Download Free Medical Physiology of Hearing and Equilibrium PowerPoint Presentation

Physiology of Hearing and Equilibrium PowerPoint Presentation

DOWNLOAD MEDICAL POWERPOINT PRESENTATION

About This Presentation

Description : The wave motion of sound can be described in terms of Amplitude, Frequency, Velocity and Wavelength

Tags : physiology of hearing,equilibrium,hearing and equilibrium,physiology

Published on : Aug 25, 2024 | Views : 1440 | Downloads : 1

Shareable Url :

Embed Code :

Comment

Transcript

Slide 1-

PHYSIOLOGY OF HEARING ANDEQUILIBRIUM Dr Harjitpal Singh Assistant Professor(ENT), Dr RKGMC, Hamirpur
Slide 2-

SOUND Sound is a form of energy It is transmitted through a medium as a longitudinal pressure wave The wave consists of a series of compressions and rarefactions of the molecules in the medium The ear is capable of capturing this energy and perceiving it as sound information
Slide 3-

PROPERTIES OF SOUND The wave motion of sound can be described in terms of Amplitude, Frequency, Velocity and Wavelength
Slide 4-

TERMS TO REMEMBER Strength of the sound Loudness denotes the appreciation of sound intensity Amplitude/intensity/loudness Number of cycles per second Pitch /Tone denotes the appreciation of frequency Frequency/Pitch/Tone Resistance offered by a medium to sound waves [Pressure/velocity], [Density x Speed] Impedence
Slide 5-

Resonance is the tendency of a system to oscillatewith larger amplitude at some frequencies than at others RESONANCE Attenuation is a general term that refers to any reduction in the strength of a signal ATTENUATI ON TERMS TO REMEMBER
Slide 6-

AMPLITUDE/ LOUDNESS The intensity of sound is perceived as loudness It is measured on a relational scale with the unit of measurement being the decibel (after Alexander Graham Bell) Sound intensities require a standard sound level against which they are compared The standard sound pressure level (SPL = 0dB) is 0.0002 dynes/cm2 The decibel(dB) is a numeric value that represents sound intensity with respect to the reference sound pressure level
Slide 7-

AMPLITUDE/ LOUDNESS
Slide 8-

FREQUENCY/PITCH Frequency is perceived as the pitch of a sound The higher the frequency, the higher the pitch and vice versa The range of human hearing is said to be from 20 - 20,000 Hz The speech frequencies; those frequencies most important for human hearing are from approximately 250 - 4000 Hz
Slide 9-

THEORIES OF HEARING Helmholtz Resonance Theory (Place Theory): According to this theory, sound waves entering the internal ear set up vibrations of particular fibers of basilar membrane depending upon the tones Lower tones stimulate longer fibers at the apex of cochlea and higher tones stimulate smaller fibers at the base of cochlea So each pitch will cause vibration of a particular place on basilar membrane.
Slide 10-

THEORIES OF HEARING Rutherford’s Telephone Theory: Rutherford’s telephone theory says that cochlea acts like a telephone transmitter Hence, basilar membrane gets stimulated by every frequency of sound Pitch depends upon the action potential of the nerve fibers.
Slide 11-

THEORIES OF HEARING George von Bekesy’s Traveling Wave Theory: George von Bekesy was awarded Nobel Prize (1961) for physiology of ear. George von Bekesy’s traveling wave theory is a combination of above two theories and is most widely accepted. It explains the response of whole cochlea to low frequencies and also explains the cochlear analysis of higher frequencies above 5000 Hz at the basal turn of cochlea So high frequency stimulates basal part of basilar membrane, while low tones displace whole of basilar membrane.
Slide 12-

TRANSMISSION OF SOUNDS External ear Mostly through air (External acoustic meatus) Middle ear Through solid medium - bone (ossicles) Inner ear Through fluid medium – endolymph (cochlea)
Slide 13-

TRANSMISSION OF SOUNDS External Auditory Meatus: Resonator Provides about 10 decibels (dB) of gain to the eardrum at around 3,300 Hertz (Hz). The net effect of the head, pinna, and ear canal is that sounds in the 2,000 to 4,000 Hz region are amplified by 10 to 20 dB. Sensitivity to sounds greatest in this frequency region Noises in this range are the most hazardous to hearing
Slide 14-

