Anatomy and Physiology of the Ear Notes

Mechanism Of Hearing Introduction

• The sound waves travel through the external auditory meatus and produce vibrations in the tympanic membrane. The vibrations travel through the malleus and incus and reach the stapes resulting in the movement of stapes. The movements of stapes produce vibrations in the fluids of the cochlea.
• The vibrations stimulate the hair cells in the organ of Corti. This, in turn, causes the generation of action potential (auditory impulses) in the auditory nerve fibers. When the auditory impulses reach the cerebral cortex, the perception of hearing occurs.
• Thus, during the process of hearing, the ear converts the sound waves into action potentials in the auditory nerve fibers. This process is called sound transduction.

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Role Of External Ear

• The external ear directs the sound waves toward the tympanic membrane. The sound waves produce pressure changes over the surface of the tympanic membrane. Accumulation of wax prevents the conduction of sound.
• In many animals, the auricle (pinna) can be turned to locate the source of the sound. The auricle can be folded to avoid unwanted sounds. But in man, the extrinsic and intrinsic muscles of the auricle are rudimentary and movement is not possible.

Role Of Middle Ear

1. Role Of Tympanic Membrane: Due to the pressure changes produced by sound waves, the tympanic membrane vibrates, i.e. it moves in and out of the middle ear. Thus, the tympanic membrane acts as a resonator that reproduces the vibration of sound.

2. Role Of Auditory Ossicles: The vibrations set up in the tympanic membrane are transmitted through the malleus and incus and reach the stapes, causing to and fro movement of stapes against the oval window and against the perilymph present in the scala vestibule of the cochlea.

• Impedance Matching:
  • Impedance matching is the process, in which the tympanic membrane and auditory ossicles convert the sound energy into mechanical vibrations in the fluid of the internal ear with minimum loss of energy by matching the impedance offered by the fluid.
  • Impedance means obstruction or opposition to the passage of sound waves. When sound waves reach the inner ear, the fluid (perilymph) in the cochlea offers impedance, i.e. the fluid resists the transmission of sound due to its own inertia. The tympanic membrane and the auditory ossicles effectively reduce the sound impedance.
  • The sound waves are conducted from the external ear to the inner ear, with an impedance of only 40%. The remaining 60% of sound energy developed in the tympanic membrane is transmitted to the cochlear fluid by the ossicles. Thus, along with the help of the tympanic membrane, the ossicles match the impedance offered by a fluid to a great extent.
  • It is because the ossicles act like a lever system so that stapes exert a greater force (pressure) against the cochlear fluid. This results in the generation of vibrations in the cochlear fluid. The increased force is very essential to set up the vibrations in cochlear fluid because of the higher inertia of the fluid.
  • The force exerted by the footplate of stapes on the cochlear fluid is, 17-22 times greater than the force exerted by sound waves at the tympanic membrane. It is because of two structural features of ossicles.

1. 1. 1. The head of the malleus is longer than the long process of the incus so that a higher force is generated in a small structure
      2. The surface area of the tympanic membrane (55 sq. mm) is larger compared to that of the footplate of stapes (3.2 sq. mm). So the pressure increases when force is applied to a small area.

• • Thus, the tympanic membrane and auditory ossicles are capable of converting sound energy into mechanical vibrations in the cochlear fluid with minimum loss of energy.
  • Significance of impedance matching: Impedance matching is the most important function of the middle ear. Because of impedance matching the sound waves (stimuli) are transmitted to the cochlea with minimum loss of intensity. Without impedance matching conductive deafness occurs.

• Types Of Conduction:

Conduction of sound from the external ear to the internal ear through the middle ear occurs by three routes:

1. Ossicular conduction
2. Air conduction
3. Bone conduction.

1. Ossicular conduction: Ossicular conduction is the conduction of sound waves in the middle ear through auditory ossicles. In normal conditions, the sound waves are conducted through auditory ossicles.

2. Air conduction: It is the conduction of sound waves through air in the middle ear. If the ossicular chain is broken, conduction occurs in an alternate route of air conduction. Air conduction is common in otosclerosis. Otosclerosis is a disease associated with the fixation of stapes to oval windows.

