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Chapter 11

PSL440Y1 Chapter Notes - Chapter 11: Superior Colliculus, Dorsal Cochlear Nucleus, Neural Coding


Department
Physiology
Course Code
PSL440Y1
Professor
A
Chapter
11

Page:
of 10
INTRODUCTION
The sense of hearing is known as audition and the sense of balance is regulated by the
vestibular system.
THE NATURE OF SOUND
Sounds are audible variations in air pressure. When an object moves toward a patch of air, it
compresses the air, increasing the density of the molecules. The air is rarefied when an object
moves away on the other hand (see Fig. 11.1).
The speed of sound is about 343 m/sec (767 mph) for air at room temperature
Frequency of sound is the number of compressed or rarefied patches of air that pass by our
ears each second. One cycle of the sound is the distance between successive compressed
patches and it is expressed in hertz (Hz).
Sound waves all propagate at the same speed so high frequency sound waves have
more compressed and rarefied regions packed into the same space than low-frequency
waves (see Fig. 11.2)
Our auditory system can respond to pressure waves between 20 Hz to 20 000 Hz.
Whether a sound is perceived to have a high or low tone (i.e. pitch) is determined by its
frequency.
Intensity is the difference in pressure between compressed and rarefied patches of air (Fig.
11.2b). Sound intensity determines the loudness we perceive loud sounds having higher
intensity.
THE STRUCTURE OF THE AUDITORY SYSTEM
See Fig, 11.3
The visible portion of the ear consists primarily of cartilage covered by skin forming a funnel
called the pinna which collects sounds from a wide area.
Its shape makes us more sensitive to sounds coming from ahead than from behind
Is more or less fixed in position
The entrance to the internal ear is the auditory canal which extends about 2.5 cm inside skull
and ends at the tympanic membrane (eardrum). Ossicles are connected to the tympanic
membrane to its medial surface and they transfer movements of the tympanic membrane into
movements of the oval window. The cochlea is located behind the oval window and it contains
the apparatus to transform physical motion into a neural response.
Sound waves moves the tympanic membrane tympanic membrane moves the ossicles
ossicles move oval window membrane motion at the oval window moves fluid in the cochlea
fluid movement causes sensory neurons to respond
The pinna to tympanic membrane make the outer ear, the tympanic membrane and
ossicles make up the middle ear and the apparatus medial to the oval window make up
the inner ear.
Once a neural response to sound is generated, the signal is transferred to and processed by a
series of nuclei in the brain stem. The output is sent to the medial geniculate nucleus (MGN)
which projects to the primary auditory cortex (A1).
THE MIDDLE EAR
Variations in air pressure are converted into movements of the ossicles.
Components of the Middle Ear
Tympanic membrane conical in shape
Three ossicles
Two tiny muscles attached to the ossicles
There three ossicles: the malleus is attached to the tympanic membrane and forms a rigid
connection with the incus which forms a flexible connection with the stapes. The flat portion of
the stapes, footplate, moves like a piston at the oval window and transmits sound vibrations to
the fluids of the cochlea.
The air in the middle ear is continuous with the air in the nasal cavities via the Eustachian tube.
Sound Force Amplification by the Ossicles
Why isn’t the ear arranged so sound waves directly move the membrane at the oval window?
The problem is that the cochlea is filled with fluid (not air) so if sound waves impinged
directly on the oval window, the membrane would barely move and all but 0.1% of the
sound energy would be reflected
The fluid in the inner ear resists being moved much more than air does, so more
pressure is needed to vibrate the fluid than air can provide and thus ossicles provide this
necessary amplification in pressure.
The pressure at the oval window will become greater than the pressure at the tympanic
membrane if:
The force on the oval window membrane > force on tympanic membrane
Surface area of the oval window < area of the tympanic membrane
By using both mechanisms, the pressure increases at the oval window and the force at the oval
window become greater since the ossicles act like levers.
Sound causes large movements of the larger tympanic membrane which transformed
into smaller but stronger vibrations of the smaller oval window.
The pressure at the oval window is about 20 times greater than at the tympanic
membrane and this is sufficient to move the fluid in the inner area.
The Attenuation Reflex
Two muscles attached to the ossicles effect sound transmission to the inner ear (see Fig. 11.6):
Tensor tympani muscle: attaches to the malleus
Stapedius muscle: attaches to the stapes
When these muscles contract, the chain of ossicles become rigid and sound conduction to the
inner ear is greatly diminished.
Loud sound causes these muscles to contract -- attenuation reflex which is much
greater at low frequencies than at high frequencies.
Attenuation reflex protects the ear from loud sounds that would damage it. However, this reflex
has a delay of 50-100 msec from the time that sound reaches the ear so it can`t protect the ear
from very sudden loud sounds. The reflex also suppresses low frequencies more than higher
ones so high frequencies are easier to discern in an environment of a lot of low frequencies.
THE INNER EAR
Consists of the cochlea (auditory system) and cochlea (vestibular system).
Anatomy of the Cochlea (see Fig. 11.6)
In the cochlea, the hollow tube has walls made of bone. The central pillar of the cochlea is a
conical bony structure, modiolus. The base of the cochlea consists of two membrane-covered
holes: the oval window and the round window.
The cochlea is divided into three fluid-filled chambers (see Fig. 11.7): the scala vestibuli, the
scala media, and the scala tympani. The Reissner’s membrane divides the scala vestibuli
and scala media; the basilar membrane separates the scala tympani form the scala media.
The organ of Corti sits on the basilar membrane and contains auditory receptor neurons.
Hanging over this structure is the tectorial membrane.
The perilymph is the fluid in the scala vestibule and scala tympani. It has an ionic content
similar to cerebrospinal fluid: low K+ (7nM) and high Na+ (140 nM) concentrations. The scala
media contains endolymph with ionic concentrations similar to ICF: high K+ (150 nM) and low
Na+ (1nM). This difference in ion content is created by active transport at the stria vascularis
which reabsorbs sodium and secretes potassium against their concentration gradients.
The endolymph has an electrical potential that is 80 mV more positive than that of the
perilymph. This is the endocochlear potential.
Physiology of the Cochlea (See Fig. 11.8)
Inward motion at the oval window pushes perilymph into the scala vestibuli. If the membranes
inside the cochlea were completely rigid, then the increase in fluid pressure at the oval window
would reach up the scala vestibuli and back down the scala tympani to the round window.
Since the fluid pressure has nowhere to escape, the round window would bulge out in
response to the inward movement oval window.
o Any motion at the oval window must be accompanied by a complementary motion at the
round window. This movement must occur because the cochlea is filled with
incompressible fluid held in a solid bony container
The Response of the Basilar Membrane to Sound
The basilar membrane has two structural properties that determine the way it responds to sound
It is wider at the apex than at the base by a factor of about 5
Its stiffness decreases from base to apex; the base about 100 times stiffer