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SCC Afferents

SCC Afferents. Kim McArthur Vestibular Classics November 3, 2006. Overview. Review: SCC Mechanics Afferent Peripheral Morphology Afferent Physiology Proposed Mechanisms. Review: SCC Mechanics. P = endolymph displacement. Q = head/canal displacement. Initial Position. CW moment:

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SCC Afferents

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  1. SCC Afferents Kim McArthur Vestibular Classics November 3, 2006

  2. Overview • Review: SCC Mechanics • Afferent Peripheral Morphology • Afferent Physiology • Proposed Mechanisms

  3. Review:SCC 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(Q–P)  elasticity of cupula (spring) G. Melvill Jones (1972)

  4. Review:SCC Transfer Function Q-P (s) = ___αT1T2s____ Qvel (T1s+1)(T2s+1) T1>>T2 T1 = B/K ; T2 = I/B ; T1T2 = I/K

  5. 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/T2 1/T1 G. Melvill Jones (1972)

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

  7. Peripheral Morphology Dimorphic/HC/Intermed Dimorphic/HC/R Calyx/HC/I Bouton/AC/R Dimorphic/AC/I Baird et al 1988

  8. Peripheral Morphology Haque, Huss & Dickman (2006)

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

  10. Physiology:Spontaneous Discharge Goldberg & Fernandez 1971

  11. Physiology:Sinusoidal Response Goldberg & Fernandez 1971

  12. Physiology:Sinusoidal Response Goldberg & Fernandez 1971

  13. Physiology:Spatial Tuning Haque, Angelaki & Dickman 2004

  14. Physiology:Spatial Tuning Haque, Angelaki & Dickman 2004

  15. Physiology:Discharge Regularity Goldberg & Fernandez 1971

  16. Physiology:Discharge Regularity Goldberg & Fernandez 1971 Baird et al 1988

  17. Physiology:CV & Galvanic Sensitivity Baird et al 1988

  18. Physiology:CV & Gain/Phase Haque, Angelaki & Dickman 2004 Baird et al 1988

  19. Physiology:Adaptation Goldberg & Fernandez 1971

  20. Physiology:Dynamics Goldberg & Fernandez 1971

  21. Physiology:Dynamics Goldberg & Fernandez 1971

  22. Physiology:Dynamics Haque, Angelaki & Dickman 2004 Baird et al 1988

  23. 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

  24. 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

  25. 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 Mechanisms:Co-variation of Properties

  26. 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

  27. Mechanisms:Discharge 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

  28. 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

  29. Mechanisms:Discharge 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

  30. Therefore …

  31. 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)

  32. 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)

  33. Therefore …

  34. 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

  35. 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 (?)

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