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NROC64H3 Study Guide - Vestibular Duct, Ventral Cochlear Nucleus, Vestibulocochlear Nerve

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Matthias Niemeier

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Chapter 11
The auditory and vestibular systems
Sense of hearing: Audition. Allows us to detect and locate sound, as well as perceive and interpret its
Mechanism of audition:
1. Translating sounds in our environment into meaningful neural signals
2. Transformations are carried out in stages rather than all at once
3. Within the inner ear, neural responses are generated by auditory receptors from the mechanical
energy in sound
4. At subsequent stages in the brain stem and thalamus, signals from the receptors are integrated
before they ultimately reach the auditory cortex
Sense of balance: Regulated by the vestibular system, it is a personal and internalized process. The
vestibular system informs our nervous system where our head and body are and how they are moving.
This information is used without conscious effort to control muscular contractions that will put our body
where we want it to be, to reorient ourselves when something pushes us aside, and to move our eyes so
that our visual world stays fixed on our retinas even when our head is bouncing around.
Mechanism of the vestibular system:
1. Translating the movements of our head into a sense of where we are
2. Transformations are carried out in stages rather than altogether.
3. Within the inner ear, neural responses are generated by vestibular receptors form the tilts and
rotations of the head
4. At subsequent stages in the brain stem and thalamus, signals from the receptors are integrated
before they ultimately reach the vestibular cortex
The nature of sound
Sounds: Sounds are audible variations in air pressure.
1. When an object moves toward a patch of air, it compresses the air, increasing the density of the
2. When an object moves away from a patch of air, the air is rarefied (made less dense).
Frequency: Frequency of a sound is the number of compressed or rarefied patches of air that pass by
our ears each second. Sound frequency is expressed in hertz (Hz); the number of cycles per second
1. Because sound waves all propagate at the same speed, high frequency sound waves have more
compressed and rarefied regions packed in to the same space than low-frequency waves

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Cycle: One cycle of sound is the distance between successive compressed patches
Pitch: The sensation of a frequency is commonly referred to as the pitch of a sound.
1. A high pitch sound corresponds to a high frequency
2. A low pitch sound corresponds to a low frequency
Intensity: The difference in pressure between compressed and rarefied patches of air. Sound intensity
determines the loudness we perceive; loud sounds have higher intensity.
The structure of the auditory system
Pinna: The visible portion of the ear made up of cartiledge covered by skin, forming a sort of funnel.
1. Helps collect sounds from a wide area
2. Makes us more sensitive from sounds coming from ahead than from behind
3. Convolutions in the pinna play a role in localizing sounds
Auditory canal: The entrance to the internal ear. It extends about 1 inch into the ear before it ends at
the tympanic membrane.
Tympanic membrane: Also known as the eardrum. It is a thin, cone shaped membrane that separates
the external ear from the middle ear. Its function is to transmit sound from the air to the ossicles inside
the middle ear, and then to the oval window in the fluid filled cochlea. It ultimately converts and
amplifies vibration in air to vibration in fluid.
Ossicles: Connected to the medial surface of the tympanic membrane. It is located in a small air filled
chamber and transfers movements of the tympanic membrane into movements of a second membrane
covering a hole in the bone of the skull called the oval window.
Cochlea: Located behind the oval window. It contains the apparatus for transforming the physical
motion of the oval window membrane into a neuronal response.
The basic auditory pathway:
1. Sound wave moves the tympanic membrane
2. Tympanic membrane moves the ossicles
3. Ossicles move the membrane at the oval window
4. Motion at the oval window moves fluid in the cochlea
5. Movement of fluid in the cochlea causes a response in sensory neurons
The ear: Has three main divisions:
1. Outer ear
a. Pinna
b. Tympanic membrane
2. Middle ear

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a. Tympanic membrane
b. Ossicles
3. Inner ear
a. Apparatus medial to the oval window
Generation of neural response in the inner ear:
1. Results in the transfer and processing of the signal by a series of nuclei in the brain stem.
2. Output from these nuclei is sent to a relay in the thalamus; the medial geniculate nucleus
3. The MGN projects to primary auditory cortex, or A1, located in the temporal lobe.
The middle ear
Components of the middle ear:
1. Tympanic membrane
a. Conical in shape with the point of the cone extending into the cavity of the middle ear
2. Three ossicles
a. Malleus
i. Attached to the tympanic membrane
ii. Forms a rigid connection with the incus
b. Incus
i. Forms a flexible connection with the stapes
c. Stapes
i. The flat bottom portion of the stapes called the footplate moves in and out like
a piston at the oval window
1. Thus transmitting sound vibrations to the fluids of the cochlea in the
inner ear
3. Two tiny muscles that attach to the ossicles
4. Eustachian tube
a. Passage that connects the air in the middle ear with the air in the nasal cavities
b. This tube is usually open and closed by a valve
i. Opening this valve relieves pressure build up associated with changes in air
1. E.g. high altitudes in airplane;
Sound force amplification by the ossicles:
1. Sound waves move the tympanic membrane
2. Ossicles move another membrane at the oval window
a. Ossicles provide the necessary amplification in pressure to influence the fluid filled
membrane of the oval window
b. Amplification is necessary because fluid resists sound changes more than air
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