3. Bone conduction: It is the conduction of sound waves in the middle ear through bones. When the middle ear is affected, bone conduction occurs. In this type of conduction, the sound waves are transmitted to the cochlear fluid by the vibrations set up in the skull bones. Bone conduction is tested by placing vibrating tuning forks or other vibrating bodies directly on the skull. This route plays a role in the transmission of extremely loud sounds.

3. Role Of Eustachian Tube: The Eustachian tube is not concerned with hearing directly. However, it is responsible for equalizing the pressure on either side of the tympanic membrane.

Role Of Inner Ear

1. Traveling Wave:

• The movement of the footplate of the stapes against the oval window causes movement of perilymph in scala vestibuli. The fluid need not move all the way from the oval window toward the round window through the helicotrema. The immediate effect is on the basilar membrane near the oval window.
• The movement of fluid in scala vestibuli causes displacement of fluid in scala media, as the vestibular membrane is flexible. This causes bulging of the basal portion of the basilar membrane towards the round window. This in turn moves the fluid in the scala tympani towards the round window and the bulging of the round window into the middle ear.
• The elastic tension developed in the basilar fibers in the bulged portion of the basilar membrane initiates a wave that travels along the basilar membrane towards the helicotrema like that of an arterial pulse wave. It is called a traveling wave.

Resonance Point:

• The resonance point is the part of the basilar membrane which is activated by the traveling wave. In the beginning, each traveling wave is weak. While traveling through the basilar membrane from the base towards the apex (helicotrema), the wave becomes stronger and stronger, and, at one point (resonance point) of the basilar membrane, it becomes very strong and activates the basilar membrane.
• This resonance point of the basilar membrane immediately vibrates back and forth. The traveling wave stops here and does not travel further.
• The distance between the stapes and resonance point is inversely proportional to the frequency of sound waves reaching the ear. The traveling wave generated by high-pitched sound dies near the base of the cochlea. Wave generated by medium-pitched sound reaches half of the way, and the wave generated by low-pitched sound travels the entire distance of the basilar membrane.

2. Excitation Of Hair Cells:

• The stereocilia of hair cells in the organ of Corti are embedded in the tectorial membrane. The hair cells are tightly fixed by cuticular lamina reticularis and the pillar cells or rods of Corti.
• When a traveling wave causes vibration of the basilar membrane at the resonance point, the basilar fiber, rods of Corti, hair cells, and lamina reticularis move as a single unit. It causes movements of stereocilia leading to the excitement of hair cells and the generation of the receptor potential.

Electrical Events During Process Of Hearing

1. Sound Transduction: Sound transduction is a type of sensory transduction (Chapter 139) in the hair cell (receptor) by which the energy (movement of cilia in hair cell) caused by sound is converted into action potentials in the auditory nerve fiber.

Three types of electrical events occur during the process of sound transduction:

1. Receptor potential or cochlear microphonic potential
2. Endocochlear potential or endolymphatic potential
3. Action potential in the auditory nerve fiber.

2. Receptor Potential Or Cochlear Microphonic Potential:

• Receptor potential or cochlear microphonic potential is the mild depolarization that is developed in the hair cells of the cochlea when sound waves are transmitted to the internal ear. The resting membrane potential in hair cells is about – 60 mV. The sensory transduction mechanism in cochlear receptor cells is different from the mechanism in other sensory receptors.
• When sound waves reach the internal ear traveling wave is produced. It causes the vibration of the basilar membrane which moves the stereocilia of hair cells away from the modiolus (towards kinocilium).
• It causes the opening of mechanically gated potassium channels and the influx of potassium ions from endolymph which contains a large amount of potassium ions. The influx of potassium ions causes the development of mild depolarization (receptor potential) in hair cells up to -50 mV.
• The cochlear microphonic potential is nonpro- negative. But, it causes the generation of action potential in auditory nerve fibers. Due to depolarization hair cells release a neurotransmitter which generates the action potential in the auditory nerve fibers. The probable neurotransmitter may be glutamate.
• The movement of stereocilia away from the modiolus (towards kinocilium) causes depolarization in hair cells. Movement of stereocilia in the opposite direction (away from kinocilium) causes hyperpolarization. The ionic basis of hyperpolarization is not clearly known. It is suggested that calcium plays an important role in this process. Hyperpolarization in hair cells stops the generation of action potential in auditory nerve fibers.

