Physiology of Hearing

Physiology of Hearing. Hearing involves the neural perception of sound energy, which includes the identification (what?) and localization (from where?) of sounds. The ear is composed of the external ear, middle ear, and cochlea in the inner ear, all of which are crucial for hearing. The inner ear’s semicircular canals, utricle, and saccule are responsible for maintaining equilibrium or balance.

What is Sound? Sound waves are produced by the compression and decompression of air molecules by a vibrating object.

Formation of Sound Waves:

• Structure of Sound Waves: Sound waves are composed of alternating areas of compressed and spread-out air molecules.
• Tuning Fork Example: A vibrating tuning fork compresses air molecules in front of it and spreads out molecules behind it, creating sound waves.
• Propagation of Sound Waves: Disturbed air molecules collide with further molecules, causing new areas of disturbance and allowing sound waves to travel progressively further from the source. Sound waves eventually fade when they become too weak to affect the surrounding region.

 

Speed of Sound

• In Air: Sound travels at 344 meters (1120 feet) per second in air at 20°C and at sea level.
• In Liquids and Solids: Sound moves faster in liquids and solids than in the air.
• Transition from Air to Liquid: Sound waves don’t travel efficiently from air to a liquid medium due to reflection caused by the high inertia of the liquid.

 

Other important areas in the temporal lobe are: Primary auditory cortex (areas 42) and Auditory association areas (areas 22, 21 and 20).  To remember auditory pathway, remember the mnemonic E COLI-MA (Eight nerve, cochlear nucleus, olivary complex, lateral lemniscus, inferior colliculus Medial geniculate body, Auditory cortex).

Auditory pathways from the right cochlea. Note bilateral route through brainstem and bilateral cortical representation.

 

Therefore, the mechanism of hearing can be broadly divided into:

1. Mechanical conduction of sound (conduction apparatus).
2. Transduction of mechanical energy to electrical impulses (sensory system of cochlea).
3. Conduction of electrical impulses to the brain (neural pathways).

Mechanical conduction of sound (conduction apparatus). 

1. Role of External Ear in mechanical conduction of sound.
(i) Pinna: This part plays a vital role in localizing, capturing, and directing sound into the external auditory meatus.
(ii) External auditory meatus: It effectively conducts sound waves to the tympanic membrane, leading to its vibration.
(iii) Tympanic membrane: This essential component acts as a sensitive pressure receiver, resonator, and critically dampens vibrations, ensuring efficient sound processing.

2. Role of Middle ear in mechanical conduction of sound: There are four mechanism through which middle ear plays important role in hearing mechanism.
(i) Impedance matching mechanism (also known as Transformation action),
(ii) Attenuation reflex,
(iii) Phase differential between cochlear windows,
(iv) The natural resonance of external ear and middle ear.

(i) Impedance matching mechanism. The vibration transfer is not as simple as it seems because the acoustic impedance (resistance) of inner ear fluids is much more than the air in the middle ear (i.e., impedance mismatch). This impedance mismatch requires a greater force to cause vibration in the fluid. When sound travels from air into water, 99.9% of the sound energy is reflected away from the water’s surface. To overcome this loss, the tympanic membrane and the ear ossicles collaborate to convert the sound from one of greater amplitude but lesser force to one of lesser amplitude but greater force. This crucial function is known as the “impedance matching mechanism” or the “transformer action.“

It is accomplished by following three mechanisms:

• The lever action of ossicles. The handle of malleus is 1.3 times longer than the long process of the incus, giving it a significant mechanical advantage. As a result, the middle ear ossicles can increase the force of movement by 1.3 times.
• The hydraulic action of the tympanic membrane. The surface area of the tympanic membrane is much larger than the surface area of the stapes footplate, with an average ratio of 21:1. However, the effective vibratory area of the tympanic membrane is only two-thirds of the total surface area, reducing the effective areal ratio to 14:1. This size difference provides a mechanical advantage, concentrating the force produced by sound over a smaller area, thus amplifying the pressure exerted on the oval window. The product of areal ratio and lever action of ossicles is 18:1 (i.e., 18 times amplification). Detailed calculations are provided in the diagram below.
• Curved tympanic membrane effect. Movements of the tympanic membrane are more at the periphery than at the centre where malleus handle is attached. This too provides some leverage.

Another study (Wever and Lawrence) concluded that out of a total of 90 mm2 area of the human tympanic membrane, only 55 mm sq is functional and given the area of stapes footplate (3.2 mm sq), the areal ratio is 17:1 and total transformer ratio (17× 1.3) is 22.1.

