Unit Ten: The Nervous System: B. Special Senses

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 Membraneto 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 isa total of 22x on the fluid of the cochlea

c. Without the tympanic membrane and ossiclesthe sound waves would still pass through to thecochlea, but at a greatly reduced sensitivity

Tympanic Membrane and Ossicles

• Attenuation of Sound by Contraction of the TensorTympani and Stapedius Muscles

a. When a loud sound is transmitted the stapediusmuscle contracts and to a lesser extent, the tensor contracts

b. Can reduce decibels by 30-40

c. Function of the attenuation reflex

1. To protect the cochlea from damaging vibrations2. 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 fluidvibrations in the cochlea

b. Energy available in loud sound is generally notenough 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 thevestibular membrane

c. Scala tympani and scala media are separated by thebasilar 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 forDifferent Sound Frequencies

Cochlea

Fig. 52.5 “Traveling waves” along the basilar membrane for high, medium, and low frequency sounds

Cochlea

Fig. 52.6 Amplitude pattern of vibration of the basilar membrane for medium frequency sound

• Amplitude Pattern of Vibration of the

Basilar Membrane

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 directionthe hair cells are bent

Cochlea

• Determination of Loudness

a. As sound becomes louder, the amplitude of vibration of the basilar membrane and hair cells alsoincreases so that the hair cells excite the nerveendings at more rapid rates

b. Causes more and more hair cells on the fringes tobecome stimulated, thus causing spatial summationof impulses

Cochlea

• Determination of Loudness

c. Outer hair cells do not become stimulated untilthe vibration of the basilar membrane reaches high intensity

• Detection of Changes in Loudness

a. The Power Law- a person interprets changes inintensity 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 detectdifferent sound frequencies is to determinethe position along the basilar membranethat is most stimulated