3. Role Of Hair Cells: Inner hair cells and outer hair cells have different roles during sound transduction.

• Role of Inner Hair Cells: Inner have cells are responsible for sound transduction, i.e. these receptor cells are the primary sensory cells which cause the generation of action potential in auditory nerve fibers.
• Role of Outer Hair Cells: Outer hair cells have different actions. These hair cells are shortened during depolarization and elongated during hyperpolarization. This process is called electro¬motility or electrical to mechanical transduction. This action of outer hair cells facilitates the movement of the basilar membrane and increases the amplitude and sharpness of sound. Hence, the outer hair cells are collectively called cochlear amplifiers. The electromotility of hair cells is due to the presence of a contractile protein, prestin (named after the musical notation presto).
• Role of Efferent Nerve Fibers to Hair Cells: The efferent nerve fibers of hair cells also play an important role during sound transduction by releasing acetylcholine.
  • The efferent nerve fiber to the inner hair cell terminates on the auditory (afferent) nerve fiber where it leaves the inner hair cell. It controls the generation of action potential in auditory nerve fibers by inhibiting the release of glutamate from inner hair cells. The efferent nerve fiber to the outer hair cell terminates directly on the cell body. It inhibits the electromotility of this cell.

4. Endocochlear Potential Or Endolymphatic Potential: Endocochlear or endolymphatic potential is the electrical potential developed in the fluids outside the hair cells.

• Cochlear Fluids: Cochlear fluids are the extracellular fluids in the inner ear. These fluids are perilymph and endolymph which have different compositions.
  • Perilymph: Scale vestibule and scala tympani are filled with pemymh similar to ECF in composition with high mryreniraVori of sodium ions.
  • Endolymph: Scaia media is filled with endolymph which contains a high concentration of potassium and a low concentration of sodium. It is due to the continuous secretion of potassium ions by stria vascularis into scala media.
• Electrical Potential: The difference in potassium concentration is responsible for the development of an electrical potential difference between endolymph and perilymph. The potential in endolymph is positive up to + 80 mV.
• Significance of endocochlear Potential: The lower portion of the hair cells is bathed by the perilymph. The head portion of the hair cells penetrates the lamina reticularis and it is bathed by endolymph. The endolymph has a positive potential (+ 80 mV).
  • So inside the hair cells, the electrical potential is -60 mV in comparison to that of perilymph and -140 mV in comparison to that of endolymph. The high potential difference sensitizes the hair cells so that, the excitability of hair cells increases. It also increases the response of the cells even to slight movement of stereocilia.

5. Action Potential In Auditory Nerve Fiber:

• The action potential in auditory nerve fiber is generated by cochlear microphonic potential. It obeys all or none law and has a definite threshold and refractory period.
• The action potential to a click sound with a moderate intensity level consists of three successive spike potentials called N-, N2 N3 representing synchronous repetitive firing in many fibers.
• At high frequencies, the synchronization of action potential disappears and a single spike occurs. The action potential appears 0.5-1 m sec after the development of cochlear microphonic potential.

Properties Of Sound

Sound has two basic properties:

1. The pitch which depends upon the frequency of sound waves. The frequency of sound is expressed in hertz. The frequency of sound audible to the human ear lies between 20 and 20,000 Hz or cycles/second. The range of greatest sensitivity lies between 2,000 and 3,000 Hz (cycles/second).
2. The loudness or intensity which depends upon the amplitude of sound waves. It is expressed in decibel (dB). The threshold intensity of sound wave is not constant. It varies in accordance to the frequency of the sound.