TRANSMISSION OF SOUND WAVES The outer ear and external auditory canal act passively to capture the acoustic energy and direct it to the tympanic membrane There, the sound waves strike the tympanic membrane causing it to vibrate These mechanical vibrations are then transmitted via the ossicles to the perilymph of the inner ear The perilymph is stimulated by the mechanical (vibrations) energy vibrations to form a fluid wave within the cochlea
Slide 15-

MIDDLE EAR The middle ear acts as an impedance-matching device Sound waves travel much easier through air (low impedance) than water (high impedance) If sound waves were directed at the oval window (water) almost all of the acoustic energy would be reflected back to the middle ear (air) and only 1% would enter the cochlea. This would be a very inefficient method. To increase the efficiency of the system, the middle ear acts to transform the acoustic energy to mechanical energy which then stimulates the cochlear fluid
Slide 16-

MIDDLE EAR The middle ear acts to increase the acoustic energy reaching the cochlea by essentially these mechanical phenomenon The area of tympanic membrane is much larger than the area of stapes footplate, the average ratio between the two being 21:1. As the effective vibratory area of tympanic membrane is only two-thirds, the effective areal ratio is reduced to 14:1, and this is the mechanical advantage provided by the tympanic membrane Lever action of the ossicles. Handle of malleus is 1.3 times longer than long process of the incus, providing a mechanical advantage of 1.3. The product of areal ratio and lever action of ossicles is 18:1.
Slide 17-

MIDDLE EAR 3. Curved Membrane Effect: Movements of the tympanic membrane are more at the periphery than at the centre where malleus handle is attached.
Slide 18-

MIDDLE EAR
Slide 19-

MIDDLE EAR Acoustic coupling is caused by the middle ear pressure that results from EAC sound pressure and motion of eardrum. Ossicular coupling (coupled motion of tympanic membrane and ossicles including stapes footplate) is 60 dB more than acoustic coupling. Oval and round windows are spatially separated Ossicular & Acoustic Coupling Pathways
Slide 20-

MIDDLE EAR Phase Differential Between Oval and Round Window
Slide 21-

PHASE DIFFERNTIAL BETWEEN OVAL AND ROUND WINDOW Sound waves striking the T.M do not reach the oval and the round windows simultaneously. There is a preferential pathway to the oval window because of the ossicular chain. Thus when oval window is receiving waves of compression, the round window is at the phase of rarefaction. If the sound waves were to strike both the windows simultaneously; they would cancel each others effect with no movement of the perilymph and no hearing. This acoustic separation of windows is achieved by the intact T.M and the cushion of air in the middle ear around the round window. Phase differential effect contributes 40db when T.M is intact.
Slide 22-

MIDDLE EAR Functions: Conduction Conduct sound from the outer ear to the inner ear Protection Creates a barrier that protects the middle and inner areas from foreign objects Middle ear muscles may provide protection from loud sounds Transducer Converts acoustic energy to mechanical energy Converts mechanical energy to hydraulic energy Amplifier Transformer action of the middle ear only about 1/1000 of the acoustic energy in air would be transmitted to the inner-ear fluids (about 30 dB hearing loss)
Slide 23-

ORGAN OF CORTI
Slide 24-

ORGAN OF CORTI Attached to the medial aspect of the scala media is a fibrous structure called the tectorial membrane It lies above the inner and outer hair cells coming in contact with their stereocilia The fluid in the space between the tectorial membrane and reticular lamina is endolymph Thus the endolymp bathes the stercocillia But the body of the hair cells which lies below the reticular lamina is bathed by perilym
Slide 25-

ORGAN OF CORTI Synapsing with the base of the hair cells are dendrites from the auditory nerve The auditory nerve leaves the cochlear and temporal bone via the internal auditory canal and travels to the brainstem
Slide 26-

OHC VS IHC FUNCTION
Slide 27-

FUNCTIONS OF HAIR CELLS Inner Hair Cells: They are believed to be the classic auditory receptor cells which signal the brain about the presence of specific sound. Outer Hair Cells: They have been shown to shorten and lengthen when stimulated by sound. A protein called prestin provides OHCs their ability to contract.
Slide 28-