(ii) Attenuation reflex (acoustic reflex) is a protective reflex. The muscles in the middle ear respond to loud sounds (70-80 dB above the hearing threshold) by contracting, which tightens the eardrum and restricts the movement of the ossicular chain. This protects the inner ear from damage caused by loud sound. 

Mechanism: The tensor tympani muscle pulls the malleus inwards, while the stapedius muscle pulls the stapes outwards, making the ossicular system rigid and preventing intense sound from entering the inner ear. This reflex also helps open the eustachian tube. 

Advantages:

• It prevents cochlear damage from loud music and loud noises like jet aircraft, etc.
• It masks the low-frequency environmental sounds and allows us to concentrate on the sound above 1000 Hz.          
• It reduces the intensity of the sounds occurring just before vocalisation and chewing.

Note: It’s important to note that this attenuation reflex is a slow protective reflex has a latent period of 40 milliseconds and occurs with a slight delay after an extremely loud sound (gunshot or bomb explosion), which may cause deafness due to cochlear damage. Therefore, it provides protection only from prolonged intense sounds, not from sudden sounds like an explosion. As a historical example, anti-aircraft guns during World War II were designed to make a loud pre-firing sound to protect the gunner’s ears from the much louder sound of the actual firing.

(iii) Phase differential between cochlear windows. The fluids in the inner ear are usually incompressible. The round window vibrates in the opposite phase to the oval window, allowing fluid in the cochlea to move. This means that when the oval window receives a wave of compression, the round window is at the phase of rarefaction. If the sound waves were to strike both windows at the same time and phase, they would cancel each other out, resulting in no movement of perilymph and no hearing. The phase differential between the windows contributes 4 dB when the tympano-ossicular system is intact. This acoustic separation of windows is attributed to the

• “Ossicular coupling” effect is produced by an intact tympano-ossicular system (preferential pathway to the oval window).
• “Acoustic coupling” effect is produced by a cushion of air in the middle ear around the round window also leads to phase difference.

(iv) Natural resonance of external and middle ear. The external ear and middle ear, due to the inherent anatomic and physiologic properties, allows certain frequencies of sound to pass more easily to the inner ear. The natural resonance of important structures is:

• External auditory canal is 3000 Hz
• Tympanic membrane is 800–1600 Hz
• Middle ear is 800 Hz
• Ossicular chain is 500–2000 Hz

Thus, the greatest sensitivity of the sound transmission is between 500 and 3000 Hz, and used in daily talk and conversation.

Minimum audibility curve:

• The amplification of sound intensity is highest between 1000 and 3000 Hz.
• The sounds below 16Hz or above 20,000Hz are never amplified.
• This is the reason, why human ear can perceive the pitch of sound between 16 and 20,000 Hz. Maximum sensitivity is between 1000 and 3000 Hz. This effect is depicted in the minimum audibility curve 

 

Transduction of mechanical energy to electrical impulses (sensory system of the cochlea). 

The process of converting mechanical energy into electric nerve impulses in the hair cells of the organ of Corti is called ‘auditory transduction’. Here’s how it works: The stapes footplate is connected to the oval window, which is in close contact with the scala vestibuli, containing perilymph. Movements of the stapes footplate cause pressure changes in the perilymph, which are transmitted to Reissner’s membrane, leading to compression of scala media, which contains endolymph. The pressure wave in the endolymph causes the basilar membrane to deflect relative to the stationary tectorial membrane, resulting in the development of hearing forces that distort and bend the stereocilia (hairs). The bending of stereocilia opens mechanically-gated cation channels, leading to the movement of ions (K+ and Ca+) and the generation of a receptor potential. When the organ of Corti moves upward, the tectorial membrane slides forward relative to the basilar membrane, bending the stereocilia away from the limbus, causing depolarization (resting membrane potential is -60mV, and depolarization is -50mV). When the organ of Corti moves downward, the tectorial membrane slides backward relative to the basilar membrane, bending the stereocilia toward the limbus, causing hyperpolarization.

Auditory transduction pathway

 

Electrical Response of Hair Cells

The electrical response of hair cells can be recorded using microelectrodes to measure four distinct potentials from the ear:

(i) Endo-cochlear Potential: This is a direct current potential (+80 mV) recorded from the scala media and is present under resting conditions, meaning it exists without any external auditory stimulus. This potential provides an energy source for cochlear transduction. The endolymph has a higher concentration of K+ (150 mEq/liters) compared to the perilymph (3 mEq/liters), due to the presence of the Na+/K+-ATPase pump in the stria vascularis of the scala media.

