12. Auditory System: Structure and Function
Part 2 of 2
Lincoln Gray, Ph.D.
.
Physical characteristics of the basilar membrane cause different frequencies to reach maximum amplitudes at different positions. Much as on a piano, high frequencies are at one end and low frequencies at the other. High frequencies are transduced at the base of the cochlea whereas low frequencies are transduced at the apex. Figure 12.7 illustrates the way in which the cochlea acts as a frequency analyzer. The cochlea codes the pitch of a sound by the place of maximal vibration. Note the position of the traveling wave at different frequencies. (Beware! It may initially seem backwards that low frequencies are not associated with the base.) Select different frequencies by turning the dial. If audio on your computer is enabled, you will hear the sound you selected. Hearing loss at high frequencies is common. The average loss of hearing in American males is about a cycle per second per day (starting at about age 20, so a 50-year old would likely have difficulty hearing over 10 kHz). If you can't hear the high frequencies, it may be due to the speakers on your computer, but it is always worth thinking about hearing preservation.
Figure 12.7 |
Tonotopic organization of the mature human Cochlea. |
As you listen to these sounds, note that the high frequencies seem strangely similar. Think about cochlear-implant patients. These patients have lost hair-cell function. Their auditory nerve is stimulated by a series of implanted electrodes. The implant can only be placed in the base of the cochlea, because it is surgically impossible to thread the fine wires more than about 2/3 of a turn. Thus, cochlear implant patients probably experience something like high frequency sounds.
The Range of Sounds to Which We Respond; Neural Tuning CurvesFigure 12.8 shows the range of frequencies and intensities of sound to which the human auditory system responds. Our absolute threshold, the minimum level of sound that we can detect, is strongly dependent on frequency. At the level of pain, sound levels are about six orders of magnitude above the minimal audible threshold. Sound pressure level (SPL) is measured in decibels (dB). Decibels are a logarithmic scale, with each 6 dB increase indicating a doubling of intensity. The perceived loudness of a sound is related to its intensity. Sound frequencies are measured in Hertz (Hz), or cycles per second. Normally, we hear sounds as low as 20 Hz and as high as 20,000 Hz. The frequency of a sound is associated with its pitch. Hearing is best at about 3-4 kHz. Hearing sensitivity decreases at higher and lower frequencies, but more so at higher than lower frequencies. High-frequency hearing is typically lost as we age.
Audiometric curve for a normal hearing subject and some neural tuning curves. |
The neural code in the central auditory system is complex. Tonotopic organization is maintained throughout the auditory system. Tonotopic organization means that cells responsive to different frequencies are found in different places at each level of the central auditory system, and that there is a standard (logarithmic) relationship between this position and frequency. Each cell has a characteristic frequency (CF). The CF is the frequency to which the cell is maximally responsive. A cell will usually respond to other frequencies, but only at greater intensities. The neural tuning curve is a plot of the amplitude of sounds at various frequencies necessary to elicit a response from a central auditory neuron. The tuning curves for several different neurons are superimposed on the audibility curves in Figure 12.8. The depicted neurons have CFs that vary from low to high frequencies (and are shown with red to blue colors, respectively). If we recorded from all auditory neurons, we would basically fill the area within the audibility curves. When sounds are soft they will stimulate only those few neurons with that CF, and thus neural activity will be confined to one set of fibers or cells at one particular place. As sounds get louder they stimulate other neurons, and the area of activity will increase.
- Question 1
- A
- B
- C
- D
- E
High frequencies are transduced
A. at the apex of the cochlea
B. at the base of the cochlea
C. throughout the cochlea
D. by vibrations of the stapes
E. at the superior temporal gyrus
High frequencies are transduced
A. at the apex of the cochlea This answer is INCORRECT.
It may seem "backwards" but although the Cochlear duct seems to get smaller toward the apex, the basilar membrane actually gets wider.B. at the base of the cochlea
C. throughout the cochlea
D. by vibrations of the stapes
E. at the superior temporal gyrus
High frequencies are transduced
A. at the apex of the cochlea
B. at the base of the cochlea This answer is CORRECT!
C. throughout the cochlea
D. by vibrations of the stapes
E. at the superior temporal gyrus
High frequencies are transduced
A. at the apex of the cochlea
B. at the base of the cochlea
C. throughout the cochlea This answer is INCORRECT.
High frequencies do not travel far along the basilar membrane. (As an aside, low frequencies traverse the length of the Cochlea, and hence cause the most damage if they are sufficiently loud.)D. by vibrations of the stapes
E. at the superior temporal gyrus
High frequencies are transduced
A. at the apex of the cochlea
B. at the base of the cochlea
C. throughout the cochlea
D. by vibrations of the stapes This answer is INCORRECT.
Sound is transmitted to the fluid of the inner ear through vibrations of the tympanic membrane, malleus, incus and stapes. Transduction, the change from mechanical energy to neural impulses, takes place in the hair cells, specifically through potassium channels at the tips of the stereocilia.E. at the superior temporal gyrus
High frequencies are transduced
A. at the apex of the cochlea
B. at the base of the cochlea
C. throughout the cochlea
D. by vibrations of the stapes
E. at the superior temporal gyrus This answer is INCORRECT.
Auditory afferents eventually reach the primary auditory cortex in Heschel's gyrus within insular cortex, and this area is tonotopically organized. Stimulation of this area leads to conscious awareness of the sound, but the transduction from mechanical vibrations to neural activity occurs in the inner ear.
