SPAA 648 - Auditory Nerve Physiology - E5

Cochlear Graded Potential

• Graded Potential → exceeds threshold of auditory nerve fiber and creates an action potential

• Amplitude increases in proportional to the amount of the stimulation

  • Ex. cochlear microphonic (OHCs) and summating potential (IHCs)

• HC needs to reach threshold for depolarization for an action potential to be triggered

Auditory Nerve Action Potential

• Resting → deporlaized → repolarizing → resting

• All-or-none electrical discharge → meaning it MUST reach threshold to be triggered

*A stimulus increases the firing rate

Action Potential (Firing)

• The discharge of a spike (action) potential

Action Potential (Firing/Discharge Rate)

• The number of action potentials discharged per second

Action Potential (Spontaneous Rate (SR))

• The rate at which a neuron fires on its own when there is no stimulation

Auditory nerve bundles encode what information?

• Frequency and intensity of sound input

  • Need to preserve information from the cochlea to be send to a higher nuclei (auditory nerve)

Intensity Coding (Continued)

• Fibers working together to encode a wide intensity range but for the same center frequency!

Frequency Coding (Depends On?)

• The rate at which an auditory nerve fiber discharges depends on the intensity and frequency of the sound input

• Depends on:

  • Characteristic Frequency (CF) of nerve fibers - where does it happen along the nerve fiber?

  • Firing Pattern of nerve fiber - afferent → Type 1 (1-8 fibers per IHC)

Characteristic Frequency (CF) of a Neuron (Tuning Curve)

• The frequency of the lowest threshold/greatest firing rate

• Tuning Curve - the threshold of an auditory nerve fiber plotted as a function of frequency

  • Low CFs - broader (not sharp) & symmetric

  • High CFs - narrower (more sharp) & asymmetric

Frequency Coding (Firing Pattern & PSTH)

• Firing Pattern - the manner in which spikes are elicited over time (after there’s a stimulus)

  • Shows were the highest amplitude for the peak is

• Post-Stimulus Time Histogram (PSTH) - number of spikes that occurred during a period of time after an acoustic stimulus is applied

  • Can revealed firing patterns

  • Spikes are phase-locked to cycles of the sine wave

Frequency Coding (Phase Locking)

• Phase Locking of Auditory Nerve Fiber - the time locking of neural discharges to the acoustic waveform

  • Low Frequency - might be okay to fire and stop at each peak

  • High Frequency - maximum firing rate is around 5000 Hz and anything higher will be hard for the nerve fibers to fire this fast at each peak

Frequency Coding (Volley Principle)

• For a high frequency sound, other nerve fibers are recruited into action when one nerve fiber reaches its maximum firing rate

• Group of neurons have to work together for high freq. sounds to encode the sound wave

Intensity Coding (Dynamic Range & Saturation)

• Dynamic Range of a Neuron: (humans -10 dB to 110 dB; total range of 120 dB)

  • The range of intensity over which the auditory nerve fiber continues to respond with increasing magnitude

  • Dynamic range for MOST neurons is about 40 dB

• Saturation - when the neuron’s response no longer increases as the stimulus level goes up

• Related to the spontaneous rates (SRs) of the nerve fibers, until we have an action potential!

*Nerve fibers work together to encode for the intensity of a sound!

Intensity Coding (Spontaneous Rates)

• Low: 0-0.5 spikes/s

  • Low SR Fibers - smaller in diameter, usually towards the modiolus, high thresholds, and wide dynamic range (40-60 dB range)

• High: > 18 spikes/s

  • High SR Fibers - larger in diameter, usually towards the tunnel of Corti, low thresholds (0 dB), and narrow dynamic range (around 20-30 dB they STOP firing)