Lecture: Physiology of Hearing and Equilibrium
I.
Physical Characteristics of Sound
A.
Sound as Vibration of Air Molecules Traveling in Waves
1. vibration of medium - sound travels in compression waves through a particular
medium
a.
solid-------------> liquid ----------------> gas
fastest
slowest
2. sound as a wave - the series of high pressure and low pressure areas are called
“compressions” and “rarefactions”, respectively
a.
sine wave - graphic representation of areas of compression and
rarefaction of a sound wave
b.
wavelength - the distance between 2 areas of compression for a
given sound wave
c.
frequency - the number of waves that pass a given point in one
second (1/s = 1 Hertz)
i.
ii.
iii.
d.
short wavelength/high frequency - high pitched tones
long wavelength/low frequency - low pitched tones
human frequency range - 20Hz - 20,000 Hz (2-3 Hz
distinction)
amplitude - intensity of energy in a given wave of sound; signified by
height of sine wave
i.
ii.
loudness - subjective interpretation of the intensity of a sound
decibel - logarithmic scale to measure the intensity of sound waves
Energy in the Sound Wave
0 dB
threshold for audibility
10 dB
l0
X
0 dB
20 dB
100
X
0 dB
30 dB
1000 X
0 dB
40 dB
10,000 X
0 dB
iii.
II.
Perceived Loudness
barely audible
2
X
0 dB
4
X
0 dB
8
X
0 dB
16
X
0 dB
human amplitude range - 0 dB - 120 dB (130 dB = pain level)
Transmission of Sound to the Inner Ear
air -->
external auditory canal -->
tympanic membrane (ear drum) -->
ossicles (malleus, incus, stapes.) -->
oval window of cochlea -->
vibration of cochlear fluid -->
basilar membrane of cochlea
III.
Resonance of Basilar Membrane & Excitation of Hair Cells
A.
Resonance of Basilar Membrane
1.
2.
3.
4.
5.
B.
vibration of oval window -> perilymph vibration
for 20 - 20,000 Hz only, vibration of vestibular membrane
vestibular membrane vibration -> endolymph vibration
endolymph vibration -> vibration of basilar membrane
basilar membrane “fibers” of different length, thickness, and tension like strings of a
piano
a. resonance - different fibers of basilar membrane have different “natural
frequencies”
b. SPECIFIC parts of basilar membrane vibrate only at SPECIFIC frequency (pitch)
Excitation of Hairs Cells of Organ of Corti
1.
cochlear hair cells - rest on the basilar membrane, contain "stereocilia" which project into
the "tectorial membrane" just above
a.
b.
c.
d.
IV.
basilar m. vibration -> hair cell vibration
hair cell vibration -> opening/closing channels
depolarization/hyperpolar -> cochlear nerve
cochlear nerve impulses -> to brain
Anatomical Pathway to the Brain
cochlear nerve (vestibulocochlear VIII)->
spiral ganglion -->
cochlear nuclei (medulla) -->
superior olivary nucleus -->
lateral lemniscal tract -->
inferior colliculus -->
medial geniculate body of thalamus -->
auditory cortex (superior temporal lobe)
V.
Processing of Auditory Information
A.
Perceiving Pitch (Frequency) - location of vibration on the basilar membrane
B.
Perceiving Differences in Loudness (Intensity) - amplitude increases, more hair cells of the
basilar membrane (with same pitch) are activated
C.
localizing Source of Sound
1.
superior olivary nucleus - first point where sound from both ears come together
a.
b.
VI.
Typical Hearing Disorders
A.
conduction deafness - disruption in sound vibrations to basilar membrane (ext & mid ear)
1.
2.
3.
4.
B.
C.
loss of hair cells (explosion, chronic loud noise)
damage to vestibulocochlear nerve (VIII)
damage to nuclei/tracts to the cortex
tinnitus - chronic perception of clicking or ringing
1.
2.
D.
blocked auditory canal (wax, fluid)
perforated tympanic membrane (eardrum)
otitis media - middle ear infection/inflammation
otosclerosis - hardening of the earbone joints
sensorineural deafness - disruption anywhere in pathway from hair cells to the auditory cortex
1.
2.
3.
sudden blow to the tympanic membrane
gradual deterioration of afferents in cochlear nerve
Menierre's Syndrome - effects both hearing and balance; results in tinnitus, vertigo, and
interspersed nausea and vomiting
1.
2.
3.
4.
VII.
relative intensity - the amplitude of sound waves hitting the different ears
relative timing - the difference in timing in which a sound reaches both ears
may be too much endolymph beneath basilar membrane
symptoms can be treated somewhat with drugs
endolymph may be drained periodically
hearing loss is progressive
Equilibrium and Balance: The Vestibular Apparatus
A.
Linear Movement: The Maculae of the Vestibule
1.
vestibule - bony cavity of the inner ear between the cochlea and the semicircular canals
a.
saccule and utricle - smaller sacs housed within the vestibule
b.
maculae - patch of "supporting cells" and "hair cells" along the utricles and
saccules
c.
B.
i. hair cells - like hair cells of basilar membrane, respond when bent
otolithic membrane - jelly-like sheet that abuts the "stereocilia" of the hair cells
i. otoliths - "ear stones" that rest on top of the otolithic membrane
2.
horizontal acceleration - maculae of UTRICLE is in the horizontal plane; hairs bend
when motion is FORWARD/BACKWARD
3.
vertical acceleration - maculae of SACCULE is in the vertical plane; hairs bend when
motion is UP/DOWN
Angular Movement: The Crista of Semicircular Canals
1.
semicircular canals - three bony "hula-hoop" extensions of vestibule in three different
planes
2.
crista ampullaris - like maculae, contain hair cells that respond to flow of endolymph in
canals
a. cupula - like otolith membrane, gelatinous "cap" into which hair cells project
3.
change in angular (rotational) acceleration - movement of the head in non-linear (circular
or angular) direction is monitored by three canals
4.
vestibular nystagmus - movement of eyes to remain fixed on object when on "merry-goround"
5.
vertigo - false feeling of gravity or motion
C. Equilibrium Pathway: Coordinating Inputs in Brain
activated hair cells of crista ampularis ->
afferent axon fibers (vestibulocochlear nerve) -> vestibular nuclear complex OR cerebellum
1. vestibular nuclei - also receive input from eyes and somatic proprioceptors; coordinates
information to help control motion of eyes, neck, limbs
2. cerebellum - also receives input from eyes and somatic proprioceptors; coordinates
information to help regulate head position, posture, and balance
D. Problems with Equilibrium
1.
dizziness, nausea, imbalance, vomiting
2.
motion sickness - conflict between visual/somatic inputs and action of the vestibular
apparatus
a. Bonine, Dramamine, Scopolamine - block inputs from vestibular apparatus to the brain