- Question 2
- A
- B
- C
- D
- E
Transduction of mechanical to neural signals occurs
A. at the base of the outer hair cells
B. at K+ channels in stereocilia
C. between the oval and round windows
D. in the scala vestibuli
E. in the scala tympani
Transduction of mechanical to neural signals occurs
A. at the base of the outer hair cells This answer is INCORRECT.
Transduction occurs in both outer and inner hair cells. Most auditory afferents synapse on inner hair cells.B. at K+ channels in stereocilia
C. between the oval and round windows
D. in the scala vestibuli
E. in the scala tympani
Transduction of mechanical to neural signals occurs
A. at the base of the outer hair cells
B. at K+ channels in stereocilia This answer is CORRECT!
Movement of the cilia opens potassium channels. The influx of potassium causes a subsequent influx of calcium and a receptor potential that can cause an action potential in the afferent dendrites.C. between the oval and round windows
D. in the scala vestibuli
E. in the scala tympani
Transduction of mechanical to neural signals occurs
A. at the base of the outer hair cells
B. at K+ channels in stereocilia
C. between the oval and round windows This answer is INCORRECT.
A pressure difference between the oval window (scala vestibuli) and the round window (scala tympani) is important for generating the traveling wave along the basilar membrane, but at this stage of auditory processing the signal is still mechanical.D. in the scala vestibuli
E. in the scala tympani
Transduction of mechanical to neural signals occurs
A. at the base of the outer hair cells
B. at K+ channels in stereocilia
C. between the oval and round windows
D. in the scala vestibuli This answer is INCORRECT.
A pressure difference between the oval window (scala vestibuli) and the round window (scala tympani) is important for generating the traveling wave along the basilar membrane, but at this stage of auditory processing the signal is still mechanical.E. in the scala tympani
Transduction of mechanical to neural signals occurs
A. at the base of the outer hair cells
B. at K+ channels in stereocilia
C. between the oval and round windows
D. in the scala vestibuli
E. in the scala tympani This answer is INCORRECT.
A pressure difference between the oval window (scala vestibuli) and the round window (scala tympani) is important for generating the traveling wave along the basilar membrane, but at this stage of auditory processing the signal is still mechanical.
- Question 3
- A
- B
- C
- D
- E
Primary auditory cortex is located in
A. parietal lobe
B. lateral surface of occipital lobe
C. superior temporal gyrus
D. parahippocampal gyrus
E. middle frontal gyrus
Primary auditory cortex is located in
A. parietal lobe This answer is INCORRECT.
The parietal lobe is not part of the primary auditory cortex. Primary auditory cortex is in the superior back of the superior temporal gyrus; the transverse temporal gyri of Heschl.B. lateral surface of occipital lobe
C. superior temporal gyrus
D. parahippocampal gyrus
E. middle frontal gyrus
Primary auditory cortex is located in
A. parietal lobe
B. lateral surface of occipital lobe This answer is INCORRECT.
The lateral surface of the occipital lobe is not part of primary auditory cortex. Primary auditory cortex is in the superior back of the superior temporal gyrus; the transverse temporal gyri of Heschl.C. superior temporal gyrus
D. parahippocampal gyrus
E. middle frontal gyrus
Primary auditory cortex is located in
A. parietal lobe
B. lateral surface of occipital lobe
C. superior temporal gyrus This answer is CORRECT!
D. parahippocampal gyrus
E. middle frontal gyrus
Primary auditory cortex is located in
A. parietal lobe
B. lateral surface of occipital lobe
C. superior temporal gyrus
D. parahippocampal gyrus This answer is INCORRECT.
The parahippocampal gyrus is not part of the primary auditory cortex. Primary auditory cortex is in the superior back of the superior temporal gyrus; the transverse temporal gyri of Heschl.E. middle frontal gyrus
Primary auditory cortex is located in
A. parietal lobe
B. lateral surface of occipital lobe
C. superior temporal gyrus
D. parahippocampal gyrus
E. middle frontal gyrus This answer is INCORRECT.
The middle frontal gyrus is not part of the primary auditory cortx. Primary auditory cortex is in the superior back of the superior temporal gyrus; the transverse temporal gyri of Heschl.
- Question 4
- A
- B
- C
- D
- E
Which of the following participate in audition?
A. trigeminal nerve
B. lateral lemniscus
C. medial lemniscus
D. pontine nuclei
E. oculomotor nerve
Which of the following participate in audition?
A. trigeminal nerve This answer is INCORRECT.
Nerve V is the general somatic sensory nerve for the head.B. lateral lemniscus
C. medial lemniscus
D. pontine nuclei
E. oculomotor nerve
Which of the following participate in audition?
A. trigeminal nerve
B. lateral lemniscus This answer is CORRECT!
C. medial lemniscus
D. pontine nuclei
E. oculomotor nerve
Which of the following participate in audition?
A. trigeminal nerve
B. lateral lemniscus
C. medial lemniscus This answer is INCORRECT.
The dorsal column-medial lemniscus system is associated with the somatosensory system.D. pontine nuclei
E. oculomotor nerve
Which of the following participate in audition?
A. trigeminal nerve
B. lateral lemniscus
C. medial lemniscus
D. pontine nuclei This answer is INCORRECT.
The pontine nuclei have axons that project to the cerebellum.E. oculomotor nerve
Which of the following participate in audition?
A. trigeminal nerve
B. lateral lemniscus
C. medial lemniscus
D. pontine nuclei
E. oculomotor nerve This answer is INCORRECT.
Motor fibers in III innervate eye muscles.
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