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.

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Physiology of Hearing

1. The Nature of Sound

Sound is a form of mechanical energy consisting of alternating compressions and rarefactions propagating through an elastic medium. In air at 20°C at sea level, sound travels at approximately 344 m/s (some sources cite 340 m/s). Sound travels faster through liquids and solids. When sound energy must cross from air to a fluid medium, nearly 99.9% of the acoustic energy is reflected due to the difference in acoustic impedance between the two media - this is the fundamental engineering problem the ear must solve.

2. Overview: Three Functional Divisions

The mechanism of hearing is divided into three sequential stages:

Stage	Structures	Function
Conductive	Pinna, ear canal, tympanic membrane, ossicles	Capture and amplify sound; impedance matching
Sensory (Transduction)	Cochlea, organ of Corti, hair cells	Convert mechanical energy to electrical signals
Neural	CN VIII, central auditory pathways	Transmit and process signals to cortex

3. Anatomy of the Ear

Structure of the human ear showing outer, middle, and inner ear anatomy

Fig. 1 - Structure of the human ear (Kandel, Principles of Neural Science)

External ear:

The auricle (pinna) is a cartilage-supported skin fold that captures sound and directs it into the external auditory meatus. Its corrugated surface is important for vertical sound localization - each person's auricle has a unique topography whose effect on sound reflection is learned early in life.
The external auditory canal ends at the tympanic membrane (diameter ~9 mm, thickness ~50 µm).
Natural resonance of the external auditory canal: ~1000 Hz

Middle ear:

An air-filled pouch connected to the nasopharynx via the Eustachian tube (equalizes pressure).
Contains three ossicles: malleus (hammer), incus (anvil), stapes (stirrup) - linked in a chain.
The handle of the malleus is attached to the tympanic membrane; the footplate of the stapes sits in the oval window of the cochlea.
Natural resonance of the middle ear: ~800 Hz; ossicular chain most efficient at 500-2000 Hz; tympanic membrane most efficient at 800-1600 Hz.

Inner ear (cochlea):

A coiled, snail-shaped bony structure (~9 mm across in humans), embedded in the temporal bone.
Three fluid-filled chambers:
- Scala vestibuli (top) - perilymph; begins at the oval window
- Scala media (middle) - endolymph (high K+, low Na+, similar to intracellular fluid)
- Scala tympani (bottom) - perilymph; ends at the round window
Scala vestibuli is separated from scala media by Reissner's membrane.
Scala media is separated from scala tympani by the basilar membrane.

4. Impedance Matching: The Transformer Action of the Middle Ear

The key problem: sound energy traveling from air to cochlear fluid would lose 99.9% at the interface. The middle ear compensates by acting as an impedance-matching transformer, converting sound of large amplitude/low force into sound of small amplitude/high force. This is accomplished by three mechanisms:

(a) Hydraulic (Area) Ratio of the Tympanic Membrane

The tympanic membrane (~90 mm² total, ~55 mm² functional) is far larger than the stapes footplate (~3.2 mm²).

Effective areal ratio = ~14:1 (using two-thirds of the TM as the functional area)
Per Wever and Lawrence: 55 mm² functional TM / 3.2 mm² footplate = 17:1

(b) Lever Action of the Ossicular Chain

The handle of the malleus is 1.3 times longer than the long process of the incus, providing a mechanical advantage of 1.3:1.

(c) Curved Membrane Effect

The tympanic membrane moves more at its periphery than at its center (where the malleus is attached), providing additional leverage.

Total transformer ratio:

Basic: 14 × 1.3 = ~18:1
Per Wever and Lawrence: 17 × 1.3 = ~22:1
Harrison's 22nd ed. states the eardrum and ossicles boost sound energy ~200-fold (accounting for all mechanical effects combined).

