Using ephaptic coupling to estimate the synaptic cleft resistivity of the calyx of Held synapse
Fig 4
Relation between the calcium current and the prespike.
(A) from top to bottom: The presynaptic voltage-clamp waveform (vpre), the inverted second derivative of the waveform (-vpre”), the presynaptic calcium current (iCa; after P/5 subtraction), its inverted first derivative (-iCa’), the active (black, thick) and passive prespike (ipost; magenta, P/5-scaled), and the calcium prespike which is the difference between the active and the passive prespike (Δipost). The passive prespike is generated by injecting the presynaptic AP at 1/5th of its amplitude. While this generates passive currents across the presynaptic cleft-facing membrane, it does not elicit the presynaptic calcium current. Voltage-gated sodium and potassium currents were blocked (see Materials and Methods). Vertical scale bars: 110 mV, 7 MV/s2, 2 nA, 15 nA/ms, 0.15 nA, 0.1 nA. Horizontal: 0.5 ms. (B) Relation between the inverted first derivative of the presynaptic calcium current and the calcium prespike (black dots). Maximum correlation was obtained by introducing a delay of 60 μs to the first derivative. The slope of the regression line (magenta) is shown in the bottom right corner.