At what voltage do the terminal's calcium channels open, and when during the spike does the calcium actually flow in? · Lobeworks/17

They are high-voltage-activated: they begin to open around −30 to −20 mV and are most open near 0 mV, so they need the spike but not its +30 mV peak.

At what voltage do the terminal's calcium channels open, and when during the spike does the calcium actually flow in?

They are high-voltage-activated: they begin to open around −30 to −20 mV and are most open near 0 mV, so they need the spike but not its +30 mV peak. The calcium flows mostly on the falling flank. The current is ICa=gCa(V)(V−ECa)I_{\text{Ca}} = g_{\text{Ca}}(V)(V - E_{\text{Ca}})ICa​=gCa​(V)(V−ECa​), and with ECaE_{\text{Ca}}ECa​ near +130 mV the driving force is small at the peak and grows as the voltage drops, while the channels are still open; recordings from a mammalian terminal put most of the calcium current in the repolarisation.