FUNCTIONS OF HAIR CELLS Outer Hair Cells: They are thought to have following functions: Amplification: OHCs amplify effect of sound stimuli to their adjacent IHCs. Sharpening: OHCs sharpen the frequency response of adjacent IHCs. Inhibitory: Efferent stimulation of OHCs is responsible for decreasing the responsiveness of cochlea. Cochlear microphonics: OHCs are responsible for cochlear microphonic effect of electrocochleography. Otoacoustic emissions: OHCs produce otoacoustic emissions that can be recorded and used to screen newborns for hearing loss.
Slide 29-

COCHLEA The cochlea consists of a fluid filled bony canal within which lies the cochlear duct containing the sensory epithelium Energy enters the cochlea via the stapes bone at the oval window and is dissipated through a second opening (which is covered by a membrane) the round window Vibrations of the stapes footplate cause the perilymph to form a wave This wave travels the length of the cochlea It takes approximately 5 msec to travel the length of the cochlea
Slide 30-

COCHLEA As it passes the basilar membrane of the cochlear duct, the fluid wave causes the basilar membrane to move in a wave-like fashion (i.e. up and down) The wave form travels the length of the cochlea and is dissipated at the round window Due to changes in the mechanical properties of the basilar membrane, the amplitude of vibration changes as one travels along the basilar membrane
Slide 31-

COCHLEA As the basilar membrane is displaced superiorly by the perilymph wave, the stereocilia at the apex of each inner and outer hair cell, which are imbedded in the tectorial membrane undergo a shearing force (i.e. they are bent) This shearing force causes a change in the resting membrane potential of the hair cell which is transmitted to its basal end
Slide 32-

COCHLEA There a synapse is formed with a dendrite from the auditory nerve The hair cell membrane potential change is transmitted across this synapse (? via acetylcholine) causing depolarization of the nerve fiber This neural impulse is then propagated to the auditory centres of the brain
Slide 33-

COCHLEA Low frequency stimuli cause the greatest vibration of basilar membrane at its apex, high frequency stimuli at its base-The place principle
Slide 34-

THE PLACE PRINCIPLE
Slide 35-

FUNCTION OF INNER EAR Convert mechanical sound waves to neural impulses that can be recognized by the brain for: Hearing Balance
Slide 36-

Transduction of Mechanical Energy to Electrical Impulses Round Window Reflex: The round window membrane moves in response to the movement of footplate of stapes. When stapes is pressed pressure is exerted to scala vestibuli perilymph which is transferred to scala media and than to scala tympani. The pressure is ultimately transferred to round window which bulges into middle ear.
Slide 37-

TRANSDUCTION Transduction is the conversion of mechanical energy to electrical energy. Movements of the stapes footplate are transmitted to the cochlear fluids, which move the basilar and tectorial membranes differentially and sets up shearing force that bends the stereocilia. Movement of stereocilia opens and closes ion channels and produces receptor potential in the IHCs. This cochlear microphonics triggers the nerve impulse by releasing neurotransmitters onto afferent nerve fibers
Slide 38-

MAJOR STEPS OF TRANSDUCTION
Slide 39-

PROCESSING OF AUDITORY SIGNAL Auditory nerve The place principle Intensity of the stimulus is coded as an increase in the frequency of action potentials There is also recruitment of additional nerve fibres as the intensity increases Cochlear nuclei There is tonotopic organisation (neurons are arranged according to the sensitivity to each frequency) Further processing happens
Slide 40-

ppt slide no 40 content not found
Slide 41-

PROCESSING OF AUDITORY SIGNAL Superior olivary complex Impulses from both ears are compared This is necessary for the localisation of sound Lateral leminscus, inferior colliculus, medial geniculate body Further processing happens Temporal lobe Unique feature of cortical neuronal response to auditory stimulus is the brief duration of the response Localisation of sound and sound discrimination based on the sequence of sounds in the stimulus occurs in the cortex
Slide 42-

PERCEPTION OF DIFFERENT CHARACTERISTICS OF SOUND Frequency Starts at the basilar membrane and frequency sharpening occurs throughout the auditory pathway Intensity Starts at the hair cells (OHC are stimulated by weaker stimulus) Frequency of impulses Direction Inter-aural time difference Pattern recognition Cortical function Interpretation of speech Complex cortical phenomenon
Slide 43-

