Download Auditory Physiology - Dr. Costanzo

Auditory Physiology
Richard M. Costanzo, Ph.D.
OBJECTIVES
After studying the material of this lecture, the student should be able to:
1. Describe the morphology and function of the following structures:
a. outer ear
b. middle ear
c. inner ear
2. Define the frequency range over which human auditory threshold is at its lowest level.
3. Explain what is meant by the term acoustical impedance matching. Give two examples of
how this is accomplished in the human auditory system.
4. Discuss the physical characteristics of the basilar membrane and how they function to
encode the frequency components of sound.
5. Discuss what is meant by the tonotopic organization of the auditory system.
6. Describe two clinical tests used to evaluate auditory function.
I.
PROPERTIES OF SOUND
A.
Sound Waves
Sound waves are produced by an oscillating body or mass. Oscillations result in
compression of air molecules (increased pressure) followed by rarefaction
(decrease in pressure). These fluctuations are propagated over long distances and
are referred to as sound waves.
Figure 1: Periodic Pressure Fluctuations of a Sound Wave
Two parameters of a sound wave are:
1. Sound pressure (amplitude of periodic fluctuations)
2. Pitch (frequency of the pressure fluctuations)
Sound waves of a single frequency are called tones.
B.
Decibels
The intensity of sound is expressed as a relative measure called the decibel.
X decibel = 20 log P/Po
P is the sound pressure being measured and Po is the sound pressure it is being
referred to (i.e., the initial pressure or the reference). If a sound pressure P is 10
times that of its initial value Po, it is said to have increased 20 decibels.
In audition the reference pressure Po most often used is .0002 dynes/cm2 ("tripleO-two") --this value is equivalent to the average human detection for a 1000 Hz
tone. When this reference is used to calculate sound intensities, the resulting value
is referred to as a sound pressure level (SPL) and has units of decibels (dB SPL).
C.
Audibility Curve
The minimum sound pressure amplitude that a subject can perceive is called the
auditory threshold. A graph of auditory threshold over a range of frequencies or
tones is called the audibility curve.
Figure 2: Audibility Curve and Equal Loudness Contours
(From Schmidt, Fundamentals of Sensory Physiology, 1986)
The normal audibility curve shows that threshold for a 1000 Hz tone is about 0
decibels Sound Pressure Level (0dB SPL). Human auditory threshold is at its
lowest level however, between 2,000 and 4,000 Hz.
The normal range for hearing is 0 to 120 dB SPL. Above 120 dB sounds become
unpleasant and even painful. This range of sound pressure levels represents a
million-fold change in sound pressure (106 or 6 log units). From an engineering
point of view the auditory system responds over a very large dynamic range of
stimulus intensities.
II.
THE OUTER AND MIDDLE EAR
Figure 3: Diagram of the Ear, (from Costanzo, 2006, Fig. 3-19)
A. Outer Ear
The outer ear collects airborne auditory vibrations and directs them to the
tympanic membrane. It consists of the pinna, a structure that helps direct or
funnel sound waves and the external auditory meatus (or auditory canal). The
external auditory meatus has a resonant frequency of about 3,000 Hz and in part
explains why human thresholds are very low at this frequency. This structure also
acts as a buffer to changes in the external environment such as temperature,
humidity, etc.
B. Middle Ear
The middle ear functions to match the acoustical impedance of airborne sound
vibrations being transferred to the fluid-filled chambers of the inner ear. The
middle ear consists of a small air-filled chamber connected to the pharynx by a
narrow tube called the eustachian tube.
Figure 4: The Middle Ear (From Ganong, 19952)
1. Eustachian tube
This narrow passageway to the pharynx serves to equalize pressure between
the middle ear and the outer ear. Sudden changes in external pressure (such as
climbing to high altitudes) cause the eardrum to stretch, altering its response
characteristics and in some cases causing discomfort or pain. By swallowing,
however, the eustachian tube reflexively opens and permits air to equilibrate,
thereby eliminating pressure differences across the tympanic membrane.
2. Malleus, incus, and stapes
Within the middle ear there are 3 bones or ossicles. They are the malleus
(hammer), incus (anvil) and stapes (stirrup). These bones transmit airborne
sound vibrations from the tympanic membrane to the oval window.
3. Impedance matching
Opposite the oval window is the fluid-filled auditory organ, the cochlea. The
middle ear must transmit airborne sound vibrations to the fluid environment of
the cochlea. Energy losses due to reflection are not insignificant and can be
as high as 99% in some cases. The middle ear compensates for some of these
losses in two ways:
Table 1
i.
