slow AHP (sAHP): Calcium

This channel is driven by a M-type mechanism operating on calcium sensor pathways that have longer time constants, updated at the theta cycle level so that guarantees a long baseline already. The logistic gating function operates with a high cutoff on the underlying Ca signal

The logistic operates on integrated Ca:

	co = (ca - Off)
	a = co / TauMax * (1 - exp(-co/Slope))
	b = -co / TauMax * (1 - exp(co/Slope))

   tau = 1 / (a + b)
   ninf = a / (a + b)

The Time Run plot (above) shows how the mAHP current develops over time in response to spiking, in comparison to the KNa function, with the default "fast" parameters of 50msec and rise = 0.05. The mAHP is much more "anticipatory" due to the direct V sensitivity, whereas KNa is much more "reactive", responding only the the Na after spiking.