EAR AS A TRANSDUCER SOUNDENERGY MECHANICAL ENERGY ELECTRICAL ENERGY
Slide 44-

PHYSIOLOGY OF HEARING The hearing mechanism can be divided into Mechanical conduction of the sound - the conductive apparatus. Transduction of mechanical energy into electrical impulses - the sensory system of cochlea. Conduction of electrical impulses to the brain - neural pathways.
Slide 45-

PHYSIOLOGY OF HEARING Thus greater sensitivity of the sound transmission is between 500 and 3000 HZ and these are the frequencies most important to man in day to day life conversation. Audible range of frequencies: It lies between 20 Hz and 20,000 Hz. Above 20,000 Hz (supersonic) are heard by dogs and below 20 Hz (subsonic) are heard by bats
Slide 46-

NATURAL RESONANT FREQUENCY EXTERNAL AUDITORY CANAL---------3000Hz TYMPANIC MEMBRANE---------------800-1600Hz OSSICULAR CHAIN------------------------500-2000Hz MIDDLE EAR ---------------------------------800Hz The noise induced hearing loss usually occurs between 3 and 6 kHz with a peak at 4 kHz because that is the resonant frequency of EAC.
Slide 47-

ACOUSTIC IMPEDENCE If we had no outer or middle ear and the oval window of the inner ear were directly exposed to sound waves transmitted through the air, only 0.1% of the sound energy would be transmitted through to the cochlea and the other 99% would be reflected. As a consequence, our hearing would be less sensitive by about 30 dB. The reason is that the inner ear is filled with fluid and consequently it has a much greater acoustic impedance than air.
Slide 48-

ACOUSTIC IMPEDENCE Acoustic impedance is a measure of how readily the particles of the conducting medium can be displaced by the sound waves, which is much more difficult for water than for air. The acoustic impedance is equal to the ratio of the sound pressure to the flow velocity of the particles of the transmitting medium. It is analogous to electrical impedance, which is the ratio of voltage to current flow.
Slide 49-

PHYSIOLOGY OF EQUILIBRIUM Along with hearing, the inner ear is responsible for encoding information about equilibrium (the sense of balance), which it does in semicircular canals and the vestibule, structures that are sometimes collectively referred to as the vestibular apparatus. Several types of sensory receptors provide information to the brain for the maintenance of equilibrium: Eyes Proproceptors in joints, tendons, and muscles
Slide 50-

PHYSIOLOGY OF EQUILIBRIUM Two types of equilibrium: Static (gravitational) equilibrium, which involves the movement of the head with respect to gravity. Dynamic (rotational) equilibrium, which involves acceleration of the head in rotation, horizontal and vertical movements.
Slide 51-

PHYSIOLOGY OF EQUILIBRIUM Static Equilibrium: The information for static equilibrium and linear acceleration (dynamic) comes from the utricle and saccule within the vestibule. The saccule and utricle each contain a sense organ, called the macula, where stereocilia and their supporting cells are found. These maculae (plural) are oriented 90 degrees to one another so that they respond to positions in different planes
Slide 52-

PHYSIOLOGY OF EQUILIBRIUM Static Equilibrium: They respond to changes in position/acceleration because the tips of their stereocilia project into a dense otolithic membrane made up of a mixture containing granules of calcium and protein, called otoliths. When the head moves, gravity causes the stones to move. The movement of the stones within the membrane causes the stereocilia to bend, initiating action potentials in the vestibular nerve fibers that innervate them.
Slide 53-

PHYSIOLOGY OF EQUILIBRIUM Static Equilibrium: Bundles of stereocilia are arranged in various directions, so that any direction of inclination will depolarize a subset of the hair cells. How the body senses head position and the linear (horizontal or vertical) direction of acceleration is determined by the specific pattern of hair-cell activity across the maculae.
Slide 54-

PHYSIOLOGY OF EQUILIBRIUM Dynamic Equilibrium The semicircular canals are three ring-like extensions from the vestibule and are mostly responsible for dynamic equilibrium. One ring is oriented in the horizontal plane and two others are in the vertical plane. At the base of each semicircular canal, where it meets with the vestibule is an enlarged region known as the ampulla, which contains a hair-cell containing structure, called the crista ampullaris that responds to rotational movement. The stereocilia of the hair cells extend into the cupula, a membrane that attaches to the top of the ampulla
Slide 55-

