1. Tuning in the basilar membrane
Vibration of basilar membrane can be described by a ‘tuning curve’
Amplitude required for the basilar membrane to vibrate at a certain constant displacement, as a function of the frequency of the input sound 1 1

2. Neural connections in the cochlea
Afferent and efferent fibers of the VIIIth cranial nerve (auditory nerve)
Afferent: From organ of Corti to brain
Efferent: From brain to organ of Corti
Peripheral processes of auditory nerve neurons enter the cochlea through small openings on the edge of the osseous spiral lamina
These openings are called ‘habenulae perforata’
These fibers are then gathered in the modiolus 2

3. Organization of the auditory nerve bundle in the modiolus
Fibers from the apex in the middle
Fibers from the base on the outside
This nerve bundle then goes to the cochlear nucleus in the brain 3

4. Afferent fibers Around 30,000 neurons in man
Only 5-15% of these innervate the OHC
These are called Type II or outer spiral fibers
One neuron connects to one OHC (one-to-many)
Rest innervate the IHC
These are called Type I or radial fibers
Many neurons connect to one IHC (many-to-one) 4

5. Originate from the olivocochlear bundle in the auditory brainstem
Efferent fibers
Originate from the olivocochlear bundle in the auditory brainstem
Efferent fibers synapse on the afferent nerves innervating the IHC
Efferent fibers synapse directly on the OHC 5

6. Discharge pattern of a neuron
Neural spike has an initial large potential shift
Following this, refractory or rest period of around 1 msec 6

7. Other terms
Spontaneous discharge rate: Neuron’s discharge rate without a stimulus
Threshold: Minimum stimulus level needed to cause an increase in the discharge rate above the spontaneous discharge rate 7

8. Spontaneous rate and thresholds
Neurons with high spontaneous rate: Low threshold
Neurons with low spontaneous rate: High threshold 8

9. Rate-Level function Also called input-output or intensity function
Increase the level of the acoustic stimulus and measuring changes in the discharge rate of a single neuron

10. Response area Also called isolevel or isointensity curve Iso: “Same”
Plotting how a neuron fires in response to sounds of different frequencies at a fixed intensity 10

11. Tuning curve Define a certain threshold for a neuron
Plot the level of the tone required for this neuron to discharge at this threshold, as a function of the frequency of the tone 11

12. Encoding of frequency Tonotopic organization
Two theories:
Place theory
Tonotopic organization
Temporal theory
Based on the periodic nature of nerve firing

13. Place theory
Different neurons in the nerve respond to different frequencies
Frequency of input determined by which neuron(s) in the auditory nerve fires at the greatest rate

14. Temporal theory
For lower frequencies (< 5000 Hz), discharge rates of neurons are proportional to the period of the input stimulus.
So for lower frequencies, rate of discharge of the neuron also provides information about the frequency of the stimulus.

15. Encoding of intensity
Increase in discharge rate with increase in stimulus intensity
However, for most neurons, increase in discharge rate only occurs for a limited range of input intensity
Possible that discharge rate of many neurons may be combined to account for the 140 dB dynamic range of the ear