Download Lecture: Physiology of Hearing and Equilibrium

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. short wavelength/high frequency - high pitched tones
ii. long wavelength/low frequency - low pitched tones
iii. human frequency range - 20Hz - 20,000 Hz (2-3 Hz distinction)
d. amplitude - intensity of energy in a given waveof sound; signified by height of sine wave
i. loudness - subjective interpretation of the intensity of a sound
ii. decibel - logrithmic scale to measure the intensity of sound waves
Energy in the Sound Wave
Perceived Loundness
O dB
threshold for audibility
barely audible
10 dB
20 dB
30 dB
40 dB
10 X 0 dB
100 X 0 dB
1000 X 0 dB
10,000 X 0 dB
2 X 0 dB
4 X 0 dB
8 X 0 dB
16 X 0 dB
iii. human amplitude range - 0 dB - 120 dB (130 dB = pain level)
II. 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. vibration of oval window -> perilymph vibration
2. for 20 - 20,000 Hz only, vibration of vestibular membrane
3. vestibular membrane vibration -> endolymph vibration
4. endolymph vibration -> vibration of basilar membrane
5. 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)
B. 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. basilar m. vibration -> hair cell vibration
b. hair cell vibration -> opening/closing channels
c. depolarization/hyperpolar. -> cochlear nerve
d. cochlear nerve impulses -> to brain
IV. 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 soundfrom both ears come together
a. relative intensity - the amplitude of sound waves hitting the different ears
b. relative timing - the difference in timing in which a sound reaches both ears
VI. Typical Hearing Disorders
A. conduction deafness - disruption in sound vibrations to basilar membrane (ext & mid ear)
1. blocked auditory canal (wax, fluid)
2. perforated tympanic membrane (eardrum)
3. otitis media - middle ear infection/inflammation
4. otosclerosis - hardening of the earbone joints
B. sensorineural deafness - disruption anywhere in pathway from hair cells to the auditory cortex
1. loss of hair cells (explosion, chronic loud noise)
2. damage to vestibulocochlear nerve (VIII)
3. damage to nuclei/tracts to the cortex
C. tinnitus - chronic perception of clicking or ringing
1. sudden blow to the tympanic membrane
2. gradual deterioration of afferents in cochlear nerve
D. Menierre's Syndrome - effects both hearing and balance; results in tinnitus, vertigo, and
nausea and vomiting
1. may be too much endolymph beneath basilar membrane
2. symptoms can be treated somewhat with drugs
3. endolymph may be drained periodically
4. hearing loss is progressive
VII. 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
i. hair cells - like hair cells of basilar membrane, respond when bent
c. 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
interspersed
B. 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 otoltih 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-go-round"
5. vertigo - false geeling 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