PHYSIOLOGY OF EQUILIBRIUM When the head rotates in a plane parallel to the semicircular canal, the fluid in the canal does not move as quickly as the head is moving. This pushes the cupula in the opposite direction, deflecting the stereocilia and creating a nerve impulse. Considering the semicircular canals on either side of the head, three orthogonal planes are defined, Horizontal plane with both horizontal canals, Two vertical planes 90o to each other with the anterior canal from one side and the posterior canal from the other.
Slide 56-

PHYSIOLOGY OF EQUILIBRIUM In each pair, deflection of the cupula on one side of the body causes depolarization of the hair cells while the same movement causes hyperpolarization of the hair cells on the other side of the body. For example, when the head rotates to the right, the horizontal canals are active and the right side depolarizes while the left hyperpolarizes, indicating the direction of the movement. By comparing the relative movements of all six semicircular canals, the vestibular system can establish movement in any direction within three-dimensional space.
Slide 57-

PHYSIOLOGY OF EQUILIBRIUM Vestibular system is traditionally divided into two parts: Peripheral Vestibular System: It consists of: Semicircular ducts (dynamic labyrinth), Utricle and Saccule (static labyrinth) and Vestibular nerve. Central Vestibular System: It includes: Vestibular nuclei and tracts It integrate vestibular impulses with other systems to maintain body balance.
Slide 58-

Vestibular Receptors and Direction/Plane Of Head Movement
Slide 59-

PHYSIOLOGY OF EQUILIBRIUM Peripheral Receptors: They are of two types: CRISTAE They are located in the ampullated ends of the three semicircular ducts. These receptors respond to angular acceleration. MACULAE They are located in otolith organs (i.e. utricle and saccule). Macula of the utricle lies in its floor in a horizontal plane. Macula of the saccule lies in its medial wall in a vertical plane. They sense position of head in response to gravity and linear acceleration.
Slide 60-

PHYSIOLOGY OF EQUILIBRIUM Cristae Ampullaris: It is a ridge of neuroepithelium, which has type I (flask shaped) and type II (cylindrical) cells.
Slide 61-

PHYSIOLOGY OF EQUILIBRIUM Cristae Ampullaris: When movements of endolymph occur towards kinocilium, the discharge increases, while it occurs towards stereocilia, the discharge decreases. That is, normally the ear of one side tends to push in the opposite direction. Because of this, sensory nerve endings are stimulated and send impulses upwards to the brain giving information about the movements of head.
Slide 62-

PHYSIOLOGY OF EQUILIBRIUM Vestibular Hair Cells.
Slide 63-

PHYSIOLOGY OF EQUILIBRIUM Cristae Ampullaris: During rest with normal vestibular apparatus, the eyes remain in the center due to cancellation effect of right and left vestibular apparatus. Now, if left vestibular apparatus is destroyed, the normal tone exerted by right vestibular apparatus pushes the eyes to the left side. This is corrected to the central position with a quick movement and nystagmus to right side appears.
Slide 64-

PHYSIOLOGY OF EQUILIBRIUM Semicircular Canals: Semicircular canals respond to angular acceleration and deceleration. The three canals, which lie in three different planes, are situated at right angles to each other. The one that lies at right angle to the axis of rotation is most stimulated. Stimulation of semicircular canals produces nystagmus. The stimulus to semicircular canal is flow of endolymph which displaces the cupula.
Slide 65-

PHYSIOLOGY OF EQUILIBRIUM Semicircular Canals: The flow of endolymph towards the ampulla or utricle is called ampullopetal or utriculopetal. The flow of endolymph away from ampulla or utricle is called ampullofugal or utriculofugal) Direction of nystagmus is determined by the plane of the canal being stimulated. The quick component of horizontal nystagmus is always opposite to the direction of flow of endolymph in the horizontal SCC. Nystagmus is horizontal from horizontal canal, rotatory from the superior canal and vertical from the posterior canal.
Slide 66-

PHYSIOLOGY OF EQUILIBRIUM Semicircular Canals: The flow of endolymph displaces the cupula and stimulates the epithelial hair cells of crista in the ampulla of the SSC. In lateral SCC ampullopetal displacement of stereocilia increases (stimulatory) the firing rate whereas ampullofugal displacement decreases (inhibitory) the firing rate. The opposite happens in posterior and superior canals.
Slide 67-

