scc afferents kim mcarthur vestibular classics november 3, 2006

SCC Afferents

Kim McArthur

Vestibular Classics

November 3, 2006

Overview

Review: SCC Mechanics Afferent Peripheral Morphology Afferent Physiology Proposed Mechanisms

Review:SCC Mechanics

G. Melvill Jones (1972)

InitialPosition

Q = head/canal displacement

P = endolymphdisplacement

CW moment:IPaccel like ma

CCW moments:B(Qvel-Pvel) viscosity of

endolymph (damping)K(Q–P) elasticity of cupula (spring)

Review:SCC Transfer Function

Q-P (s) = ___αT1T2s____

Qvel (T1s+1)(T2s+1)

T1>>T2

T1 = B/K ; T2 = I/B ; T1T2 = I/K

Review:SCC Transfer Function

HF range (ω>1/T2)

responsive to angular position (dominated by inertia)

MF range (1/T1<ω<1/T2)

responsive to angular velocity (dominated by endolymph viscosity)

LF range (ω<1/T1)

responsive to angular acceleration (both dominated by cupular elasticity)

1/T1

G. Melvill Jones (1972)

1/T2

Peripheral Morphology

Dickman in Fundamental Neuroscience, 2nd ed. (2002)


Baird et al 1988

Dimorphic/HC/R Dimorphic/HC/Intermed

Bouton/AC/RCalyx/HC/I

Dimorphic/AC/I


Haque, Huss & Dickman (2006)

Physiology

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

Physiology:Spontaneous Discharge

Goldberg & Fernandez 1971

Physiology:Sinusoidal Response


Physiology:Spatial Tuning

Haque, Angelaki & Dickman 2004

Physiology:Discharge Regularity


Physiology:Discharge Regularity

Goldberg & Fernandez 1971 Baird et al 1988

Physiology:CV & Galvanic Sensitivity

Baird et al 1988

Physiology:CV & Gain/Phase

Baird et al 1988 Haque, Angelaki & Dickman 2004

Physiology:Adaptation


Physiology:Dynamics


Physiology:Dynamics

Baird et al 1988 Haque, Angelaki & Dickman 2004

To re-cap …

Morphology:Type I hair cells – calyx (& dimorphic)

afferent terminals in the central zoneType II hair cells – bouton (&

dimorphic) afferent terminals in the peripheral zone

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

Mechanisms:Co-variation of PropertiesIrregular 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

Mechanisms:Discharge 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

Mechanisms:Discharge Regularity

General Model: Variability in the SD of ISI due to:

Synaptic noiseSlope of the recovery function

Galvanic sensitivity will be tied to the recovery function, but will be independent of synaptic noise

Goldberg, Smith & Fernandez 1984


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




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)

Therefore …

K+ AHP

Slowly decaying Slow recovery function

Rapidly decaying Rapid recovery function

Regular discharge (low CV)Low galvanic sensitivity

Occurs more in peripheral zone

Irregular discharge (high CV)High galvanic sensitivity

Occurs more in central zone

Mechanisms:Response 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)

Mechanisms:Synaptic 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)

Therefore …

Hair cell innervation

Calyx units - Type I only Low input impedance

Bouton/Dimorphic units – also Type II Higher input impedance

Smaller synaptic gains Larger synaptic gains

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

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 (?)

scc afferents kim mcarthur vestibular classics november 3, 2006

Documents

dynamicsgoldberg fernandez

adaptationgoldberg fernandez

angelaki dickman

peripheral morphologybaird

scc mechanicsg

dynamicsbaird et

neuroepitheliumbaird

cv gainphasebaird et