle is displaced in direction away from the tallest stereocilium hyperpolarization occurs When the hair bundle is displaced in direction perpendicular to stereocilium provides no change in membrane potential When the hair bundle is displaced in direction intermediate between these two directions produces depolarization or hyper-polarization that is proportionate to the degree to which the direction is toward or away from the Kino-cilium. The function of inner hair cells The function of inner hair cells is transmission of hearing signals The function of outer hair cells Outer hair cells contact by relatively few sensory axons and therefore accounted for only a small fraction of the auditory information that is sent onto the brain If the outer cells are damaged while the inner cells remain fully functional, a large amount of hearing loss occurs. The principle function of outer hair cells is to amplify the sound-induce vibration and thus increase our ability to hear very quiet sounds Unlike the inner hair cells, the outer hair cells do not signal the brain about incoming sounds. They instead actively and rapidly shortening occurs during depolarization and elongation occurs during hyperpolarization. They do this due to: 1. a very flexible basal membrane, 2. prestin a transmembrane protein that changes the length of outer hair cell. This behavior is known as electromotility. When the outer hair cells elongate, the motion of the basilar membrane is amplified. This modification of the basilar membrane is believed to improve and tune the stimulation of the inner hair cells. The outer hair cells therefore enhance the receptors of the inner hair cells, increasing their sensitivity to sound intensity and rendering them highly discriminatory between various pitches of sound. In support of this concept, a large number of retrograde nerve fibers pass from the brain stem to the vicinity of the outer hair cells (i.e. this will explain why there is afferent and efferent nerve fiber to both outer and inner hair cells) The differences between endolymph and perilymph Perilymph Endo-lymph Found in bony labyrinth Found in membranous labyrinth Like extracellular fluid Like intracellular fluid 6

7 Ultra-filtrated of plasma Low K and Protein, High Na and Ca Produced by stria vascularis High K and Protein, low Na, and Ca An electrical potential of about +80 millivolts exists all the time between endolymph and perilymph, with positivity inside the scala media and negativity outside. This is called the endocochlear potential, and it is generated by continual secretion of positive potassium ions into the scala media by the stria vascularis. Function of cerebral cortex in hearing 1) Primary auditory cortex (AI; area 41) The primary auditory cortex is directly excited by auditory nerve The primary auditory cortex has tonotopic maps "frequency map" for different tones. The primary auditory cortex unilateral lesions do not effect hearing because of completely bilateral sound representation. Sound frequency perception in the primary auditory cortex. Neurons in the auditory cortex are organized according to the frequency of sound to which they respond best. Neurons at one end of the auditory cortex respond best to low frequencies; neurons at the other respond best to high frequencies. The purpose of this frequency map (known as a tonotopic map) is unknown The frequency range to which each individual neuron in the auditory cortex responds is much narrower than that in the cochlear (i.e. sharpen the frequency). It is believed that this sharpening effect is caused mainly by the phenomenon of lateral inhibition. The same effect has been demonstrated to be important in sharpening patterns of somesthetic images, visual images, and other types of sensations. 2) Auditory association cortex or the secondary auditory cortex (AII; area 42) The auditory association areas surrounding the primary auditory cortex The auditory association areas are excited secondarily by impulses from the primary auditory cortex, as well as by some projections from thalamic association areas The auditory association areas are involved in the interpretation of sound. Many of the neurons in the auditory cortex, especially in the auditory association cortex, do not respond only to specific sound frequencies in the ear. It is believed that these neurons associate different sound frequencies with one another or associate sound information with information from other sensory areas of the cortex. Indeed, the parietal portion of the auditory association cortex partly overlaps somatosensory area II, which could provide an easy opportunity for the association of auditory information with somatosensory information. 3) Wernicke's area (Brodmann s area 22) Wernicke's area is concerned with the processing of auditory signals related to speech. Wernicke's area in the dominant hemisphere surrounding the auditory cortex Wernicke's area in the non-dominant hemisphere may be involved in understanding the tone, pitch, and لحن sound intensity and melody Wernicke's area is much more active on the left side than on the right side during language processing 7

