PHYSIOLOGY OF HEARING: PHYSIOLOGY OF HEARING BY:INDERDEEP SINGH ARORA DEPT. OF OTORHINOLARYNGOLOGY G.R.M.C. GWALIOR GUIDE: DR.A.K.JAIN(M.S.) PROF.&HEAD CO -GUIDE-- DR.V.P.NARVE(M.S.) DR.R.NIGAM(M.S.) DR.R.CHATURVEDI DR.A.SAXENA DR.A.GUPTA SOUND: SOUND Sound is a form of energy that propagates in the form of waves The speed of sound depend on the medium through which the wave pass Speed of sound in air is 343m/ s,in water is 1482m/sec The sound frequencies audible to humans range from about 20 to a maximum of 20,000 cycles per second (cps, Hz). PowerPoint Presentation: The human ear is sensitive to sound over wide range of amplitudes:0.0002—200 dyne/cm2 It can detect the difference between two sounds occuring 10micro seconds apart in time. EAR AS A TRANSDUCER: EAR AS A TRANSDUCER Terms to remember: Terms to remember Natural resonant frequency: Natural resonant frequency EXTERNAL AUDITORY CANAL--------------- 3000Hz TYMPANIC MEMBRANE----------------------- 800-1600Hz MIDDLE EAR---------------------------------------- 800Hz OSSICULAR CHAIN------------------------------ 500-2000Hz EXTERNAL EAR: EXTERNAL EAR Pinna,concha and external auditory meatus have two main influences on incoming sound Sound collection: Sound collection Pinna - concha system catches sound over large area and concentrate it to smaller area of ext. auditory meatus . This increases the total energy available to the tympanic membrane Pressure increase by EAC: Pressure increase by EAC If a tube which is closed at one end and open at other is placed in a sound field then pressure is low at open end and high at closed end. This phenomenon is seen in EAC at 3kHz frequency,and at concha at 5kHz The two main resonance are complementory,and increases sound pressure in range of 2-7kHz. TOTAL GAIN: TOTAL GAIN The total effect of reflection of sound from head,pinna and external canal resonances is to add 15-20dB to sound pressure, over frequency range of 2-7kHz. Sound localization: Sound localization Because of its shape , the pinna shield the sound from rear end,change timbre,and helps to localize sound from infront or back Cues for sound localization from right/left Sound wave reaches the ear closer to sound source before it arise in farthest ear Sound is less intense as it reaches the farthest ear because head act as barrier Auditory cortex integrates these cues to determine location. FUNCTIONS OF MIDDLE EAR: FUNCTIONS OF MIDDLE EAR Impedence mismatch: Impedence mismatch IF THERE WAS NO MIDDLE EAR SYSTEM ,99% OF SOUND WAVES WOULD HAVE REFLECTED BACK FROM OVAL WINDOW MIDDLE EAR BY ITS IMPEDENCE MATCHING PROPERTY ALLOWS 60% OF SOUND ENERGY TO DISSIPATE IN INNER EAR “Impedance Matching” by the middle ear System: “Impedance Matching” by the middle ear System HYDRauLIC ACTION OF TYMPANIC MEMBRANE: HYDRauLIC ACTION OF TYMPANIC MEMBRANE Total area of tympanic membrane 90mm2 Functional area of tympanic membrane is two third Area of stapes footplate is 3.2mm2 Effective areal ratio is 14:1 Thus by focusing sound pressure from large area of tympanic membrane to small area of oval window the effectiveness of energy transfer between air to fluid of cochlea is increased Lever action of ossicles : Lever action of ossicles Handle of malleus is 1.3 times longer than long process of incus Overall this produces a lever action that converts low pressure with along lever action at malleus handle to high pressure with a short lever action at tip of long process of incus action of tympanic membrane: action of tympanic membrane Eustachian tube equilibriates the air pressure in middle ear with that of atmospheric pressure,thus permitting tympanic membrane to stay in its most neutral position. A buckling motion of tympanic membrane result in an increased force and decreased velocity to produce a fourfold increase in effectiveness of energy transfer Total gain: Total gain Total transformer ratio=14x1.3x4=73:1 Attenuation reflex: Attenuation reflex When loud sounds are transmitted through the ossicular system and from there into the central nervous system, a reflex occurs after a latent period of only 40 to 80 ms to cause contraction of the stapedius muscle and the tensor tympani muscle The tensor tympani muscle pulls the handle of the malleus inward while the stapedius muscle pulls the stapes outward. These two forces oppose each other and thereby cause the entire ossicular system to develop increased rigidity, thus greatly reducing the ossicular conduction of low frequency sound Function of attenuation reflex: Function of attenuation reflex To protect the cochlea from damaging vibrations caused by excessively loud sound. To mask low-frequency sounds in loud environments. This usually removes a major share of the background noise To decrease a person’s hearing sensitivity to his or her own speech PHASE DIFFERENTIAL EFFECT: PHASE DIFFERENTIAL EFFECT Sound waves striking the tympanic membrane do not reach the oval and round window simultaneously. There is preferential pathway to oval window due to ossicular chain. This acoustic separation of windows is achieved by intact tympanic membrane and a cushion of air around round window This contributes 4dB when tympanic membrane is intact COCHLEA ---TWO FUNCTIONS….: COCHLEA ---TWO FUNCTIONS…. A TRANSDUCER that translates sound energy into a form suitable for stimulating the dendrites of auditory nerve. AN ENCODER that programs the features of an acoustic stimulus so that the brain can process the information contained instimulating sound. Travelling wave theory: Travelling wave theory The movements of the footplate of the stapes set up a series of traveling waves in the perilymph of the scala vestibuli High-pitched sounds generate waves that reach maximum height near the base of the cochlea; low-pitched sounds