Appreciation Of Pitch Of The Sound Theories Of Hearing

• Many theories are postulated to explain the mechanism by which the pitch of the sound is appreciated or the frequency is analyzed. These theories are generally classified into two groups. According to the first group, the analysis of sound frequency is the function of the cerebral cortex; and the cochlea merely transmits the sound.
• According to the second group of theories, the frequency analysis is done by the cochlea, which later sends the information to the cerebral cortex.

1. Theories Of First Group:

• Telephone Theory of Rutherford:
  • It was postulated by Rutherford in 1880. It is also called frequency theory. According to this theory, the cochlea plays a simple role of a telephone transmitter.
  • In a telephone, sound vibrations are converted into electrical impulses. The electrical impulses are trans¬mitted by cables to the receiving end. There the receiver instrument converts the electrical impulses back into sound waves.
  • Similarly, cochlea just converts the sound waves into electrical impulses of the same frequency. The impulses are transmitted by auditory nerve fibers to cerebral cortex, where perception and analysis of sound occur.
  • It is believed that the nerve fibers can transmit a maximum of 1000 impulses per second. Thus, the telephone theory fails to explain the transmission of sound waves with a frequency above 1000/second. So, a second theory was postulated.
• Volley Theory: In 1949, Wever postulated this theory. Volley means groups. According to this theory, the impulses of sound waves with frequency above 1000 cycles per second are transmitted by different groups of nerve fibers. However, this theory has no evidence to prove it. Thus, these two theories were not accepted by many physiologists.

2. Theories Of Second Group:

• Resonance Theory of Helmholtz
  • It was the first theory of hearing to emerge in 1863. According to Helmholtz, the analysis of sound frequency is the function of the cochlea. The basilar membrane contains many basilar fibers. Helmholtz named the basilar fibers resonators and compared them with the resonators of the piano.
  • When a string in the piano is struck, a sound with a particular note is produced. Similarly, when sound with a particular frequency is applied, the basilar fibers in a particular portion of the basilar membrane are stimulated.
  • The resonance theory was not accepted because the individual resonators could not be identified in the cochlea. Gradually, this theory was modified into another theory called the place theory, which is more widely accepted.
• Place Theory:
  • According to this theory, the nerve fibers from different portions (places) of the organ of Corti on the basilar membrane give responses to sounds of different frequencies. Accordingly, the corresponding nerve fiber from the organ of the Corti gives information to the brain regarding the portion of the organ of the Corti that is stimulated.
  • Many experimental pieces of evidence are available to support place theory.
    • If a person is exposed to a loud noise of a parti¬cular frequency for a long period, he becomes deaf for that frequency. It is found that the specific portion of the organ of Corti is destroyed as in the case of Boiler maker’s disease.
    • In experimental animals, the destruction of a portion of the organ of Corti occurs by exposing the animal to loud noise of a particular frequency.
    • In human high tone deafness, there is a de¬generation of the organ of Corti near the base of co¬chlea or degeneration of the nerve supplying the cochlea near the base.
    • During exposure to high-frequency sound, the cochlear microphonic potentials show greater voltage in hair cells near the base of the cochlea. Also, during the exposure to low-frequency sound, the cochlear microphonic potentials show greater voltage in hair cells near the apex of the cochlea.
    • There is a point-to-point representation of the basilar membrane in the auditory cortex.
• Traveling Wave Theory: From place theory, emerged yet another theory called the traveling wave theory. This theory explains how the traveling wave is generated in the basilar membrane. The generation, movement, and disappearance of traveling waves are already described earlier in this chapter.

Appreciation Of Loudness Of Sound

Appreciation of the loudness of sound depends upon the activities of auditory nerve fibers. The intensity or loud¬ness of sound correlates with two factors:

1. The rate of discharge from individual fibers of auditory nerve
2. Total number of nerve fibers discharging.

When the loudness of sound increases, it produces longer vibrations which spread over longer area of the basilar membrane. This activates a large number of hair cells and recruits more number of auditory nerve fibers. So, the frequency of action potential is also increased.

Localization Of Sound

Sound localization is the ability to detect the source from where the sound is produced or the direction through which the sound wave is traveling. It is important for survival and it helps to protect us from moving objects such m vehicles. Cerebral cortex and the medial geniculate body play an important role in the localization of sound.