SCC Afferents

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SCC Afferents

• Kim McArthur
• Vestibular Classics
• November 3, 2006

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Overview

• Review SCC Mechanics
• Afferent Peripheral Morphology
• Afferent Physiology
• Proposed Mechanisms

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ReviewSCC Mechanics
P endolymph displacement
Q head/canal displacement
Initial Position
CW moment IPaccel ? like ma
CCW moments B(Qvel-Pvel) ? viscosity
of endolymph (damping) K(QP) ? elasticity of
cupula (spring)
G. Melvill Jones (1972)
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ReviewSCC Transfer Function

• Q-P (s) ___aT1T2s____
• Qvel (T1s1)(T2s1)
• T1gtgtT2
• T1 B/K T2 I/B T1T2 I/K

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ReviewSCC Transfer Function

• HF range (?gt1/T2)
• ? responsive to angular position (dominated by
  inertia)
• MF range (1/T1lt?lt1/T2)
• ? responsive to angular velocity (dominated by
  endolymph viscosity)
• LF range (?lt1/T1)
• ? responsive to angular acceleration (both
  dominated by cupular elasticity)

1/T2
1/T1
G. Melvill Jones (1972)
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Peripheral Morphology
Dickman in Fundamental Neuroscience, 2nd ed.
(2002)
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Peripheral Morphology
Dimorphic/HC/Intermed
Dimorphic/HC/R
Calyx/HC/I
Bouton/AC/R
Dimorphic/AC/I
Baird et al 1988
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Peripheral Morphology
Haque, Huss Dickman (2006)
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Physiology

• Spontaneous discharge
• Spatial tuning
• Discharge regularity
• Sensitivity to galvanic stimulation
• Adaptation to constant velocity
• Dynamics (transfer function)

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PhysiologySpontaneous Discharge
Goldberg Fernandez 1971
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PhysiologySinusoidal Response
Goldberg Fernandez 1971
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PhysiologySinusoidal Response
Goldberg Fernandez 1971
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PhysiologySpatial Tuning
Haque, Angelaki Dickman 2004
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PhysiologySpatial Tuning
Haque, Angelaki Dickman 2004
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PhysiologyDischarge Regularity
Goldberg Fernandez 1971
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PhysiologyDischarge Regularity
Goldberg Fernandez 1971
Baird et al 1988
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PhysiologyCV Galvanic Sensitivity
Baird et al 1988
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PhysiologyCV Gain/Phase
Haque, Angelaki Dickman 2004
Baird et al 1988
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PhysiologyAdaptation
Goldberg Fernandez 1971
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PhysiologyDynamics
Goldberg Fernandez 1971
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PhysiologyDynamics
Goldberg Fernandez 1971
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PhysiologyDynamics
Haque, Angelaki Dickman 2004
Baird et al 1988
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To re-cap

• Morphology
• Type I hair cells calyx ( dimorphic) afferent
  terminals in the central zone
• Type II hair cells bouton ( dimorphic)
  afferent terminals in the peripheral zone

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To re-cap

• Physiology
• Cosine tuning to canal planes
• Discharge regularity (CV) varies across the
  population
• Dynamics may differ from prediction based on
  torsion-pendulum model of SCC mechanics
• Adaptation ? low-frequency phase lead
• Cupular velocity sensitivity ? high-frequency
  phase lead and gain enhancement

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MechanismsCo-variation of Properties

• Irregular afferents
• Calyx/dimorphic terminals in the central zone
• Phasic-tonic response dynamics (adaptation
  cupular velocity sensitivity)
• Large responses to efferent fiber stimulation
• Large, low threshold responses to galvanic
  stimulation

• Regular afferents
• Bouton/dimorphic terminals in the peripheral zone
• Tonic response dynamics (resemble expectation
  from canal dynamics)
• Small responses to efferent fiber stimulation
• Small, high threshold responses to galvanic
  stimulation

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MechanismsDischarge Regularity

• Compartmental cable calculations indicate that
  electronic distance has only a small effect on
  discharge regularity
• Dimorphic units with similar terminal branching
  patterns may be regular or irregular
• ? Terminal branching pattern is not causally
  related to discharge regularity (may be causally
  related to location of the terminal within the
  neuroepithelium)

Baird et al 1988
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MechanismsDischarge Regularity

• General Model
• Variability in the SD of ISI due to
• Synaptic noise
• Slope of the recovery function
• Galvanic sensitivity will be tied to the recovery
  function, but will be independent of synaptic
  noise

Goldberg, Smith Fernandez 1984
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MechanismsDischarge Regularity

• Prediction If the shape of the recovery
  function is an important contributing factor in
  discharge regularity, then CV should correlate
  with galvanic sensitivity.
• ? Irregular afferents will have higher
  sensitivity to galvanic stimulation

Goldberg, Smith Fernandez 1984
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MechanismsDischarge Regularity

• Afferent irregularity is causally related to its
  post-spike voltage recovery function
• (Irregular afferents have faster recovery, due to
  a smaller, more rapidly decaying K AHP)

Goldberg, Smith Fernandez 1984
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Therefore
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MechanismsResponse Dynamics

• Dynamics in response to galvanic currents are
  similar for regular and irregular afferents
  (Goldberg, Fernandez Smith 1982)
• Dynamics in response to natural stimulation
  differ (as previously shown)
• Dynamics do not arise from the same mechanism as
  discharge regularity
• Dynamics arise from transduction prior to the
  afferent spike encoder (probably during hair cell
  transduction)

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MechanismsSynaptic Gain

• Synaptic gain system gain / encoder gain
  (galvanic sensitivity)
• Bouton and dimorphic afferents have higher
  synaptic gains than calyx units, possibly due to
  the low input impedance of type I hair cells
• ? Synaptic gain is causally linked to hair cell
  innervation (calyx units innervate type I hair
  cells lower gain)

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Therefore
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SUMMARY

• Afferent discharge regularity and galvanic
  sensitivity are determined by the slope of the
  recovery function (K AHP), which may be
  determined by location within the crista
• Peripheral zone slow recovery regular
• Central zone fast recovery irregular
• Synaptic gains are determined by hair cell
  innervation
• Type I HC (calyx) low synaptic gains
• Type II HC (bouton) higher synaptic gains
• Response dynamics are probably determined by hair
  cell transduction (either intrinsic to the HC or
  characteristic of the synapse)
• Regular afferents tend to have more canal-like
  dynamics
• Irregular afferents exhibit more adaptation
  (low-frequency phase lead) and more cupular
  velocity sensitivity (high-frequency phase lead
  and gain enhancement)
• HOWEVER dynamics are not determined by the
  recovery function, but by some correlated
  property prior to the spike encoder

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Some Notes on Function

• Most secondary neurons receive mixed regular and
  irregular input
• VOR Driven by regular afferents, modified by
  irregular afferents (?)
• VCR Driven by irregular afferents (?)