scc afferents kim mcarthur vestibular classics november 3, 2006
Post on 03-Jan-2016
214 Views
Preview:
TRANSCRIPT
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
Baird et al 1988
Dimorphic/HC/R Dimorphic/HC/Intermed
Bouton/AC/RCalyx/HC/I
Dimorphic/AC/I
Physiology
Spontaneous discharge Spatial tuning Discharge regularity Sensitivity to galvanic stimulation Adaptation to constant velocity Dynamics (transfer function)
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
Mechanisms:Discharge 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
Mechanisms:Discharge Regularity
Goldberg, Smith & Fernandez 1984
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
top related