1. The Neural Response and the Auditory Code
CHAPTER 9
The Neural Response
and the Auditory Code

2. The Neuron Dendrites receive synaptic stimulation (neurotrans.)
Action Potential generated in soma near axon
AP conducted along axon from Node to Node (saltatory conduction)
AP produces release of neurotransmitter at terminal boutons

3. An Action Potential (or Spike)

4. Two Descriptors for Neurons
Afferent (sensory)-- carrying signals toward the brain
Efferent (motor) -- carrying signals from brain to periphery

5. Afferent & Efferent Neurons

6. 4 Types of Cochlear Neurons
INNER HAIR CELLS
Multiple (10 to 20) Afferent synapses
(Efferents synapse on afferent dendrites)
OUTER HAIR CELLS:
Large Efferent synapses engulf base of cell
Small (& not very active) Afferent synapses

7. IHC Innervation Pattern

8. OHC Innervation Pattern

9. Inner hair cells Synapse at the base with up to 20 afferent neurons
“Divergence”
Efferents synapse on afferent dendrites under IHCs

10. IHC activation alters firing rate

11. Afferent neurons have their cell bodies in the Spiral Ganglion (4)

12. Tuning Curves
Iso-Rate Function -- Shape similar to what we’ve already described (Fig 6.12 b)
Iso-level Function -- Shows spike rate as a function of frequency-- peak at a single frequency (Fig 6.12a)

13. Bekesy’s Theory describes Passive Mechanics
Based on work in “dead” cochleae
Highly damped -- not sharply tuned
Active Undamping occurs in live and healthy cochleae
Like pumping on a swing--adds amplitude

14. The Active Component Adds to Bekesy’s Traveling Wave

15. The Active Component Improves Sensitivity for soft sounds
Improves frequency resolution

16. Frequency Tuning Curves Show these Effects
= plots of response threshold as a function of frequency
They have a characteristic shape
sharp tip (shows best sensitivity at one freq)
steep high frequency tail
shallow low frequency tail

17. Tuning Curves
Passive Only
Active + Passive

18. More on Tuning & Tuning Curves:
Frequency of “tip” is called the CHARACTERISTIC FREQUENCY
Can be seen for:
basilar membrane,
hair cells,
nerve cells

19. Single-Unit Measures
Post-Stimulus Time Histogram-- Shows firing rate changes over time
Period or Interval Histograms-- Show phase-locking of neural firing

20. Tuning Curves
Iso-Rate Function -- Shape similar to what we’ve already described (Fig 6.12 b)
Iso-level Function -- Shows spike rate as a function of frequency-- peak at a single frequency (Fig 6.12a)

21. Two-Tone Suppression
The response to one tone can be reduced or eliminated by introducing a second tone near the neuron’s CF.
(Fig 6.16)
Second tone can be either one which normally would excite the neuron or not

22. Two-tone Suppression
Regular Tuning Curve

23. Frequency Coding
The Place Code-- each neuron has a characteristic frequency
Periodicity Pitch-- neurons phase-lock to stimuli

24. Intensity Coding Firing rate increases in single neurons
Spread of activation to a wider range of neurons-- “Density of Discharges”
Latency of Firing (shorter delay at higher levels)

25. Efferent (Descending) Control
Cochlear Efferents come from Superior Olivary Complex --The Olivo-Cochlear Bundle (OCB)
Uncrossed OCB-- synapses on dendrites under inner hair cells
Crossed OCB-- synapses on outer hair cells
Both use inhibitory neurotransmitters

26. Uncrossed OCB-- synapses on dendrites under inner hair cells

27. Crossed OCB-- synapses on outer hair cells

28. Efferent Control (cont’d)
The Acoustic Reflex
Auditory Cortex and Thalamus also send descending fibers to auditory brainstem locations

29. The Acoustic Reflex Afferent: Efferent: VIIIth nerve Cochlear Nucleus
Superior Olivary Complex
Efferent:
VIIth nerve nucleus
VIIth nerve
Stapedius muscle