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

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

An Action Potential (or Spike)

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

Afferent & Efferent Neurons

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

IHC Innervation Pattern

OHC Innervation Pattern

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

IHC activation alters firing rate

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

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)

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

The Active Component Adds to Bekesy’s Traveling Wave

The Active Component Improves Sensitivity for soft sounds Improves frequency resolution

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

Tuning Curves Passive Only Active + Passive

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

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

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)

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

Two-tone Suppression Regular Tuning Curve

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

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)

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

Uncrossed OCB-- synapses on dendrites under inner hair cells

Crossed OCB-- synapses on outer hair cells

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

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