Physiology of Hearing 1 Physiology Lecture SlideshowPhysiology of Hearing 1 Physiology Lecture Slideshow

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
  • Describe “Impedance Matching” and its importance
  • Discuss the process of attenuation of sounds
  • Explain the working of cochlea
  • Discuss Place principle


  • Bridge the ear drum and the inner
  • 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
  • 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

  1. To mask low-frequency sounds in loud environments.
  2. To decrease a person’s hearing sensitivity to his or her own

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


  • 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

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.


Due to the movement of foot of
stapes the oval window bulges

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


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
The wave dies at this point and fails to
travel the remaining distance along the
basilar membrane.


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
  • 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
  • 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

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
  • Low-frequency sounds, from 20 to 1500 to 2000 cycles/sec, can
    cause volleys of nerve impulses synchronized at the same
  • These volleys are transmitted by the cochlear nerve into the
    cochlear nuclei of the brain.

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