1. Nervous System Physiology
Special Senses By
Dr. SHAHAB SHAIKH
PhD MD MBBS
Lecture: Physiology of Hearing
••••••••••••••••••••••••••••••••••
Faculty of Medicine
Al Maarefa Colleges of Science & Technology

2. Introduction
Hearing, Auditory Perception, or Audition is the ability to perceive sound by detecting vibrations or changes in the pressure of the surrounding medium through the ear.
Hearing is the neural perception of sound energy.
Hearing mainly involves two aspects:
the identification of the sounds (what) and
their localization (where).
Hearing is performed primarily by the auditory system: mechanical waves, known as vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain.

3. Sound
Sounds are produced when vibrating objects, such as the plucked tuning fork or guitar strings, produce pressure pulses of vibrating air molecules, better known as sound waves. 
Sound waves are often simplified as plane waves, which are characterized by these generic properties:
Frequency,
Amplitude
Direction
Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz.
Humans can detect the difference between two sounds occurring 10micro seconds apart in time

4. Audible Sound Range

5. Sound
Generally loudness of sound is correlated with the amplitude of sound wave (subjective interpretation of the intensity of a sound)
Decibel:
logarithmic scale to measure the intensity of sound waves
human amplitude range - 0 dB dB
avg sound intensity of human speech is 65 dB
Greater than 100 dB can cause damage to auditory apparatus
The pitch is correlated with the frequency ( no of waves or cycles per second / Hz)
The sound frequencies audible in humans - 20 Hz to Hz ,with greatest sensitivity from 1000 to 4000 Hz
Normal pitch: Male – 120 Hz
Female HZ
Pure tone result from sinusoidal waves of single frequency
Most of the sound are mixture of pure tones
Sound waves which have repeating pattern even though individual waves are complex are perceived as musical sound
Pure tone sound is sound wave with single frequency
Most of the sound are mixture of pure tones

6. Sound Terminologies Amplitude/loudness Frequency/Pitch/Tone Impedence
Strength of the sound
Loudness denotes the appreciation of sound intensity
Expressed in decibel (dB)
Amplitude/loudness
Number of cycles per second
Pitch /Tone denotes the appreciation of frequency
Expressed in Hertz(Hz)
Frequency/Pitch/Tone
Resistance offered by a medium to sound waves
Impedence
Resonance is a phenomenon that occurs when a vibrating system or external force drives another system to oscillate with greater amplitude at a specific preferential frequency.
Resonance
Attenuation is a general term that refers to any reduction in the strength of a signal.
Attenuation

7. Functional Anatomy External (Outer) ear: Middle ear.: Inner ear
Auricle
External acoustic meatus
Tympanic membrane
Ceruminous glands
Middle ear.:
Tympanic cavity
Osicles
Malleus (hammer)
Incus (anvil)
Staples (stirrup).
Eustachian tube.
Inner ear
Bony and membranous labyrinth
Cochlea: hearing
Vestible: equilibrium
3 perpendicular semicircular canals: equilibrium

8. Sound Transmission
In the human ear, a sound energy is transmitted through four separate mediums along the auditory system before a sound is perceived: in the outer ear—air, in the middle ear— mechanical, in the inner ear liquid and to the brain—Electrical.
SOUNDENERGY
MECHANICAL ENERGY
ELECTRICAL ENERGY

9. Sound Transmission

10. Sound Transmission External Ear:
Air transmitted sound waves are directed toward the Tympanic Membrane
Functions of External Air:
Sound collection
Increasing pressure on the tympanic membrane in a frequency sensitive way.
Sound localization

11. Sound Transmission Functions: Middle Ear:
When sound wave strikes the tympanic membrane (also called EAR DRUM), the tympanic membrane moves.
Motion of the eardrum sets the ossicular chain i.e. Malleus, Incus & Stapes into motion.
Functions:
Maintains resting air pressure on both sides of TM equal
Transfers movements of the tympanic membrane to the inner ear
Impedance matching
Attenuation

12. Impedance Matching
The Impedance (resistance) for the sound waves to travel in air and fluid is different.
The middle ear plays role in matching this impedance as follows:
The amplitude of movement of the stapes footplate with sound vibration is only three fourth as much as amplitude of the handle of the malleus
This increases the force of movement by 1.3 times
The surface area of tympanic membrane is about 55 square mm.& that of stapes is 3.2 sq mm
This 17 fold difference times the 1.3 fold ratio of the lever system causes about 22 times as much total force exerted on the fluid of the cochlea

13. Attenuation Reflex Function:
When loud sounds are transmitted through the ossicular system to the central nervous system, a reflex occurs to cause contraction of the stapedius muscle and the tensor tympani muscle.
This cause the entire ossicular system to develop increased rigidity, thus greatly reducing the sound conduction
This reflex occurs after a latent period of about 40 to 80 ms.
Function:
To protect the cochlea from damaging vibrations caused by excessive sound
To mask background noise in loud environments.

