Chapter 52: The Sense of Hearing
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Tympanic Membrane and Ossicles
• Conduction of Sound from the Tympanic Membrane
to the Cochlea
Fig. 52.1 Tympanic membrane, ossicular system of the middle and inner ear
Tympanic Membrane and Ossicles
• “Impedance Matching” by the Ossicular System
a. Reduces the distance between the ossicles but
increases the force of movement
b. Because of size difference, the force exerted is
a total of 22x on the fluid of the cochlea
c. Without the tympanic membrane and ossicles
the sound waves would still pass through to the
cochlea, but at a greatly reduced sensitivity
Tympanic Membrane and Ossicles
• Attenuation of Sound by Contraction of the Tensor
Tympani and Stapedius Muscles
a. When a loud sound is transmitted the stapedius
muscle contracts and to a lesser extent, the tensor
contracts
b. Can reduce decibels by 30-40
c. Function of the attenuation reflex
1.
2.
To protect the cochlea from damaging vibrations
Mask low frequency sounds in loud environments
Tympanic Membrane and Ossicles
• Transmission of Sound Through Bone
a. Because the inner ear is embedded in bone,
vibrations of the entire skull can cause fluid
vibrations in the cochlea
b. Energy available in loud sound is generally not
enough to cause hearing via bone conduction
Cochlea
• Functional Anatomy
Fig. 52.2 Cochlea
Cochlea
• Functional Anatomy
Fig. 52.3 Section through one of the turns of the cochlea
Cochlea
• Functional Anatomy- consists of three tubes
a. Scala vestubli, scala media, scala tympani
b. Scala vestubli and scala media are separated by the
vestibular membrane
c. Scala tympani and scala media are separated by the
basilar membrane
d. On the surface of the basilar membrane lies the
organ of Corti (contains hair cells; the receptors)
Cochlea
• Functional Anatomy
Fig. 52.4 Movement of fluid in the cochlea after forward thrust of the stapes
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
Cochlea
Fig. 52.5 “Traveling waves” along the basilar membrane for high,
medium, and low frequency sounds
Cochlea
• Amplitude Pattern of Vibration of the
Basilar Membrane
Fig. 52.6 Amplitude pattern of vibration of the basilar membrane for medium frequency sound
Cochlea
• Function of the Organ of Corti
a. Receptor organ that generates nerve impulses
in response to vibration of the basilar
membrane
b. Actual receptors are called “hair” cells
c. Nerve fibers that are stimulated lead to the
spiral ganglion of Corti which sends axons to
the cochlear nerve
Cochlea
Fig. 52.7 Organ of Corti showing hair cells and the tectorial membrane
Cochlea
Fig. 52.8 Stimulation of the hair cells by movement of hairs projecting
into the gel casing of the tectorial membrane
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
Cochlea
• Determination of Loudness
a. 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
b. Causes more and more hair cells on the fringes to
become stimulated, thus causing spatial summation
of impulses
Cochlea
• Determination of Loudness
c. Outer hair cells do not become stimulated until
the vibration of the basilar membrane
reaches high intensity
•
Detection of Changes in Loudness
a. 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
Central Auditory Mechanisms
• Auditory Nervous Pathways
Fig. 52.10
Central Auditory Mechanisms
• Function of the Cerebral Cortex in Hearing
Fig. 52.11 Auditory cortex
Central Auditory Mechanisms
• Function of the Cerebral Cortex in Hearing
a. Sound frequency perception
b. Discrimination of sound patterns
c. Determination of direction from which sound comes
Cochlea
• Determination of Sound Frequency
a. The “Place” Principle- major method to detect
different sound frequencies is to determine
the position along the basilar membrane
that is most stimulated