Learning Objectives
- At the end of the lecture the students should be able to
- Discuss the physiological Anatomy of ear
- Explain the mechanism of conduction of sound waves through the ear to the
cochlea - Describe “Impedance Matching” and its importance
- Discuss the process of attenuation of sounds
- Explain the working of cochlea
- Discuss Place principle
Ossicles
- Bridge the ear drum and the inner
ear - Tiny ligaments attach bones to wall
of tympanic cavity - Are covered by mucous membrane
- Help increase(amplify) the force of
vibrations from eardrum to oval
window
- Attached to the tympanic membrane is the
handle of the malleus. - The malleus is bound to the incus by minute
ligaments, so whenever the malleus moves, the
incus moves with it. - The opposite end of the incus articulates with
the stem of the stapes, and the faceplate of the
stapes lies against the membranous labyrinth of
the cochlea in the opening of the oval window. - The tip end of the handle of the malleus is
attached to the center of the tympanic
membrane, and this point of attachment is
constantly pulled by the tensor tympani muscle,
which keeps the tympanic membrane tensed.
Impedance Matching
Impedance: Resistance offered by a medium for
transmission of sound.
Impedance Matching
The amplitude of movement of the stapes faceplate with each sound vibration
is only three fourths as much as the amplitude of the handle of the malleus.
The system actually reduces the distance but increases the force of movement
about 1.3 times.
In addition, the surface area of the tympanic membrane is about 55 square
millimeters, whereas the surface area of the stapes averages 3.2 square
millimeters. . (17 times smaller)
Total Force applied by Stapes on Fluid in Cochlea = 1.3 x 17 = 22 times
What will happen if there was no impedance matching?
Attenuation Reflex
Attenuation Reflex
When loud sounds are transmitted through the ossicular system and
from there into the central nervous system, a reflex occurs to cause
contraction of the stapedius muscle and the tensor tympani muscle.
The tensor tympani muscle pulls the handle of the malleus inward
while the stapedius muscle pulls the stapes outward.
Increased rigidity, thus greatly reducing the ossicular conduction of
low-frequency sound, mainly frequencies below 1000 cycles/sec.
This attenuation reflex can reduce the intensity of lower-frequency
sound transmission by 30 to 40 decibels.
The function of this mechanism is believed to be two fold:
1.To protect the cochlea from damaging vibrations caused by
excessively loud sound
- To mask low-frequency sounds in loud environments.
- To decrease a person’s hearing sensitivity to his or her own
speech.
Inner Ear
Inner (Internal)Ear
- Consists of:
- Bony labyrinth
- Membranous Labyrinth
Spiral Organ
- Organ of Corti
- Contains hearing receptors
- Receptor Cells
- Hair cells
- Function somewhat like neurons
- Move back and forth depending on pitch of sound
- Young person
- Detect sound waves ranging from 20-20,000 Db or more vibrations
per second - 2,000-3,000 is the range of greatest sensitivity
Cochlea
- It consists of three tubes coiled side by side:
- (1) the scala vestibuli,
- (2) the scala media,
- (3) the scala tympani.
- The scala vestibuli and scala media are separated from each other by Reissner’s membrane (also
called the vestibular membrane - The scala tympani and scala media are separated from each other by the basilar membrane.
- On the surface of the basilar membrane lies the organ of Corti, which contains a
series of electromechanically sensitive cells, the hair cells. - They are the receptive end organs that generate nerve impulses in response to sound vibrations.
Hair Cells
The hair cells are arranged in four rows: three rows of outer hair cells
one row of inner hair cells
There are 20,000 outer hair cells and 3500 inner hair cells in each
human cochlea.
Covering the rows of hair cells is a thin, viscous, but elastic tectorial
membrane in which the tips of the hairs of the outer but not the inner
hair cells are embedded.
Basilar Membrane
The basilar membrane is a fibrous membrane that separates
the scala media from the scala tympani.
It contains 20,000 to 30,000 basilar fibers that project from the
bony center of the cochlea, the modiolus, toward the outer wall.
These fibers are stiff, elastic and reed like.
The lengths of the basilar fibers increase progressively . From 0.04
millimeter near the oval and round windows to 0.5 millimeter at
helicotrema, a 12-fold increase in length.
The diameters of the fibers, however, decrease from the oval window
to the helicotrema, so their overall stiffness decreases more than
100-fold.
As a result, the stiff, short fibers near the oval window of the cochlea
vibrate best at a very high frequency, whereas the long, limber fibers
near the tip of the cochlea vibrate best at a low frequency.
TRANSMISSION OF SOUND WAVES IN THECOCHLEA
Due to the movement of foot of
stapes the oval window bulges
inwards.
The elastic tension that is built up in
the basilar fibers as they bend
toward the round window initiates a
fluid wave that “travels” along the
basilar membrane toward the
helicotrema.
Pattern of Vibration of the Basilar Membrane for Different Sound Frequencies
Each wave becomes strong when it
reaches the portion of natural resonant
frequency equal to the respective sound
frequency.
The wave dies at this point and fails to
travel the remaining distance along the
basilar membrane.
FUNCTION OF THE ORGAN OF CORTI
Outer ends of the hair cells are fixed
tightly in a rigid structure composed
of a flat plate, called the reticular
lamina, supported by triangular rods
of Corti, which are attached tightly to
the basilar fibers.
Generation of Signals
- Cilia are bent in the direction of the longer
ones, the tips of the smaller stereocilia are
tugged outward from the surface of the
hair cell. - Opens 200 to 300 cation-conducting
channels, allowing rapid movement of
potassium ions from the surrounding scala
media fluid into the stereocilia, which causes
depolarization of the hair cell membrane. - When the bundle of processes is pushed
in the opposite direction, the cell is
hyperpolarized. - Thus, the hair processes provide a
mechanism for generating changes in
membrane potential proportional to the
direction and distance the hair moves.
Place Principle
- There is spatial
organization of the nerve fibers in
the cochlear pathway, all the way
from the cochlea to the cerebral
cortex. - Therefore, the major method
used by the nervous system to
detect different sound
frequencies is to determine the
positions along the basilar
membrane that are most
stimulated, which is called the
place principle for the
determination of sound
frequency.
What about sound below 200cycles/sec?
Volley / Frequency Principle
- These low frequencies have been postulated to be
discriminated mainly by the so-called volley or frequency
principle. - Low-frequency sounds, from 20 to 1500 to 2000 cycles/sec, can
cause volleys of nerve impulses synchronized at the same
frequencies - These volleys are transmitted by the cochlear nerve into the
cochlear nuclei of the brain.
