BIOLOGY 2C03 Lecture Notes - Middle Ear, Kanamycin A, Hair Cell

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Published on 28 Jan 2013
Outline of Lecture 57 (03-24 B; Young)
Auditory I – Peripheral Auditory System
I. Human perceive the frequency and loudness of sound
- The dynamic range of human perception is about 100-16,000 Hz and 0-120 dB
- The dynamic range of conversational speech is a small subset of total perception
- One way to measure a tuning curve is to measure the loudness needed to mask a probe of single
tone as a function of frequency; obviously, minimum loudness is needed when mask frequency is
same as probe
II. Speech consists of a mixture of tones of different frequencies
- Different vowel sounds are the result of the shape of the vocal tract and how it modifies the tone
produced by the vocal folds
- The information needed to recognize vowel sounds is encoded as peaks of energy at certain
frequencies; these peaks are called formants
- The precise value of formants can vary from person to person (e.g. due to size of vocal tract), but
are still recognized because vowel sounds cluster in formant-space
III. Functional anatomy of the ear (this is a review, and is omitted here)
IV. Basilar membrane (BM) is responsible for tuning: separation of sound into its component frequencies
- Sound results in a traveling waveform in the BM, and the position of maximum displacement
depends on frequency (high freq near stapes, low freq near apex)
- Since auditory nerve fibers are distributed down length of cochlea, each ANF has a different best
frequency, giving rise to a tonotopic representation of sound
- Outer hair cells are responsible for sound sensitivity: they amplify BM movement in response to
soft sounds and partially damp BM movement in response to loud sounds; loss of OHCs (e.g. due
to kanamycin) results in loss of sensitivity and broadened tuning
V. Auditory nerve fiber is phase-locked to stimulus waveform, a consequence of hair cell cilia movement
VI. On the whole, complex auditory stimuli are represented tonotopically (e.g. in low threshold ANFs)
VII. Hearing impairment: can’t hear it and it sounds fuzzy
- Three basic classes of hearing loss
1) conductive: sound transmission through external and/or middle ear is lost
2) sensorineural: hair cells damaged, most common form of hearing impairment
3) retrocochlear: lesion in brain that affects hearing
- Characteristics of damage to…
- inner hair cell: loss of sensitivity
- outer hair cell: loss of sensitivity and broadening of tuning curve
- Three typical consequences of hearing impairment
- Loss of sensitivity: can’t hear it
- Broadened tuning curve: sounds fuzzy, can’t understand speech clearly
- Loudness recruitment: shortened dynamic range of loudness, still capped at 120 dB; perceived
loudness grows more rapidly with respect to actual dB
Summary of major ideas
- See extensive objectives listed on p. 1
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