1. Unit Ten: The Nervous System: B. Special Senses
Chapter 52: The Sense of Hearing
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Tympanic Membrane and Ossicles
Conduction of Sound from the Tympanic Membrane
to the Cochlea
Fig Tympanic membrane, ossicular system of the middle and inner ear

3. Tympanic Membrane and Ossicles
“Impedance Matching” by the Ossicular System
Reduces the distance between the ossicles but
increases the force of movement
Because of size difference, the force exerted is
a total of 22x on the fluid of the cochlea
Without the tympanic membrane and ossicles
the sound waves would still pass through to the
cochlea, but at a greatly reduced sensitivity

4. Tympanic Membrane and Ossicles
Attenuation of Sound by Contraction of the Tensor
Tympani and Stapedius Muscles
When a loud sound is transmitted the stapedius
muscle contracts and to a lesser extent, the tensor
contracts
Can reduce decibels by 30-40
Function of the attenuation reflex
To protect the cochlea from damaging vibrations
Mask low frequency sounds in loud environments

5. Tympanic Membrane and Ossicles
Transmission of Sound Through Bone
Because the inner ear is embedded in bone,
vibrations of the entire skull can cause fluid
vibrations in the cochlea
Energy available in loud sound is generally not
enough to cause hearing via bone conduction

6. Cochlea
Functional Anatomy
Fig Cochlea

7. Cochlea Functional Anatomy
Fig Section through one of the turns of the cochlea

8. Cochlea
Functional Anatomy- consists of three tubes
Scala vestubli, scala media, scala tympani
Scala vestubli and scala media are separated by the
vestibular membrane
Scala tympani and scala media are separated by the
basilar membrane
On the surface of the basilar membrane lies the
organ of Corti (contains hair cells; the receptors)

9. Cochlea Functional Anatomy
Fig Movement of fluid in the cochlea after forward thrust of the stapes

10. Cochlea
Basilar Membrane and Resonance- high frequency
and low frequency resonance
Transmission of Sound Waves in the Cochlea—
“Traveling Wave”
Pattern of Vibration of the Basilar Membrane for
Different Sound Frequencies

11. Cochlea
Fig “Traveling waves” along the basilar membrane for high,
medium, and low frequency sounds

12. Amplitude Pattern of Vibration of the Basilar Membrane
Cochlea
Amplitude Pattern of Vibration of the
Basilar Membrane
Fig Amplitude pattern of vibration of the basilar membrane for medium frequency sound

13. Cochlea
Function of the Organ of Corti
Receptor organ that generates nerve impulses in response to vibration of the basilar membrane
Actual receptors are called “hair” cells
Nerve fibers that are stimulated lead to the spiral ganglion of Corti which sends axons to the cochlear nerve

14. Cochlea
Fig Organ of Corti showing hair cells and the tectorial membrane

15. Cochlea
Fig Stimulation of the hair cells by movement of hairs projecting
into the gel casing of the tectorial membrane

16. Cochlea
Auditory Signals are Transmitted Mainly by
the Inner Hair Cells
Hair Cell Receptor Potentials and Excitation of
Auditory Nerve Fibers- polarization or
hyperpolarization depending on the direction
the hair cells are bent

17. Cochlea
Determination of Loudness
As sound becomes louder, the amplitude of
vibration of the basilar membrane and hair cells also
increases so that the hair cells excite the nerve
endings at more rapid rates
Causes more and more hair cells on the fringes to
become stimulated, thus causing spatial summation
of impulses

18. Cochlea
Determination of Loudness
Outer hair cells do not become stimulated until
the vibration of the basilar membrane
reaches high intensity
Detection of Changes in Loudness
The Power Law- a person interprets changes in
intensity approximately in proportion to an
inverse power of the function of the actual
intensity (can interpret an increase of 1 trillion times

19. Central Auditory Mechanisms
Auditory Nervous Pathways
Fig

20. Central Auditory Mechanisms
Function of the Cerebral Cortex in Hearing
Fig Auditory cortex

21. Central Auditory Mechanisms
Function of the Cerebral Cortex in Hearing
Sound frequency perception
Discrimination of sound patterns
Determination of direction from which sound comes

22. Cochlea
Determination of Sound Frequency
The “Place” Principle- major method to detect
different sound frequencies is to determine
the position along the basilar membrane
that is most stimulated