The Structure and Function of the Auditory Nerve

Transcription

1 The Structure and Function of the Auditory Nerve Brad May Structure and Function of the Auditory and Vestibular Systems (BME ) September 21,

2 Objectives Anatomy Basic response patterns Frequency coding Loudness coding Spectral coding 2

3 The Organ of Corti Auditory nerve fibers link the Organ of Corti to the auditory brainstem. The cell bodies of the fibers are located in the spiral ganglion. 3

4 Type 1 and II Fibers The fibers are divided into two classes based on the anatomy of the projections. Type I fibers innervate inner hair cells and encode sound. 4

5 Type 1 and II Fibers (cat) Number of AN fibers: 50, % of AN fibers are Type I 45,000-47,500 Type I fibers 2,500-5,000 Type II fibers Number of Hair Cells: 12, outer hair cells 3000 inner hair cells 5 The majority of auditory nerve fibers are type I.

6 Type 1 and II Fibers Type I fibers innervate a single inner hair cells. Type II fibers innervate several outer hair cells. 6

7 Spontaneous Activity Type I fibers fire action potentials. Most fibers have spontaneous activity in the absence of sound stimulation. 7

8 Sound-Driven Activity 8 Activity is increased during sound presentations.

9 Frequency Tuning Curves Fibers show sound-driven activity over a narrow range of stimulus frequencies at low sound levels. 9

10 Frequency Tuning Curves 10 Different fibers respond best to different frequencies.

11 Frequency Tuning Curves The frequency tuning of the auditory nerve is determined by the mechanical tuning of the Organ of Corti. 11

12 Tonotopic Organization 12 Like the basilar membrane, neural tuning is tonotopically organized.

13 Best Frequency The best frequency of each fiber is determined by the location of its terminal projection in the Organ of Corti. 13

14 Peri-Stimulus Time Histograms Physiological characterizations of the auditory nerve are usually based on responses to best frequency tones. Temporal patterns are revealed by peri-stimulus time histograms. 14

15 Phase-Locking The action potentials of auditory nerve fibers synchronize to the phase of low-frequency tone. They show sustained responses to high-frequency tones. 15

16 Period Histogram The period histogram provides a better indication of the quality of phase locking. Each spike is plotted relative to its time in the stimulus period. This format is essentially a folding of the PSTH. 15B

17 Inner Hair Cell Potentials The upper limits are determined by the membrane capacitance of inner hair cell membranes. 16

18 Rate versus Time Coding The same neuron my represent low frequencies by the timing of action potentials and high frequencies by a rate-place code. 17

19 Rate-Level Functions The magnitude of rate responses also represents stimulus level. Rate-level functions show a dynamic range where changes in level are unambiguously encoded by changes in rate. 18

20 Best Frequency Thresholds The dynamic range of hearing is produced by the variations in threshold between spontaneous rate classifications. Low levels are encoded by fibers with high spontaneous rates. High levels are encoded by fibers with low spontaneous rates. 19

21 Stimulus Coding The remainder of the lecture describes the coding of complex sounds in the auditory nerve. 20

22 The Acoustic Basis of Sound Localization Sound source location is indicated by the binaural time and level differences and spectral cues introduced by the directional filtering properties of the outer ear. 21

23 Pinna Filtering Effects in the Horizontal Plane A movie steps through a series of HRTFs that were recording for fixed locations in the horizontal plane. 22

24 Pinna Filtering Effects in the Horizontal Plane These head-related transfer functions (HRTFs) show how the sound spectrum at the eardrum is modified by moving the sound in the horizontal plane. Note the clear head-shadowing effect at low frequencies. 23

25 Pinna Filtering Effects in the Vertical Plane A movie steps through a series of HRTFs that were recording for fixed locations in the vertical plane. 24

26 Pinna Filtering Effects in the Vertical Plane These head-related transfer functions (HRTFs) show how the sound spectrum at the eardrum is modified by moving the sound in the vertical plane. Note the lack of headshadowing effect at low frequencies and the directionally dependent spectral shape at high frequencies. The frequency location of the prominent mid-frequency notch changes systematically with elevation. 25

27 The Traveling Wave Low-frequency sounds produce maximum membrane displacements in the apical cochlea. High-frequency sounds produce peak displacements in the base. 26

28 Spectral Coding in the Auditory Nerve As a result of traveling wave effects, each auditory-nerve fiber responds to a selective frequency range of the HRTF spectrum. 27

29 Auditory Nerve Responses to HRTFs This movie describes the linear rate representation of spectral shape that is conveyed by a single sharply tuned auditory-nerve fiber. As the spectrum changes near the neuron s best frequency,peak energy is encoded by peak discharge rates. 28

30 Auditory Nerve Responses to HRTFs This movie describes the linear rate representation of spectral shape that is conveyed by a population of sharply tuned auditory-nerve fibers. Peak energy is the sound spectrum is associated with peak discharge rates. 29

31 Auditory Nerve Responses to HRTFs This movie describes the linear rate representation of spectral shape that is conveyed by a population of sharply tuned auditory-nerve fibers. Peak energy is the sound spectrum is associated with peak discharge rates. 30

32 Hearing Loss Most forms of sensorineural hearing loss involve damage to outer hair cells. Without the active mechanical influences of outer hair cells, neural responses of the auditory nerve become less sensitive and more broadly tuned. 31

33 Hearing Loss Hearing aid amplification can restore sensitivity but not tuning. Individuals hear sounds but cannot distinguish their meaning. 32

34 Suggested Reading JO Pickles. Introduction to the physiology of hearing. Academic Press, MA Ruggero. Physiology and coding in the auditory nerve. In: AN Popper and RR Faye (eds), The Mammalian Auditory Pathway: Neurophysiology. Springer-Verlag,