PHYSIOLOGY OF HEARING PY 10.15 Any vibrating object causes waves of compression and rarefaction and is capable of producing sound. In the air, at 20°C and at sea level, sound travels at a speed of 344 m (1120 feet) per second. It travels faster in liquids and sol-ids than in the air. Also, when sound energy has to pass from air to liquid medium, most of it is reflected because of the impedance offered by the liquid. MECHANISM OF HEARING A sound signal in the environment is collected by the pinna, passes through external auditory canal and strikes the tympanic membrane. Vibrations of the tympanic membrane are transmitted to stapes footplate through a chain of ossicles coupled to the tympanic membrane. Movements of stapes footplate cause pressure changes in the labyrinthine fluids, which move the basilar mem-brane. This stimulates the hair cells of the organ of Corti. It is these hair cells which act as transducers and convert the mechanical energy into electrical impulses, which travel along the auditory nerve. Thus, the mechanism of hearing can be broadly divided into: 1. Mechanical conduction of sound (conductive apparatus) 2. Transduction of mechanical energy to electrical impulses (sensory system of cochlea) 3. Conduction of electrical impulses to the brain (neural pathways) 4. Conduction of Sound A person under water cannot hear any sound made in the air because 99.9% of the sound energy is reflected away from the surface of water because of the impedance of fered by it. A similar situation exists in the ear when air conducted sound has to travel to cochlear fluids. Nature has compensated for this loss of sound energy by inter posing the middle eat which converts sound sound of greater amplitude but lesser force, to that of lesser amplitude plitude but greater force. This function of the middle ear is called impedance matching mechanism or the transformer action. It is accomplished by: (a) Lever action of the outcles. Handle of malleus is 1.3 long process of the incus, providing times longer than l a mechanical advantage of 1.3. (b) Hydraulic action of tympanic membrane. 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 advan tage provided by the tympanic membrane (Fig. 2.3). The product of areal ratio and lever action of ovicles is 18:1 According to some workers (Wever and Lawrence), out of a total of 90 mm³ area of human tympanic mem brane, only 55 mm is functional and given the area of stapes footplate (3.2 mm²), the areal ratio is 17:1 and total transformer ratio (17 x 1.3) is 22.1 (c) Curved membrane effect. Movements of tympanic membrane are more at the periphery than at the centre where malleus handle is attached. This too provides some leverage. Phase Differential Between Oval and Round Windows Sound waves striking the tympanic membrane do not reach the oval and round windows simultaneously. There is a preferential pathway to the oval window because of the ossicular chain. Thus, when oval window is receiving wave of compression, the round window is at the phase of rarefaction. If the sound waves were to strike both the Axis of osaicutar movement Stapos Eflective vibratory of TM 45 mm² Footplate area 3.2 mm² Areal ratio 14:1 Tympanic membrane Lever ratio jossicles) 1.3:1 Total transformer ram 14 x 1.3 182:1 (say 18:1) Fig. 2.3 Transformer action of the middle ear. Hydraulic effect of tympanic membrane and lever action of ossicles combine to compensate the sound energy lost during its transmission from air to liquid medium windows simultaneously, they would cancel each others effect with no movement of the perilymph and n This acoustic separation of windows is achieved esence of intact tympanic membrane and a the presenc of air in the middle ear around the round windo differential between the windows contributes 4 d the tympanic membrane la intact. Natural Resonance of External and Middle Ear Sus ent anatomic and physiologic properties of the extens and middle ear allow certain frequencies of sound pass more easily to the inner car due to their natund resonances. Natural resonance of external ear cana 1000 Hz and that of middle eat 800 Hz. Fregu Frequencies most efficiently transmitted by ossicular chain are be tween 500 and 2000 Hz while that by tymраnіс пит brane is 800-1600 Hz. Thus, greatest sensitivity of sound transmission is between 500 and 3000 Hz Hz and these are the frequencies most important to man in day. to-day conversation (Table 2.2). the 2. Transduction of Mechanical Energy to Electrical Impulses Movements of the stapes footplate, transmitted to the cochlear fluids, move the basilar membrane and set up shearing force between the tectorial membrane and the hair cells. The distortion of hair cells gives rise to cochleat microphonics, which trigger the nerve impulse. A sound wave, depending on its frequency, reaches maximum amplitude on a particular place on the basile membrane and stimulates that segment (travelling wave theory of von Bekesy). Higher frequencies are represented in the basal turn of the cochlea and the progressively lower ones towards the apex (Figs 2.4 and 2.5). 3. Neural Pathways Hair cells get innervation from the bipolar cells of spiral ganglion. Central axons of these cells collect to form the co chlear nerve which goes to the ventral and dorsal cochlear nuclei. From there, both crossed and uncrossed fibres travel to the superior olivary nucleus, lateral lemniscus, inferior colliculus, medial geniculate body and finally reach the audi tory cortex situated at the middle of superior temporal gyrus (Brodmanns area 41). Fig EL F Table 2.2 Natural resonance and efficiency. of auditory apparatus External auditory canal Tympanic membrane 800-1600 Hz 800 Hz Middle ear 500-2000 Hz Ossicular chain 3000 Hz CHAPTER 2- PERIPHERAL RECEPTORS AND PHYSIOLOGY OF AUDITORY AND VESTIBULAR SYSTEMS 2000 600 1500 Apes 1001 4000 20,000 Base 1000 8000 Fig. 2.4 Frequency localization in the cochlea. Higher frequencies are localized in the basal turn and then progressively decrease to wards the apex 19 60 40 V Base Base 8000 Hz Apex 3000 Hz Apex Fig. 2.5 Travelling wave creates maximum displacement of basilar membrane according to frequency stimulus, higher frequencies towards the stapes and lower ones towards apex of cochlear. ELECTRICAL POTENTIALS OF COCHLEA AND CN VIII Four types of potentials have been recorded, three from the cochlea and one from CN VIII fibres, which are as follows: 1. Endocochlear potential 2. Cochlear microphonic } from cochlea 3. Summating potential 4. Compound action potential from nerve fibres 5. Endocochlear potential It is a direct current (DC) potential recorded from scala media. It is +80 mV and is generated from the stria vas-cularis by Na/K+-ATPase pump and provides source of energy for cochlear transduction (Fig. 2.6). It is present at rest and does not require sound stimulus. This poten-tial provides a sort of "battery" to drive the current through hair cells when they move in response to a sound stimulus. 2. Cochlear microphonic (CM) When basilar membrane moves in response to sound stimulus, electrical resistance at the tips of hair cells changes and allows flow of K through hair cells to produce voltage fluctuations called cochlear microphon ics. It is an alternating current (AC) potential. 3. Summating potential (SP) It is a DC potential and follows "envelope" of stimulat-ing sound. It is produced by hair cells. It may be Fig. 2.6 Davis' battery model of cochlear transduction. Sicala media has a DC potential of +80 mV. Stimulation of hair calls produces in tracellular potential of-40 mV. This provides flow of current of 120 mV through the top of hair cells. negative or positive. SP has been used in diagnosis of Ménière's disease. It is superimposed on VIIlth nerve action potential. Both CM and SP are receptor potentials as seen in other sensory end organs. They differ from action potentials in that they (i) are graded rather than all or none phenom-enon, (ii) have no latency, (iii) are not propagated and (iv) have no postresponse refractory period. 4. Compound action potential It is an all or none response of auditory nerve fibres. Simplified