Physiology of Hearing

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PHYSIOLOGY OF HEARING:

PHYSIOLOGY OF HEARING Dr.Abhijit Gogoi University of Fiji

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

THE HUMAN EAR:

THE HUMAN EAR

Three Major Section of the Ear:

Three Major Section of the Ear Introduction The ears are paired sensory organs comprising 1. the auditory system , involved in the detection of sound, and 2.the vestibular system , involved with maintaining body balance/ equilibrium. The ear divides anatomically and functionally into three regions: the external ear , the middle ear , and the inner ear . All three regions are involved in hearing. Only the inner ear functions in the vestibular system . 

Anatomy of the Ear :

Anatomy of the Ear The external ear (or pinna , the part you can see) serves to protect the tympanic membrane (eardrum), collects and directs sound waves through the auditory canal to the eardrum . About 3 cm. long, the canal contains modified sweat glands that secrete cerume n , or earwax. Too much cerumen can block sound transmission. 

Anatomy of the Ear:

Anatomy of the Ear The middle ear , separated from the external ear by the eardrum, is an air-filled cavity ( tympanic cavity ) carved out of the temporal bone. It connects to the throat/nasopharynx via the Eustachian tube. This ear-throat connection makes the ear susceptible to infection ( otitis media ). The eustachian tube functions to equalize air pressure on both sides of the eardrum. Normally the walls of the tube are collapsed. Swallowing and chewing actions open the tube to allow air in or out, as needed for equalization. Equalizing air pressure ensures that the eardrum vibrates maximally when struck by sound waves.

Anatomy of the Ear:

Anatomy of the Ear Adjoining the eardrum are three linked, movable bones called " ossicles ," which convert the sound waves striking the eardrum into mechanical vibrations. The smallest bones in the human body , the ossicles are named for their shape. The hammer ( malleus ) joins the inside of the eardrum. The anvil ( incus ), the middle bone, connects to the hammer and to the stirrup ( stapes ). The base of the stirrup, the footplate, fills the oval window which leads to the inner ear. 

Anatomy of the Ear:

Anatomy of the Ear

Anatomy of the Ear:

Anatomy of the Ear The inner ear consists of a maze of fluid-filled tubes , running through the temporal bone of the skull. The bony tubes, the bony labyrinth, are filled with a fluid called perilymph .  Within this bony labyrinth is a second series of delicate cellular tubes, called the membranous labyrinth , filled with the fluid called endolymph . This membranous labyrinth contains the actual hearing cells, the hair cells of the organ of Corti .  

Anatomy of the Ear:

Anatomy of the Ear There are three major sections of the bony labyrinth:  The front portion is the snail-shaped cochlea , which functions in hearing .  The rear part, the semicircular canals , helps maintain balance .  Interconnecting the cochlea and the semicircular canals is the vestibule , containing the sense organs responsible for balance, the utricle and saccule . 

Anatomy of the Ear:

Anatomy of the Ear The inner ear has two membrane-covered outlets into the air-filled middle ear - the oval window and the round window . The oval window sits immediately behind the stapes, the third middle ear bone, and begins vibrating when "struck" by the stapes. This sets the fluid of the inner ear sloshing back and forth. The round window serves as a pressure valve, bulging outward as fluid pressure rises in the inner ear. Nerve impulses generated in the inner ear travel along the vestibulocochlear nerve (cranial nerve VIII), which leads to the brain. (acoustic or auditory nerve) This is actually two nerves, somewhat joined together, the cochlear nerve for hearing and the vestibular nerve for equilibrium. 

How We Hear – The Auditory System :

How We Hear – The Auditory System All sounds (music, voice, a mouse-click, etc.) send out vibrations, or sound waves. Sound waves do not travel in a vacuum, but rather require a medium for sound transmission, e.g. air or fluid. What actually travels are alternating successions of increased pressure in the medium, followed by decreased pressure. These vibrations occur at various frequencies, not all of which the human ear can hear. Only those frequencies ranging from 20 to 20,000 Hz  (Hz = hertz = cycles/sec) can be perceived. 

PowerPoint Presentation:

In hearing, air-borne sound waves funnel down through the ear canal and strike the eardrum, causing it to vibrate. The vibrations are passed to the small bones of the middle ear ( ossicles ), which form a system of interlinked mechanical levers: First, vibrations pass to the malleus (hammer), which pushes the incus (anvil), which pushes the stapes (stirrup). The base of the stapes rocks in and out against the oval window - this is the entrance for the vibrations. The stapes agitates the perilymph of the bony labyrinth. At this point, the vibrations become fluid-borne. The perilymph, in turn, transmits the vibrations to the endolymph of the membranous labyrinth and, thence, to the hair cells of the organ of Corti . It is the movement of these hair cells which convert the vibrations into nerve impulses. The round window dissipates the pressure generated by the fluid vibrations, thus serves as the release valve: It can push out or expand as needed.  The nerve impulses travel over the cochlear nerve to the auditory cortex of the brain, which interprets the impulses as sound. 

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

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

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.

How We Balance – The Vestibular System :

How We Balance – The Vestibular System The semicircular canals and vestibule function to sense movement (acceleration and deceleration) and static position. The three semicircular canals lie perpendicular to each other, one to sense movement in each of the 3 spatial planes. At the base of the canals are movement hair cells, collectively called the crista ampullaris . Depending on the plane of movement, the endolymph flowing within the semicircular canals stimulates the appropriate movement hair cells. Static head position is sensed by the vestibule, specifically, its utricle and saccule , which contain the position hair cells. Different head positions produce different gravity effects on these hair cells. Small calcium carbonate particles (otoliths) are the ultimate stimulants for the position hair cells.

How We Balance – The Vestibular System:

How We Balance – The Vestibular System The hair cells for both position and movement create nerve impulses. These impulses travel over the vestibular nerve to synapse in the brain stem, cerebellum, and spinal cord. No definite connections to the cerebral cortex exist. Instead, the impulses produce reflex actions to produce the corrective response. For example, a sudden loss of balance creates endolymph movement in the semicircular canals that triggers leg or arm reflex movements to restore balance.

Deafness:

Deafness Two types of deafness 1. Conduction deafness Caused by interference of sound from the outside up to the oval window By obstruction,effusion,stiffness and discontinuity May result from Buildup of earwax Scarring or tearing of eardrum Damage to the middle ear ossicles Otitis media ototoxicity

Deafness:

Deafness 2. Sensori -neural deafness Caused by damage to nerve cells or sensory cells presbyacusis May result from damage to Hair cells in the organ of corti The acoustic nerve (usually a tumor) The brain (damage to the temporal lobe of the brain)

Sound Intensity:

Sound Intensity Measured in decibels (dB) 0 dB: threshold of human hearing 25 dB: quiet whisper 50 dB: normal speech 80 dB: very loud, annoying noise 100 dB: pneumatic drill 110 dB: loud music 120 dB: painful, some delayed hearing loss Over 130 dB: permanent hearing loss

Sound Frequency:

Sound Frequency Measured in hertz (Hz) Human hearing range 15 Hz to 20 000 Hz Human speech averages 1000 Hz Human scream 300 Hz Highest note on a piano 4186 Hz

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