14. Sound Transmission Inner Ear:
The ossicular chain transfers energy from air of external ear to the fluid medium of the inner ear via the stapes attached to the oval window.
Movement of the oval window creates motion in the cochlear fluid and along the Basilar membrane. Motion along the basilar membrane excites frequency specific areas of the Organ of Corti, which in turn stimulates a series of nerve endings.
Nerve impulses are relayed through the VIII C.N., through various nuclei along the auditory pathway to areas to the brain. It is the brain that interprets the neural impulses.

15. Inner Ear
Organ of Corti

16. Organ of Corti

17. Organ of Corti
Organ of corti has specialized type of nerve cells called hair cells
Single row of inner hair cells numbering about 3500 and
3 to 4 rows of outer hair cells numbering about 12,000
90-95% of cochlear nerve ending synapse on inner hair cells

18. Sound Transmission
Basilar membrane motion causes depolarization of the hair cells.
While the hair cells do not produce action potentials themselves, they release neurotransmitter at synapses with the fibers of the auditory nerve, which does produce action potentials.

19. Sensory Transduction Kinocilia - Stereocilia are Linked
Displacement Opens K+ Channels
Depolarization → release of glutamate
K+ flows through cell
Glutamate → increase spike rate in auditory nerve

20. Inner Ear Function Thus the inner ear has 2 main functions:
Sensory transduction:
pressure waves are transformed into neural impulse.
Frequency analysis – Place Principle

21. Resonance of the basilar membrane
Stapes
Scala
vestibuli
Cochlear
nerve
Oval
window
Perilymph
Round
window
Scala
tympani
Basilar
membrane
Cochlear
duct
(a)
Base
Apex
Basilar
membrane
500 Hz
Relative
lengths
of basilar
fibers
within
different
regions
membrane
4000 Hz Hz
20,000
(High notes) Hz
1500 Hz
500 Hz 20
(Low notes)
24,000 Hz
(b)
(c)

22. Place Principle
The method used by nervous system to detect different sound frequencies is to determine the position along the basilar membrane that are most stimulated. This is called the “place principle

23. Hair Cell Innervation
Hair cells are the sensory receptors of both the auditory system and the vestibular system
They derive their name from the tufts of stereocilia that protrude from the apical surface of the cell, largest one being called the Kinocilium
Nerve fiber innervation is much denser for inner hair cells than for outer hair cells. A single inner hair cell is innervated by numerous nerve fibers, whereas a single nerve fiber innervates many outer hair cells.

24. Function of Inner & Outer Hair cells
Inner Hair Cells:
They transform the mechanical forces of sound into electrical impulses.
Outer Hair cell:
Increases the sensitivity of inner hair cells for different frequency and intensity.

25. From there it passes to medial geniculate nucleus
Auditory Pathway
Auditory cortex
From there it passes to medial geniculate nucleus
Pathways passes through lateral lemniscus but many bypass this and travel to inf. colliculus
Second order neuron from here cross to opposite side of brain stem and terminate in sup olivary nucleus
Fibers from the spiral ganglion enters ventral cochlear nuclei located in the upper part of medulla

26. Peculiarities of auditory pathway
Signals from both ears are transmitted through the pathways of both sides of the brain, with a preponderance of transmission in the contralateral pathway
Many collateral fibers from the auditory tracts pass directly into the reticular activating system of the brain stem
A high degree of spatial orientation is maintained in the fiber tracts from the cochlea all the way to the cortex

27. Perceiving Pitch (Frequency): Appreciation Of Loudness Of Sound:
Sound Ananlysis
Perceiving Pitch (Frequency):
location of vibration on the basilar membrane
Appreciation Of Loudness Of Sound:
Intensity or loudness of sound correlates with two factors:
Rate of discharge from the individual fibers of auditory nerve
Total number of nerve fibers discharging.

28. Sound Ananlysis Localization Of Sound
Sound localization is the ability to detect the source from where sound is produced.
Source of sound is Localized by comparing sounds from both ear for:
relative intensity - the amplitude of sound waves hitting the different ears
relative timing - the difference in timing in which a sound reaches both ears

29. Pathophysiology of hearing
Conduction deafness
That caused by impairment of the physical structures of the ear that conduct sound to cochlea like outer ear and middle ear.
Sensorineural deafness
It is the deafness caused from impairment of the Hair cells, auditory nerve etc.

30. References
Human physiology by Lauralee Sherwood, 9th edition
Text Book Of Physiology by Guyton & Hall, 12th edition

31. THANK YOU