(ii) Cochlear Microphonic Potential: An alternating current is recorded when an auditory stimulus is presented to the ear. This potential represents the sum of receptor potentials from a large number of hair cells when recorded extracellularly. One electrode is placed in the scala media and another in the scala tympani. When the basilar membrane moves in response to a sound stimulus, the electrical resistance at the tips of the hair cells changes, allowing the flow of K+ through the hair cells and producing voltage fluctuations known as cochlear microphonics.

(iii) Summating Potential (SP): This is a direct current potential superimposed on the auditory nerve action potential. SP is used in the diagnosis of Meniere’s disease. Both cochlear microphonics and summating potentials are receptor potentials, similar to those seen in other sensory end-organs. They differ from action potentials in that:

• Both are graded potentials, not action potentials.
• They are not propagated.
• They have no post-response refractory period.
• They have no latency.

(iv) Compound Action Potential: This is an all-or-none response of auditory nerve fibers, reflecting the collective firing of these fibers in response to auditory stimuli.

Conduction of Electrical Impulses to the Brain (Neural Pathways)

Neural transmission within the auditory pathways involves a series of critical relay stations:

(i) Bipolar Cells of the Spiral Ganglion (First-Order Neurons): The dendrites of bipolar cells in the spiral ganglion innervate hair cells. The axons of these bipolar cells form the cochlear division of the eighth cranial nerve, which terminates in the cochlear nuclei located in the upper part of the medulla.

(ii) Superior Olivary Nucleus Complex and Nucleus of the Lateral Lemniscus (Second-Order Neurons): Fibers from the cochlear nuclei synapse and cross to the contralateral side of the brainstem, terminating in the superior olivary nucleus.

(iii) Third-Order Neurons: These neurons, with cell bodies in the superior olivary complex, trapezoid nucleus, and nucleus of the lateral lemniscus, project through the lateral lemniscus to the inferior colliculus.

(iv) Fourth-Order Neurons: Located in the inferior colliculus, these neurons send projections to the medial geniculate body.

(v) Fifth-Order Neurons: The final relay of neurons transmits impulses from the medial geniculate body to the auditory cortex (Brodmann’s area 41) in the temporal lobe.

 Neural Processing of Auditory Information

The human auditory system is capable of detecting sounds within the frequency range of 20-20,000 Hz. Sound intensity is encoded at the level of cochlear nerve fibers through various mechanisms, enabling higher auditory centers to respond to specific sound features. The auditory system can distinguish sounds from sources separated by as little as 1°. It uses the time lag between sound entry into both ears to determine sound origin. Lesions in the auditory cortex can disrupt sound localization.

 Key Terminology

– Amplitude (Intensity): Refers to the loudness of the sound, measured in decibels (dB).

– Frequency: Indicates the number of sound waves produced per second, measured in hertz (Hz). Human hearing ranges from approximately 20 to 20,000 Hz, with normal speech frequencies ranging from 2000 to 5000 Hz.

– Pitch: The subjective sensation corresponding to the frequency of sound. Higher frequencies result in higher pitches. The average male voice has a pitch of 120 Hz, while the average female voice has a pitch of 250 Hz. An average individual can distinguish approximately 2000 different pitches.

– Pure Tone: A sound with a single frequency, such as 250, 500, or 1000 Hz. Pure tone audiometry measures the hearing threshold in decibels for various pure tones ranging from 125 to 8000 Hz.

– Complex Sound: Comprises multiple frequencies. The human voice is an example of a complex sound.

– Sound Pressure: Expressed in decibels (dB), a relative measure on a logarithmic scale.

– Overtones: A complex sound consists of a mixture of pure tones. The lowest frequency at which a source vibrates is termed the fundamental (or primary) frequency. Other frequencies, which are multiples of the fundamental frequency, are called overtones or harmonics. Overtones determine the quality or timbre of sound, enabling the identification of different musical instruments playing the same pitch.

———— End of the chapter ————

Learning resources.

• Scott-Brown, Textbook of Otorhinolaryngology Head and Neck Surgery.
• Ganong’s Review of Medical Physiology.
• Indu Khurana, Textbook of Medical Physiology
• Lauralee Sherwood, Human Physiology: From Cells to Systems
• Glasscock-Shambaugh, Textbook of  Surgery of the Ear.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
• Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
• Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck surgery.
• Anirban Biswas, Textbook of Clinical Audio-vestibulometry.

         Author:

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

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