8 The auditory pathways are also very plastic, and, like the visual and somasthetic pathways, they are modified by experience. Discrimination of sound Patterns by the auditory Cortex. Destruction of one side primary auditory cortex a. only slightly reduces hearing in the opposite ear b. it does not cause deafness in the same ear because of many crossover connections from side to side in the auditory neural pathway c. it does affect one s ability to localize the source of a sound, because comparative signals in both cortices are required for the localization function. Destruction of both primary auditory cortices in the human being greatly reduces one s sensitivity for hearing. Lesions that affect the auditory association areas but not the primary auditory cortex do not decrease a person s ability to hear and differentiate sound tones, or even to interpret at least simple patterns of sound. However, the person is often unable to فسرinterpret the meaning of the sound heard even though he or she hears them perfectly well and can even repeat them ال نفقد القدرة على السماع اوتفر ق االصوات ولكن عدم القدرة على تفسر معان االصوات وبالتال عدم القدرة على تكرارها Therefore, the auditory cortex is especially important in the discrimination of tonal andنغمة sequential patterns. soundتسلسل Determination of the direction from which sound come A person determines the horizontal direction from which sound comes by two principal means: (1) The intensity mechanism The intensity mechanism determine the distance depending on the difference between the intensities of the sounds reaching the two ears. The intensity mechanism operates best at higher frequencies because the head is a greater sound barrier at these frequencies. If a person is looking straight toward the source of the sound, the sound reaches both ears at exactly the same instant 8

9 If the right ear is closer to the sound than the left ear First, the paths are of different length because sound has to travel past the head to get to the left ear and sound intensity decreases with distance. Second, the head interferes with the sound-wave, casting the auditory equivalent of a shadow on the far ear. And, this sound shadow is more effective for sounds of higher frequency. (2) The time lag تأخر mechanism: The time lag between the entry of sound into one ear and its entry into the opposite ear (as little as 20 μs). This mechanism functions best at frequencies below 3000 cycles/sec The time lag mechanism discriminates direction much more exactly than the intensity mechanism because it does not depend on extraneous factors but only on the exact interval of time between two acoustical signals. The two aforementioned انفا mechanismsمذكور cannot tell whether the sound is emanating from in front of or behind the person or from above or below. This discrimination is achieved mainly by the pinnae of the تأك دempha sizing two ears. The shape of the pinna changes the quality of the sound entering the ear by specific sound frequencies depending on the direction from which the sound comes قوم ص وان االذن بتغ ر طب عة الصوت من خالل تغ ر تردد الصوت الواصل لالذن بناء على اتجاه الذي قدم منه الصوت 9

10 Equilibrium The vestibular system has two parts, the otolith organs (macula) and the semicircular canals. A mechanoreceptor (a hair cell with stereocilia) senses head position, head movement, and whether our bodies are in motion. The neural signals generated in the vestibular ganglion are transmitted through the vestibulecochlear nerve to the brain stem and cerebellum. 1. Sensory organ of utricle and saccule; They are2 millimeters in diameter They give information about static head position moving linearly the in 3 axis of the head The macula in the utricle and saccule contains an array of supporting cells and hair cells. The hair cells have stereocilia each and one large kinocilium. Hair cells stereocilia project into and embedded in the Otolithic membrane Otolithic membrane, contains a gelatinous mass otoliths (literally, ear stones ) or otoconia (literally, ear dust ). Otoconia are crystals of calcium carbonate range from 3 to 19 μm in length in humans Otoconia make the otolithic membrane heavier than the structures and fluids surrounding it. Gravity pulls on the dense otoliths, which deform the gelatinous mass and subsequently press on the stereocilia and influence the firing rate of the hair cells. Since gravity pulls constantly, the hair cells in the utricle and saccule provide tonic information about the orientation of the head. The nerve 10

11 fibers from the hair cells join those from the cristae in the vestibular division of the eighth cranial nerve A. Macula of utricle: 1. Utricle senses the Horizontal plane: موازي سطح االرض and Head orientation when person is upright position a. horizontal linear acceleration (motion in a straight line) They sense how quickly you are accelerating forward or ف الس ارةbackward,كما left or right (side by side) الراس مع الجذع الى االمام او الخلف b. up-down and forward-backward movements in the sagittal plane كما ف رفع ش ء من االرض 2. Macula of Saccule: The saccule senses the Vertical plane: مع مستوى االرض andعمودي Head orientation when person is lying down) كما ف حالة صعود المصعد( downward a. vertical linear acceleration in the sagittal plane (upward or b. head-tilts in the horizontal plane such as sideways head tilts and rapid lateral displacements, الرأس لحاله الى االمام او الخلف او على الجانب ن 11