generate waves that peak near the apex The basilar membrane is not under tension, and it also is readily depressed into the scala tympani by the peaks of waves in the scala vestibuli PowerPoint Presentation: The tops of the hair cells in the organ of Corti are held rigid by the reticular lamina, and the hairs of the outer hair cells are embedded in the tectorial membrane The hairs of the inner hair cells are not attached to the tectorial membrane, but they are apparently bent by fluid moving between the tectorial membrane and the underlying hair cells. PowerPoint Presentation: 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. PowerPoint Presentation: Upward movement of the basilar fiber rocks the reticular lamina upward and inward toward the modiolus. Then , when the basilar membrane moves downward,the reticular lamina rocks downward and outward. The inward and outward motion causes the hairs on the hair cells to shear back and forth against the tectorial membrane.Thus , the hair cells are excited whenever the basilar membrane vibrates Endocochlear potential: Endocochlear potential 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 Resting potential ofhair cells: Resting potential ofhair cells Each hair cell has an intracellular potential of (-70mV) with respect to perilymph . At upper end of hair cell the potential difference between intracellular fluid and endolymph is (-150mV) This high potential difference makes the cell very sensitive. Tip links: Tip links the tops of the shorter stereocilia are attached by thin filaments to the back sides of their adjacent longer stereocilia . Depolarization/activation: Depolarization/activation when the cilia are bent in the direction of the longer ones, the tips of the smaller stereocilia are tugged outward.This causes a mechanical transduction that opens 200 to 300 cation -conducting channels, allowing rapid movement of potassium ions from the surrounding scala media fluid into the stereocilia , which causes depolarization of the hair cell membrane PowerPoint Presentation: The influx of potassium inside the cell causes activation of calcium channels This calcium drags the neurotransmitter filled vesicle to fuse with cell membrane at base of cell. Neurotransmitter (glutamate)releases and excites the dendrites of afferent nerve fibres . Tuning by outer hair cells: Tuning by outer hair cells Tuning of sound in basilar membrane requires local addition of mechanical energy There are efferent fibres from crossed olivocochlear bundle supplying the outer cells The inputs from these bundle causes contraction of outer cells located close to maximum of travelling wave give rise to extra distortion of basilar membrane This provides an extra gain of 40-50dB to the system Cochlear echoes/otoacoustic emissions: Cochlear echoes/ otoacoustic emissions Energy produced by outer hair cell motility serves as an amplifier within the cochlea, contributing to better hearing OAEs are produced by the energy from outer hair cell motility that makes its way outward from the cochlea through the middle ear, vibrating the tympanic membrane, and propagating into the external ear canal Determination of Sound Frequency— The “Place” Principle: Determination of Sound Frequency— The “Place” Principle There is spatial organization of the nerve fibers in the cochlear pathway, all the way from the cochlea to the cerebral cortex Specific brain neurons are activated by specific sound frequencies The major method used by the nervous system to detect different sound frequencies is to determine the positions along the basilar membrane that are most stimulated. This is called the place principle Central auditory pathway: Central auditory pathway nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nuclei second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus the superior olivary nucleus,the auditory pathway passes upward through the lateral lemniscus . PowerPoint Presentation: Some of the fibers terminate in the nucleus of the lateral lemniscus , but many bypass this nucleus and travel on to the inferior colliculus , where all or almost all the auditory fibers synapse From there, the pathway passes to the medial geniculate nucleus, where all the fibers do synapse Finally, the pathway proceeds by way of the auditory radiation to the auditory cortex, located mainly in the superior gyrus of the temporal lobe. Pecularities of auditory pathway: Pecularities of auditory pathway First,signals from both ears are transmitted through the pathways of both sides of the brain, with a preponderance of transmission in the contralateral pathway Second, many collateral fibers from the auditory tracts pass directly into the reticular activating system of the brain stem Third, a high degree of spatial orientation is maintained in the fiber tracts from the cochlea all the way to the cortex Function of auditory cortex: Function of auditory cortex Perception of sound Judging the intensity of the sound Analysis of different property of sound Determination of Loudness: Determination of Loudness Determined by the auditory system in at least three ways. First, as the sound becomes louder, the amplitude of vibration of the basilar membrane and hair cells also increases, so that the hair cells excite the nerve endings at more rapid rates PowerPoint Presentation: Second, as the amplitude of vibration increases, it causes more and more of the hair cells on the fringes of the resonating portion of the basilar membrane to become stimulated, thus causing spatial summation of impulses. Third, the outer hair cells do not become stimulated significantly until vibration of the basilar membrane reaches high intensity, and stimulation of these cells presumably apprises the nervous system that the sound is loud.