Menu Close

Physiology of Hearing

Physiology of Hearing. Hearing is the neural perception of sound energy, involving the identification (what?) and localization (from where?) of sounds. The ear, consisting of the external, middle, and cochlea of the inner ear, plays a key role in the mechanism of hearing. While, the semicircular canals, utricle, and saccule of the inner ear are responsible for maintaining equilibrium (balance).

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

Formation of sound waves:

(a) Sound waves consist of alternating areas of compressed and spread-out air molecules. 

(b) When a tuning fork vibrates, it creates sound waves by compressing the air molecules in front of it and spreading out the molecules behind it. 

(c) The disturbed air molecules then collide with molecules further away, creating new areas of air disturbance. This allows sound waves to travel progressively further from the source, even though each air molecule only moves a short distance when disturbed. The sound wave eventually fades when the last area of air disturbance becomes too weak to affect the surrounding region.

Sound waves 3. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing
Sound waves 2. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing
Sound waves 1. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing

 

Speed of sound:

  • Sound travels at a speed of 344 meters (1120 feet) per second in the air at 20°C and at sea level. It moves faster in liquids and solids than in the air.
  • When sound waves move from air to a liquid medium, they don’t travel as efficiently because most of the sound waves are reflected due to the high inertia offered by the liquid.

 

Mechanism of Hearing. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing

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 pathway. Dr. Rahul Bagla ENT Textbook. Physiology of HearingAuditory 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.
Impedance-matching device. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing

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.
Acoustic coupling. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing

(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 
Minimum audibility curve. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing

 

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. Dr. Rahul Bagla ENT Textbook. Physiology of HearingAuditory transduction pathway

 

Electrical response of hair cells. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing

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

(i) Endo-cochlear potential: This is a direct current potential (+80 mV) recorded from scala media. It is present in resting conditions, meaning it is present in the absence of any external auditory stimulus. This potential provides a source of energy for cochlear transduction. The endolymph contains more K+ (150 mEq/litres) than perilymph (3 mEq/litres) due to the presence of Na+/K+-ATPase pump in the stria vascularis of scala media. 

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

(iii) Summating potential (SP): It is a direct current potential superimposed on the auditory nerve action potential. SP is used for the diagnosis of Meniere’s disease. Both Cochlear Microphonics and Summating Potentials are receptor potentials as seen in other sensory end-organs. They differ from action potentials in that:

    • Both are graded potential and not, action potential
    • Are not propagated
    • Have no post-response refractory period. 
    • Have no latency

(iv) Compound action potential: It is an all or none response of auditory nerve fibres.

 

Conduction of electrical impulses to the brain (neural pathways).

Neural transmission in auditory pathways involves relay stations: 

(i) Bipolar cells of spiral ganglion (First-order neurons): Hair cells receive innervation from the dendrites of bipolar cells of the spiral ganglion. The axons of these cells form the cochlear division of the eighth cranial nerve. The cochlear nerve terminates in the cochlear nuclei in the upper part of the medulla.

(ii) Superior olivary nucleus complex and nucleus of the lateral lemniscus (Second-order neurons): Fibers synapse in the cochlear nuclei and pass to the opposite side of the brain stem to terminate in the superior olivary nucleus.

(iii) The third-order neurons have cell bodies in the superior olivary complex, trapezoid nucleus, and nucleus of the lateral lemniscus. From there, the pathway passes through the lateral lemniscus to the inferior colliculus. 

(iv) The fourth-order neurons are located in the inferior colliculus, and the fibers then reach the medial geniculate body. 

(v) Finally, the fifth-order neurons continue the pathway to the auditory cortex (Brodmann’s area 41) in the temporal lobe.

Neural processing of auditory information: 

The human auditory system can detect sounds in the range of 60-20,000 Hz. The intensity of sound is encoded at the level of cochlear nerve fibers through several mechanisms. Higher auditory centers respond to specific features of sound stimuli. Humans can distinguish sounds coming from sources separated by as little as 1°. The auditory system uses the time lag between the entry of sound in the two ears to determine the origin of the sound. Sound localization is disrupted in lesions of the auditory cortex.

Physiology of Hearing Physiology of Hearing PPhysiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing hysiology of Hearing Physiology of Hearing Physiology of HePhysiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing Physiology of Hearing aring

OTHER IMPORTANT TERMS:

  • Amplitude (intensity) determines the loudness of the sound and is measured in decibels. 
  • Frequency refers to the number of waves produced per second. The unit of frequency is hertz (Hz). Range of human hearing is approximately 20–20,000 Hz. The average range of normal speech is 2000–5000 Hz. 
  • Pitch. It is the subjective sensation produced by the frequency of sound. Higher the frequency greater is the pitch. Pitch of the average male voice is 120 Hz and that of a female is 250 Hz. An average individual can distinguish about 2000 different pitches.
  • Pure Tone. A single frequency sound is called a pure tone, e.g. a sound of 250, 500 or 1000 Hz. In pure tone audiometry, we measure the threshold of hearing in decibels for various pure tones from 125 to 8000 Hz.
  • Complex Sound refers to more than one frequency. For example, a human voice is a complex sound.
  • Sound pressure is expressed in decibels (dB), which is a relative measure on a log scale.
  • Overtones. A complex sound is a mixture of pure tones. The lowest frequency at which a source vibrates is called fundamental (or primary) frequency. All other frequencies, which are multiples of the fundamental frequency are called overtones or harmonics. The overtones determine the quality or the timbre of the sound. Variations in timbre permit us to identify the sounds of various musical instruments (e.g. guitar, piano, tabla, sarangi etc.) even though they are playing notes of 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 ENT Textbook

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Designation: Faculty, Associate Professor
Government Institute of Medical Sciences,
Greater Noida, India
Mail: msrahulbagla@gmail.com

Please read. Hearing tests. Click on this link. https://www.entlecture.com/assessment-of-hearing/

Like our Facebook page: https://www.facebook.com/Dr.Rahul.Bagla.UCMS

Join our Facebook group: https://www.facebook.com/groups/628414274439500

Leave a Reply

Your email address will not be published. Required fields are marked *