Phase Differential Between Oval and Round Windows

The ossicular chain creates a preferential pathway to the oval window - so when the oval window receives a compression wave, the round window is in the rarefaction phase. This out-of-phase relationship is essential: if both windows received simultaneous pressure waves, they would cancel each other out with no net movement of perilymph and no hearing. This acoustic separation requires an intact tympanic membrane and air in the middle ear space around the round window. The phase differential contributes approximately 4 dB when the tympanic membrane is intact.

5. Cochlear Anatomy and the Organ of Corti

Cross-section of cochlea and organ of Corti showing inner/outer hair cells, basilar membrane, tectorial membrane, and scala divisions

Fig. 2 - Cross-section of the cochlea and the organ of Corti (Costanzo Physiology, 7th ed.)

The organ of Corti lies on the basilar membrane, bathed in endolymph of the scala media. It contains two types of mechanoreceptor hair cells:

Feature	Inner Hair Cells	Outer Hair Cells
Arrangement	Single row	3 parallel rows
Number	~3,500	~20,000
Innervation	Primarily afferent (95% of CN VIII fibers)	Primarily efferent
Function	Principal sensory transducers	Cochlear amplifier (electromotility)

The cilia (stereocilia) of hair cells are embedded in the tectorial membrane above, while their cell bodies sit on the basilar membrane below. The tectorial membrane is stiffer/less elastic than the basilar membrane.

6. Mechanoelectrical Transduction: Step-by-Step

Mechanism of auditory transduction in 6 steps from sound waves to action potentials in afferent cochlear nerves

Fig. 3 - Steps in auditory transduction (Costanzo Physiology, 7th ed.)

Sound waves arrive at the tympanic membrane and are transmitted via the ossicular chain to the stapes footplate at the oval window.
Stapes movements create pressure waves in perilymph - the scala vestibuli pressure rises relative to scala tympani, causing the basilar membrane to bow downward.
The basilar membrane vibrates - because it is more elastic than the tectorial membrane, vibration creates a shearing force between the basilar membrane (moving) and the tectorial membrane (more stationary), bending the stereocilia.
Stereocilia deflection - bending toward the tallest stereocilium stretches tip links (fine filamentous connections between adjacent stereocilia), mechanically pulling open K+ channels (MET channels). K+ floods into the hair cell from endolymph (which has unusually high K+ concentration, similar to intracellular fluid). This causes depolarization - a receptor potential called the cochlear microphonic (CM).
Depolarization opens voltage-gated Ca²+ channels in the presynaptic terminal of the hair cell.
Ca²+ entry triggers vesicular release of glutamate at the ribbon synapse, generating action potentials in afferent cochlear nerve fibers.

Note on K+ paradox: Because hair cell cilia are bathed in endolymph (high K+), K+ flows into the cell when channels open - the opposite of what happens in most neurons. Bending toward the tallest stereocilium = depolarization; bending away = hyperpolarization.

Outer hair cell role (cochlear amplifier): The motility of outer hair cells (driven by the protein prestin in their lateral wall) physically amplifies basilar membrane motion, providing the cochlea with its exquisite sensitivity (~10 nm motion) and sharp frequency tuning. This active amplification mechanism explains why the cochlea is so much more sensitive than a passive system.

7. Tonotopic Organization and the Travelling Wave (von Bekesy)

According to Georg von Bekesy's travelling wave theory (Nobel Prize 1961), a sound wave entering the cochlea does not displace the entire basilar membrane equally. Instead, it creates a travelling wave that propagates from the base toward the apex, reaching its point of maximum amplitude at a location determined by the sound's frequency:

High frequencies (e.g., 20,000 Hz) → maximum displacement near the base (near the stapes)
Low frequencies (e.g., 20-200 Hz) → maximum displacement near the apex
Speech-range frequencies (~500-4000 Hz) → represented at intermediate positions

This is the basis of tonotopic (place) coding - the cochlea acts as a mechanical frequency analyzer. The basilar membrane is narrow and stiff at the base, wide and floppy at the apex, which is what creates this frequency selectivity.