12 Note that the saccular and utricular maculae on one side of the head are mirror images of those on the other side. Thus, a tilt of the head to one side has opposite effects on corresponding hair cells of the two utricular maculae. This concept is important in understanding how the central connections of the vestibular periphery mediate the interaction of inputs from the two sides of the head When the stereocilia and kinocilium bend in the direction of the kinocilium causing receptor membrane depolarization this will increase firing rated of nerve above 100 per second. The reverse will causes receptor hyperpolarization this will increase firing rated of nerve below 100 per second in same manner as explained above. It is especially important that the hair cells are all oriented in different directions in the maculae of the utricles and saccules so that with different positions of the head, different hair cells become stimulated. The patterns of stimulation of the different hair cells apprise the brain of the position of the head with respect to the pull of gravity. In turn, the vestibular, cerebellar, and reticular motor nerve systems of the brain excite appropriate postural muscles to maintain proper equilibrium. This utricle and saccule system functions extremely effectively for maintaining equilibrium when the head is in the near-vertical position. Indeed, a person can determine as little as half a degree of disequilibrium when the body leans from the precise upright position. The impulses generated from these receptors are partly responsible for labyrinth righting reflexes. These reflexes are a series of responses integrated for the most part in the nuclei of the midbrain. The stimulus for the reflex is tilting of the head, which stimulates the otolithic organs; the response is compensatory contraction of the neck muscles to keep the head level. In cats, dogs, and primates, visual cues شاارات البصار ة canأأل initiate optical righting reflexes that right the animal in the absence of labyrinthine or body stimulation. In humans, the operation of these reflexes maintains the head in a stable position and the eyes fixed on visual targets despite movements of the body and the jerks and jolts of everyday life. The responses are initiated by vestibular stimulation, stretching of neck muscles, and movement of visual images on the retina, and the responses are the vestibulo-ocular reflex and other remarkably precise reflex contractions of the neck and extra-ocular muscles. Spatial orientation Orientation in space depends in part on input from the vestibular receptors visual cues proprioceptors cutaneous extero-ceptors, especially touch and pressure receptors. These four inputs are synthesized at a cortical level into a continuous picture of the individual orientation in space. 12

13 Although most of the responses to stimulation of the maculae are reflex in nature, vestibular impulses also reach the cerebral cortex. These impulses are presumably responsible for conscious perception of motion and supply part of the information necessary for orientation in space. Vertigo is the sensation of rotation in the absence of actual rotation and is a prominent symptom when one labyrinth is inflamed. Rotational movement; Semicircular canal: The three semicircular ducts in each vestibular apparatus, known as the anterior, posterior, and lateral (horizontal) semicircular ducts, are arranged at right angles to one another so that they represent all three planes in space. Each of the three semicircular canals senses just a onedimensional component of rotational acceleration. The three semicircular canals give information about dynamic head position Each of the three semicircular canals is responsible for a specific direction of head movement: One of the canals responds to the head tilting upwards or downwards, one responds to it tilting to the right or to the left, and one responds to it turning sideways. 13

14 Inside the bony canals, the membranous canals are suspended in perilymph. A receptor structure, the crista ampullaris, is located in the expanded end (ampulla) of each of the membranous canals. Each crista consists of hair cells and supporting (sustentacular) cells surmounted by a gelatinous partition (cupula) that closes off the ampulla. The processes of the hair cells are embedded in the cupula, and the bases of the hair cells are in close contact with the afferent fibers of the vestibular division of the eighth cranial nerve. 14

15 Responses to rotational acceleration Rotational acceleration in the plane of a given semicircular canal stimulates its crista. The response passes into the following stages: First: The endolymph, because of its inertia, is displaced in a direction opposite to the direction of rotation. The fluid pushes on the cupula, deforming it. This bends the processes of the hair cells. This will increase the number of nerve impulses (depolarization) 15

16 Second: When a constant speed of rotation is reached, the fluid spins at the same rate as the body and the cupula swings back into the upright position in the middle of the ampulla because of its own elastic recoil This will returns the number of nerve impulses back to basilar level gradually Third: When the rotation suddenly stops, exactly opposite effects take place: The endolymph continues to rotate while the semicircular duct stops. This time, the cupula bends in the opposite direction, causing the hair cell to stop discharging entirely (hyperpolarization) Fourth: the endolymph stops moving and the cupula gradually returns to its resting position in 25 to 30 s, thus allowing hair cell discharge to return to its normal tonic level This will returns the number of nerve impulses back to basilar level gradually Movement of the cupula in one direction commonly causes an increase in the firing rate of single nerve fibers from the crista, whereas movement in the opposite direction commonly inhibits neural activity. Rotation causes maximal stimulation of the semicircular canals most nearly in the plane of rotation. Because the canals on one side of the head are a mirror image of those on the other side, the endolymph is displaced toward the ampulla on one side and away from it on the other. The pattern of stimulation reaching the brain therefore varies with the direction as well as the plane of rotation. Linear acceleration probably fails to displace the cupula and therefore does not stimulate the cristae. However, there is considerable evidence that when one part of the labyrinth is destroyed, other parts take over its functions. 16