PHYSIOLOGY OF EQUILIBRIUM Utricle And Saccule: They respond to the linear acceleration and deceleration or gravitational pull during the head tilts. Macula of the utricle is situated in its floor in a horizontal plane in the dilated superior portion of the utricle. Macula of saccule is situated in its medial wall in a vertical plane.
Slide 68-

PHYSIOLOGY OF EQUILIBRIUM Utricle And Saccule: During the head tilts hair cells are stimulated by displacement of otolithic membrane. The functions of saccule and utricle are similar but the saccule is also seen to respond to sound vibrations. Saccular macula responds to the tilting of head. If the head is tilted to left side, left saccular macula is stimulated while right saccular macula will remain static. Utricular macula responds to forward and backward movement of head.
Slide 69-

PHYSIOLOGY OF EQUILIBRIUM The vestibular system thus registers changes in the head position, linear or angular acceleration and deceleration, and gravitational effects. This information is sent to the central nervous system where information from other systems— visual, auditory, somatosensory (muscles, joints, tendons, skin)—is also received. Cerebellum, which is also connected to vestibular end organs, further coordinates muscle movements in their rate, range, force and duration and thus helps in the maintenance of balance
Slide 70-

PHYSIOLOGY OF EQUILIBRIUM Maintenance of Body Equilibrium: Sensory Component: The vestibular system records changes in the head position, linear or angular acceleration and deceleration and gravitational effects. Motor Component: The standing and walking need not only sensory integration (from vestibular, somatosensory and visual systems) but also motor commands, which are fine-tuned through the frontal cerebral lobes, cerebellum and basal ganglia. Disorder of any of these systems can lead to dizziness.
Slide 71-

PHYSIOLOGY OF EQUILIBRIUM Push and Pull System: The balance system, which includes vestibular, visual and somatosensory organs, can be compared with a two-sided push and pull system. In a neutral position, push and pull of one side is equal to that of the other side. If one side is pulling more than the other, the body balance is disturbed.
Slide 72-

PHYSIOLOGY OF EQUILIBRIUM Push and Pull System: During the turning or tilting, a temporary change in the push and pull force of one system is taken care of by the appropriate reflexes and motor outputs to Eyes (vestibuloocular reflex), Neck (vestibulocervical reflex) and Trunk and limbs (vestibulospinal reflex)
Slide 73-

PHYSIOLOGY OF EQUILIBRIUM Push and Pull System: Turning the head to the right direction produces an increase in the resting spontaneous outflow of action potential in the nerve coming from right horizontal SCC. Simultaneously there occurs decrease activity in left vestibular nerve. The CNS compares the input coming from each vestibule. There is no sense of movement when input is equal. The CNS interprets asymmetric input not only as a head rotation but also generates compensatory eye movements and postural adjustment.
Slide 74-

PHYSIOLOGY OF EQUILIBRIUM Push and Pull System: The vestibular inhibition on one side (e.g. acute vestibular failure, labyrinthectomy, Ménière’s disease, VIIIth nerve section) causes vertigo. Similarly, stimulation of labyrinth by thermal or rotational stimulus causes vertigo. Dizziness can similarly result from the ocular causes, e.g. high errors of refraction or acute extraocular muscle paralysis with diplopia.
Slide 75-

THANKS

User Presentation
Related Presentations

Physiology of Hearing and Equilibrium

By: HarjitpalSingh
Views: 1440

TUMORS OF NASOPHARYNX

By: HarjitpalSingh
Views: 5124

ANATOMY OF PHARYNX

By: HarjitpalSingh
Views: 8859

Esophagus and Disorders

By: HarjitpalSingh
Views: 3740

Tumours of Nose and PNS

By: HarjitpalSingh
Views: 4882

Ct Anatomy Of Paranasal Sinuses

By: evesabrina
Views: 3266

Ent Emergencies

By: medhelp
Views: 5442

Nasal cavity Pharynx and Larynx

By: drdwayn
Views: 3250

Physiology of Hearing and Equilibrium

By: HarjitpalSingh
Views: 1440

Pediatric Turbinate Hypertrophy

By: KhushbuSG
Views: 2010

HarjitpalSingh

About : Associate Professor (ENT) at Dr Radhakrishnan Govt Medical College, Hamirpur(H P)
Occupation : Medical Professional
Specialty : MD
Country : India
Website :

HEALTH A TO Z