Central auditory pathways

Question 1

Describe SOUND

Answer 1

1. Sound transduces into electrical signals in the cochlea
2. Into the IHC
3. Then information is conveyed to brain via spiking patterns in auditory nerve afferent
4. Those afferent then enter the brainstem at the level of the medulla
5. They form synaptic contacts which feed into an extensive network of auditory processing stages that reach cerebral cortex

Question 2

Amplitude of mechanical oscillations in basement membrane and electrical oscillations (IHC) correlated with….?

Answer 2

with intensity of sound waves

Question 3

Frequency tuning in cochlea and auditory nerve axons

Answer 3

• Each IHC synapses with 5-15 AN axons
• AN afferents:
  
  • cell bodies in spiral ganglion
  • peripheral axon contacts a single IHC
  • central axon projects to cochlear nucleus in brainstem
  • frequency tuning results from place of origin along cochlea

Question 4

how does a vowel show up on spectogram

Answer 4

periodic complex

Question 5

Place code

Answer 5

Characteristic frequency based on cochlear location

Question 6

Temporal code (phase locking to periodicity)

Answer 6

• Phase-locking to sound wave for individual afferent only possible for <1kHz
• Can phase-lock to alternate cycles (eg, 250 Hz, 4ms period, spikes could occur very 4, 8, 12, … etc ms)
• Period could be registered by pool of AN afferents connected to single IHC
• only possible up to 2-3 kHz (cf. human frequency response up to 20 kHz, and higher in many other animals)

Question 7

What causes phase locking to break down at high frequency?

Answer 7

Electrical properties of cell membrane

Question 8

Receptor potentials in IHCs can follow waveform of sound over lower frequency range

Answer 8

• Intracellular recording from IHC exposed to range of pure sinusoidal tones
• Oscillations in Vm are phase-locked to period of sound wave at low to middle frequencies (AC component)
• Response summates at higher frequencies – depolarization, but no phase-locked oscillations (DC component)