Hearing physiology

Dr.Basma Elgatany
Objectives
• 1. Mention the important physical properties of sound.
• 2. Explain the functional anatomy of ear
• 3. Discuss mechanism of hearing
• 4.Describe auditory pathway
• 5. Define auditory coding and explain its types
• 6. Define deafness and list its types
Two important physical properties of a
sound are pitch and intensity
• (1) Pitch=tone =frequency
It is determined by the frequency of the sound waves per second
It is measured in Hertz (Hz):
The range for human hearing is about 20 –20,000 cycle/sec (Hz).
The human voice produces sound of about 1000 –3000 Hz when
speaking.
The pitch of a sound is correlated with the frequency of the sound waves.
The greater the frequency, the higher the pitch
• (2) Amplitude= intensity=loudness
The amplitude of sound waves determines the intensity or volume
of the sound (i.e. its loudness or audibility). The greater the
amplitude, the louder will be the sound & vice versa.
Sound intensity is measured in bels (B) or decibels (dB).
Normal conversation has a typical noise level of about 60 dB.
Sounds of 80 dB or more can damage the sensitive hearing
receptors of the ear, resulting in hearing loss
• A is the record of a pure tone.
• B has a greater amplitude and islouder thanA.
• C has the same amplitude as A but a greaterfrequency,
And its pitch is higher.
• D is a complex wave form that is regularly repeated.
Such patterns are perceived as musical sounds, whereas waves
like that shown in E, which have no regular pattern, are perceived
as noise.
 The Auditory System
The Ear Has Different Functional
Components
The human ear consists of 3 parts:

(1) External ear: this consists of the following structures:

a. The ear pinna (or auricle). This collects sound waves & directs them towards the external auditory
meatus. - In man, it is of little use .
b. The external auditory meatus (a narrow canal that conducts the sound waves inwards to
• the tympanic membrane). It conducts & concentrates the sound waves to the tympanic membrane.
 The auditory canal protects the tympanic membrane:
 The hair traps the foreign bodies & prevents its entrance into the canal.
 The ear wax lubricates the tympanic membrane, prevents growth of organisms (antiseptic due to
its acidic pH) & also prevents entrance of the insects it act slightly as insect repellent .
• C. The tympanic membrane or eardrum, is a thin, semitransparent structure between the external
auditory canal and the middle ear. When sound waves strike the tympanic membrane, it vibrates
and then transmits the vibrations to the middle ear
(2) Middle ear:

- This is an air-filled cavity in the temporal bone that is connected to the nasopharynx
by the Eustachian tube.
- It contains the following structures:

a. 3 small bony ossicles : - These include (from outside inwards) the malleus (hammer),

• incus (anvil) & stapes (stirrup).

 They constitute a chain connecting the tympanic membrane to the membrane
covering the oval window.
 The main function of the auditory ossicles is to transmit and amplify vibrations from
the tympanic membrane to the oval window
b. Middle ear muscles:

• The middle ear also contains two skeletal muscles that contract
reflexively to protect the structures of the inner ear from damage by
loud noises .The tensor tympani muscle , which is attached to the
malleus, protects the inner ear from loud sounds by limiting the
movements of the tympanic membrane. The stapedius muscle ,
which is attached to the stapes, protects the inner ear from loud
noises by dampening the movements of the stapes in the oval
window. Because it takes a fraction of a second(40 msec) for the
tensor tympani and stapedius muscles to contract, they can protect
the inner ear from prolonged loud noises but not from brief ones
such as a gunshot.
• c. The Eustachian tube:
• - The middle ear is connected with the nasopharynx by the
Eustachian tube.
• It equalizes the pressure on both sides of the tympanic
membrane.
• Normally the tube is closed but it opens during swallowing,
chewing, yawing, blowing the nose, sneezing.
 Otitic Barotrauma:
• - During rapid descent of an airplane the external pressure
increases & pushes the drum inwards & may rupture it unless
the person swallows.

• d. 2 foramena, which separate the middle ear from the internal

ear: oval window & round window.
(3) Internal ear
• (= cochlea or auditory labyrinth):- The inner ear contains structures associated
with both hearing and equilibrium.

The fluid in the scala vestibuli and the scala tympani is similar in ion composition
to CSF and is known as perilymph.

The scala media is filled with endolymph secreted by epithelial cells in the duct.

Endolymph is unusual because it is more like intracellular fluid than extracellular

fluid in composition, with high concentrations
of K and low concentrations of Na
+ +
Organ of corti

Resting on the basilar membrane is the organ of Corti or spiral

organ .
It consists of supporting cells and hair cells.
There are two groups of hair cells:
 a single row of inner hair cells and three rows of outer hair cells
Organ of corti
At the apical tip of each hair cell are stereocilia, which are
actually microvilli arranged in several rows of graded height.
The stereocilia are embedded in a flexible, gelatinous
covering called the tectorial membrane.
At their basal ends, hair cells receive innervation from sensory
and motor neurons of the cochlear branch of the
vestibulocochlear (VIII) nerve.
Functions of chochlea:

1.Transmission of sound waves from the oval

window to the organ of Corti.
2.Transduction: changing sound pressure into an
action potential.
3.Pitch analysis: each group of fibres in the basilar
membrane responds to specific frequency.
Mechanism of hearing
• 1. The pinna directs sound waves into the external auditory
canal.
• 2. Sound waves will be transmitted to the tympanic membrane .
• 3. The central area of the tympanic membrane connects to the
malleus, which also starts to vibrate. The vibration is
transmitted from the malleus to the incus and then to the stapes.
Mechanism of hearing
4. As the stapes moves back and forth, it pushes the membrane
of the oval window in and out. The oval window vibrates about 20
times more vigorously than the tympanic membrane because the
auditory ossicles efficiently transmit small vibrations spread over
a large surface area (tympanic membrane) into larger vibrations
of a smaller surface (oval window).
5. The movement of the stapes at the oval window sets up fluid
pressure waves in the perilymph of the cochlea. As the oval
window bulges inward, it pushes on the perilymph of the scala
vestibuli
Mechanism of hearing
6. Pressure waves are transmitted from the scala vestibuli to the scala
tympani and eventually to the round window, causing it to bulge outward
into the middle ear
7.As the pressure waves deform the walls of the scala vestibuli and scala
tympani, they also push the vestibular membrane back and forth,
creating pressure waves in the endolymph inside the cochlear duct. the
basilar membrane move in a pattern determined by the frequency(pitch)
& intensity(amplitude=loudness) of the sound waves.
Mechanism of hearing
• 8. The pressure waves in the endolymph cause the basilar
membrane to vibrate, which moves the hair cells of the organ of
Corti against the tectorial membrane. This leads to bending of
the hair cell stereocilia, resulting in the production of receptor
potentials that ultimately lead to the generation of action
potentials
Mechanism of stimulation of hair cells:
- Bending of the hairs in one direction depolarizes the hair cell,
& bending them in the opposite direction hyperpolarizes it.
-Movement of short hairs (stereocilia) toward the direction of
the longer one (kinocilium)
• causes opening of mechanosensitive K channels
+

depolarization of hair cells & development of receptor potential

action potentials along cochlear nerve.
Auditory coding
• (Determination of sound frequency, intensity & localization)
I. Discrimination of sound frequency (pitch=tone): (The basilar place code)
- Short thick stiff basilar fibres at the base are maximally activated by the
high frequency sounds, the long thin lax basilar fibres at the apex are
maximally activated by the low frequency sounds & the fibres between
the apex & base respond to intermediate frequencies. So each frequency
causes vibration of its own particular “place” on the basilar membrane.
Auditory coding
II. Discrimination of sound intensity (amplitude or loudness):
• (The basilar resonant code)
- Discrimination of sound intensity is determined by the number of impulses
discharged from the hair cells of the organ of Corti to the auditory cortex.
This occurs by means of two different ways:
• As the intensity of the sound increases the movement of the basilar
membrane increases
1. Causing the hair cells to fire more rapidly.
2. Stimulation of more & more hair cells & so stimulating more neurons in
auditory cortex
Auditory coding
• Discrimination of the sound direction (sound localization):
• Detection of the direction of the sound depends upon:
1) The difference in time of arrival of the sound in the ears the
sound arrived first to the side close to the source of sound.
2)The difference in sound loudness the sound is louder on the
side close to the source of sound.
3) The role of auditory cortex is proved to be important because
destruction of auditory cortex on both sides of the brain causes
loss of ability to detect the direction from which the sound
comes
Auditory pathway
 Primary auditory neurons project from the cochlea to cochlear
nuclei in the medulla oblongata.
 From the medulla, secondary sensory neurons project to two
higher nuclei, one ipsilateral (on the same side of the body) and
one contralateral .
 These ascending tracts then synapse in nuclei in the midbrain
and thalamus before projecting to the auditory cortex
Auditory cortex
The Primary Auditory Cortex (areas 41 & 42)Perceives Sound
• The arrival of action potentials in the primary auditory cortex
allows you to perceive sound.
• One aspect of sound that is perceived by this area is pitch (
frequency). The primary auditory cortex is mapped according to
pitch: Input about pitch from each portion of the basilar
membrane is conveyed to a different part of the primary
auditory cortex.
Auditory cortex
• High-frequency sounds activate one part of the cortex, low-
frequency sounds activate another part, and medium-frequency
sounds activate the region in between .
• Hence, different cortical neurons respond to different pitches.
Neurons in the primary auditory cortex also allow you to
perceive other aspects of sound such as loudness and duration
The Auditory Association Area (area 22)Allows You to Recognize
a Sound
• From the primary auditory cortex, auditory information is
conveyed to the auditory association area in the temporal lobe.
This area stores auditory memories and compares present and
past auditory experiences, allowing you to recognize a particular
sound as speech, music, or noise. If the sound is speech, input in
the auditory association is relayed to Wernicke’s area in the
adjacent part of the temporal lobe, which interprets the meaning of
words, translating them into thoughts
Deafness
• - It is either partial or complete loss of hearing.
• - It is manifested by decrease in the acuity of hearing or increase in
the threshold of hearing.

• There are 3 types of deafness:

I. Conductive deafness:
- It results from interference with the proper conduction & amplification of
sound waves through the external or middle ears.
Deafness
II. Nerve deafness:
 It results from interference with the response of the hair cells of
the organ of Corti or the transmission of impulses from the cochlea
to the auditory cortex.
III. Word deafness: (auditory aphasia)
 It results from lesion of area 22. =The auditory psychic (=
association or interpretative) area .
Thank you all