TYMPANIC MEMBRANE
OVAL WINDOW
DIFFERENCE
AREA
55 mm2
3.2 mm2
(17 times)
FORCE
1 unit
1.3 units
(1.3 times)
DISPLACEMENT
1 unit
0.75 units
(0.75 times)
Concentration of force on a smaller area
The sound pressure impinging on the tympanic membrane is distributed
over approximately 55 mm2. These forces are transferred by the bones of
the middle ear to the oval window (3.2 mm2); the pressure is effectively
increased some 17 times.
Pressure = Force/Area
ii.
Increased force due to lever action of ossicles
The lever action of the ossicles provides additional amplification of force.
This is not, however, due to an increase in displacement at the oval
window (the oval window is displaced only 3/4 that of the tympanic
membrane) but rather by an increase in the force at the oval window. The
total force at the oval window is 1.3 times that at the tympanic membrane.
The overall result is that the sound pressure at the tympanic membrane is
amplified approximately 22 times (17 x 1.3). This extra force is used to
overcome the inertia of the fluid-filled cochlea and permits vibrations to
be transmitted to the basilar membrane. Matching of the acoustical
impedance of the air-fluid interface is an important function of the middle
ear.
III.
INNER EAR The inner ear contains the sensory cells of the auditory organ (the cochlea)
which transforms vibrations into neural impulses.
A. The Cochlea
Figure 5: Structure of the Cochlea and the Organ of Corti (From Costanzo, , 2006, Fig. 3-20)
Consists of three tubular canals: (1) the scala vestibuli, (2) the scala media (cochlear
duct) and (3) the scala tympani. The human cochlea is a helical structure (approx. 21/2 turns) embedded in the temporal bone.
1. Scala vestibuli - upper canal- contains perilymph and at the basal end of the
cochlea is terminated at the oval window.
2. Scala tympani - lower canal- contains perilymph and is connected to the scala
vestibuli by a small opening at the top (apex) of the cochlea called the
helicotrema. The basal end is terminated by the round window.
3. Scala media - located between the scala vestibuli and the scala tympani and is
sometimes referred to as the cochlear partition. It is bordered by the vestibular
(Reissner's) membrane above, the stria vascularis laterally, and the basilar
membrane below. The scala media contains endolymph which is rich in
potassium.
B. Organ of Corti
Located on the basilar membrane is the Organ of Corti which contains the receptor
cells for auditory stimuli.
1. Receptor cells - do not have axons and therefore, are referred to as secondary
sensory cells. There are two types: inner hair cells and outer hair cells. Inner
hair cells form a single row and are fewer in number than outer hair cells, which
are arranged in three parallel rows.
2. Auditory (8th) nerve - Cell bodies of the 8th nerve are located in the spiral
ganglion. Individual nerve fibers connect to only a single inner hair cell even
though a single inner hair cell may have several different fibers connected to it.
Nerve fibers innervating outer hair cells are branched and connected to several
different cells.
C. Transduction Process (shearing forces)
The stereocilia which protrude from the hair cells are embedded in the tectorial
membrane which is anchored to the medial wall of the scala media. The basilar
membrane is more elastic than the tectorial membrane so that deflections in the
cochlear partition produce a greater displacement of the basilar membrane than the
tectorial membrane. This results in a medially directed shearing force on the
stereocilia which in turn causes conductance changes in the receptor cell membrane.
These conductance changes lead to the release of transmitter at the basal end of the
cells which stimulate the terminal ends of the auditory nerve fibers.
D. Cochlear Dynamics
The physical characteristics of the basilar membrane make it an excellent frequency
analyzer. Sound is encoded by the relative amplitude of vibration of different
regions along the basilar membrane.
Figure 6: Diagram of the Basilar Membrane
(From Costanzo, 2006, Fig. 3-21)
1. Base - is narrow (.1 mm) and very stiff. Responds best to high frequencies
2. Apex (tip) - is widest part of basilar membrane and very compliant. Responds
best to low frequencies.
3. Traveling waves - Vibrations are introduced to the cochlea at the oval window
and are transmitted down the cochlea from the base to the tip. Due to differences
in the elastic properties along the basilar membrane, vibrations differ in amplitude
and phase. This results in a complex set of "traveling waves" and even though
each point along the basilar membrane may be vibrating at the same frequency,
these amplitude and phase differences produce an apparent movement of waves
along the surface.
E. Place Theory
The envelope of maximal amplitudes of oscillation for a set of traveling waves
changes for tones of different frequencies.
Figure 7: Envelope of Maximal Amplitude Oscillations along the Basilar
Membrane (From Schmidt, Fundamentals of Sensory Physiology, 1986)
High frequencies produce maximal oscillations near the stapes (basal end) and with
decreasing frequency there is a shift in maxima towards the apex (tip) of the cochlea.