8. Electrical Potentials of the Cochlea

Four types of electrical potentials are recorded from the cochlea and CN VIII:

(1) Endocochlear Potential (EP)

A DC potential of +80 mV recorded from the scala media.
Generated by the stria vascularis via the Na+/K+-ATPase pump.
Present at rest (no sound stimulus required).
Acts as a "battery" - provides the electrochemical driving force for K+ influx into hair cells during transduction.
The resting potential inside the hair cell is approximately -40 to -70 mV, so the combined driving force across the stereocilia tip is +80 mV + ~60 mV = ~120-140 mV (Davis' battery model).

(2) Cochlear Microphonic (CM)

An AC (alternating current) potential generated by hair cells.
Mirrors the waveform of the stimulating sound (like a microphone, hence the name).
Results from oscillating K+ conductance changes as the basilar membrane vibrates.
A receptor potential - graded (not all-or-none), no latency, not propagated, no refractory period.

(3) Summating Potential (SP)

A DC potential that follows the "envelope" of the stimulating sound.
Generated by hair cells; can be positive or negative.
Also a receptor potential (shares properties with CM above).
Clinically used in the electrocochleography (ECoG) diagnosis of Ménière's disease (elevated SP/AP ratio is a key diagnostic marker).

(4) Compound Action Potential (AP / CAP)

An all-or-none response of the auditory nerve fibers.
Reflects the summed firing of many CN VIII fibers.
Has latency, is propagated, follows all properties of a true nerve action potential.

9. Central Auditory Pathways

Hair cells synapse on bipolar neurons of the spiral ganglion (located in the modiolus of the cochlea). Their central axons form the cochlear nerve (CN VIII). The auditory pathway then ascends:

Spiral ganglion (CN VIII)
        ↓
Cochlear nuclei (ventral + dorsal) — first synapse, in medulla
        ↓ (both crossed and uncrossed fibers)
Superior olivary nucleus (pons) — key for sound localization/binaural processing
        ↓
Lateral lemniscus
        ↓
Inferior colliculus (midbrain) — reflex responses to sound
        ↓
Medial geniculate body (thalamus)
        ↓
Auditory cortex (Heschl's gyri, Brodmann's area 41)
— located at the middle of the superior temporal gyrus

Key features of the central pathway:

Tonotopic organization is preserved at every level from cochlea to cortex.
Both crossed and uncrossed fibers exist at multiple levels, meaning each cortex receives input from both ears.
At low frequencies, auditory nerve fibers fire synchronously with the sound wave (phase locking). At higher frequencies, neurons alternate their firing across phases.
Sound intensity is encoded by: firing rate of individual neurons, total number of active neurons, and which specific neurons are activated.

Cortical lateralization:

The left auditory cortex is specialized for speech recognition and production.
The right hemisphere processes emotional and tonal aspects of speech.
Left hemisphere dominance for speech: found in 95-98% of right-handed persons and 70-80% of left-handed persons.
A "right ear advantage" exists for dichotic listening tasks involving consonants and words - because right-ear signals cross to the dominant left hemisphere.

10. Summary of Frequency Transmission Efficiency

Structure	Best Frequency Range
External auditory canal	~1000 Hz (natural resonance)
Tympanic membrane	800-1600 Hz
Middle ear (overall)	800 Hz (resonance)
Ossicular chain	500-2000 Hz
Overall best range	500-3000 Hz

This 500-3000 Hz range corresponds precisely to the frequencies most important for human speech, reflecting evolutionary optimization of the auditory apparatus.

Sources:

Harrison's Principles of Internal Medicine, 22nd ed. (2025), Chapter 36 - Physiology of Hearing
Costanzo Physiology, 7th ed. - Auditory Transduction; Cochlea and Organ of Corti
Kandel, Principles of Neural Science, 6th ed. - Chapter 26: Hearing
The passage provided (Scott-Brown/ENT textbook, Chapter 2)

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