Transcript Neurophysiology - University of Florida

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
• VIIth nerve nucleus
• Cochlear Nucleus
• VIIth nerve
• Superior Olivary
Complex
• Stapedius muscle