An electrophysiological and kinematic model of Paramecium, the “swimming neuron”
Fig 3
Summary of the model, showing a cilium attached to the base, and the movements of the two main ions, Ca2+ (red) and K+ (blue).
There is more Ca2+ outside than inside, and more K+ inside than outside. Calcium enters through ciliary voltage-gated channels as a current ICa. It then quickly inactivates these channels, forming a negative feedback loop. Calcium activates motor proteins, triggering ciliary reversal, as well as a ciliary K+ channel, producing an outward K+ current IK(Ca). The motor activation results in calcium concentration [Ca2+]i modulating kinematic parameters. Calcium is then expelled, in particular by plasma membrane Ca2+ ATPases (PMCA). Depolarization opens voltage-gated K+ channels in the basilar membrane, presumably near the cilium, creating a current IKd (delayed rectifier). A linear leak current is also present. Finally, an inward rectifier current IKir opens at very hyperpolarized voltages, which has little impact on the avoiding reaction.