For each frequency component there is a point of maximal displacement along the
cochlea which excites receptor cells in that region and thereby encodes that particular
frequency. Cells in different regions of the basilar membrane encoding different
frequencies. This is known as the Place Theory of Hearing.
Evidence that the cochlea acts as a frequency analyzer comes from microelectrode
recordings from the cochlea. Since it is extremely difficult to stay in cells while they
are vibrating, electrodes have been placed in the perilymph of the scala tympani to
monitor potential changes. These potential changes fluctuate and are identical to the
stimulus waveform. Since this is similar to what happens in a microphone, the
potentials are called cochlear microphonics. Measurements of cochlear
microphonics at different points along the basilar membrane give the same maximal
excitation as predicted by the Place Theory.
IV.
TUNING CURVES
Although a given nerve fiber innervates a particular region of the cochlea and is
maximally excited by the characteristic frequency of that region, it can also be excited
by other frequencies, although to a lesser extent.
A.
The frequency requiring the least intensity to excite a fiber is called its characteristic
or best frequency.
B. Frequencies other than the best frequency require more intense stimuli to excite the
fiber. A larger range of frequencies below the best frequency than above are capable
of exciting the unit.
Figure 8: (From Schmidt, Fundamentals of Sensory Physiology, 1986)
V.
AUDITORY PATHWAYS
A. Anatomical Structures
1.
2.
3.
4.
5.
6.
7.
8th nerve
cochlear nucleus (dorsal and ventral)
superior olive, accessory nucleus
lateral lemniscus
inferior colliculus
medial geniculate body
auditory cortex
B. Bilateral Representation - occurs at most levels of the auditory system. As a
consequence many ipsilateral lesions do not produce a significant loss of hearing.
C. Tonotopic Organization - There is a spatial mapping of frequencies at all levels of
the auditory system.
Figure 10. Netter Presenter Image III.15
D. Complex Pattern Recognition - The higher in the auditory system you go, the more
complex the stimulus required to excite a cell.
VI.
CLINICAL DISORDERS
A.
Conduction and Nerve Deafness
Auditory disorders are usually due to problems in sound transmission (conduction
deafness) or damage of auditory pathways (nerve deafness). Conduction deafness
can occur due to blockage of the external auditory canal with wax, inflammatory
processes such as acute or chronic otitis media, thickening of the tympanic
membrane, otosclerosis and stiffening of the attachment of the stapes to the oval
window. Nerve deafness can occur as a result of trauma, tumors of the acoustic
nerve, spread of infections originating in the middle ear, lesions of hair cells and
degeneration of the acoustic nerve by toxic drugs. Streptomycin causes deafness
by damaging the cochlear organ.
B.
Clinical Tests
Tests are available to differentiate between lesions of the conducting system in the
middle ear (conductive deafness) and sensorineural lesions (nerve deafness).
In the Weber Test a tuning fork is applied to the forehead and the patient is asked
whether the sound is heard in the midline or localized to one ear. In conductive
deafness the tone sounds louder in the affected ear (due to absence of background
environmental noise). In nerve deafness it is louder in the normal ear
(transmission from the affected ear is impaired). In conductive deafness normal
air conduction though the external and middle ear is reduced, but bone conduction
(direct stimulation of cochlea) is relatively enhanced.
In the Rinne test a tuning fork is applied to the patient's mastoid process. When
bone conduction is no longer audible the tuning fork is placed near the external
meatus to determine if sound can be detected via air conduction. In conductive
deafness sound cannot be heard by air conduction following bone conduction. In
normal individuals and in nerve deafness the reverse is true.
Audiometry uses a range of pure tones to determine threshold values for each ear
using both bone and air conduction. Characteristic patterns in the audibility
threshold curve can be used to determine nerve deafness (for high and low
frequencies), conduction deafness and mixed types of auditory dysfunctions.
Table 2: Clinical Tests for "Conductive" and "Nerve" Deafness
Procedure
Normal
Hears the same in
both ears
Conductive Deafness
Weber Test
Vibrating tuning fork
placed on vertex of
skull
Rinne Test
Vibrating tuning fork Hears vibration in air Vibration not heard
placed on mastoid
after bone
in air after bone
process until subject
conduction is over.
conduction
no longer hears it, then
held in air next to ear.
Nerve Deafness
Louder in diseased
Louder in normal ear
ear (no masking
environmental noise)
May be detected in
air after bone
conduction
ADDITIONAL REFERENCES
Costanzo, L.S. Physiology , 3rd Edition, Saunders Elsevier, 2006, pp. 86-90.
Klinke, R. Physiology of Hearing. In Fundamentals of Sensory Physiology, 3rd ed., R.F.
Schmidt Ed. Springer-Verlag 1986, p. 199-223.
* Netter Presenter Image Copyright 2004 Icon Learning Systems. All rights reserved.