Physiology of Hearing 1

Dr. Sumera Gul

Associate Professor

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

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

• Oval window vibrations move fluid in inner ear to stimulate hearing receptors

• 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.

Inner (Internal) Ear

• Consists of:• Labyrinth

• Semicircular canals

• Cochlea

• Round window

• Spiral Organ (Organ of Corti)

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.

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 or more vibrations per second

• 2,000-3,000 is the range of greatest sensitivity

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.

Total Force applied by Stapes on Fluid in Cochlea = 1.3 x 17 = 22 times

Therefore, the tympanic membrane and ossicular system provide impedance matching between the sound waves in air and the sound vibrations in the fluid of the cochlea.

What will happen if there was no impedance matching?

Attenuation Reflex

Frequency of Sound

• The frequency of a sound is the number of cycles of a sound wave in one second. The unit of measurement is hertz (Hz).

• The frequency of a sound increases as the number of cycles per second increase.

• Vibrations between 20 and 20,000 cycles per second are interpreted as sound by a normal healthy person.

Intensity of Sound

• The term dB (deciBel) and the dB scale are used world-wide for the measurement of sound level

• Here are some examples of different sound intensities as expressed in dB

• 180 dB: Rocket at take-off140 dB: Jet engine at take-off120 dB: Rock band110 dB: Loud thunder90 dB: City traffic80 dB: Loud radio60 dB: Ordinary conversation30 dB: Soft whisper0 dB: Softest sound a person can hear

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.

Latent period of only 40 to 80 milliseconds

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.

Attenuation Reflex

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 twofold:

To protect the cochlea from damaging vibrations caused by excessively loud sound

To mask low-frequency sounds in loud environments.

Another function of the tensor tympani and stapedius muscles is to decrease a person’s hearing sensitivity to his or her own speech.