Table 1.

Effects of persistent inward currents on motoneuron discharge properties

PIC Effect	Effect on Motoneuron Discharge	Biophysical Mechanism	Putative Function
Amplification of synaptic inputs	Produces an initial acceleration in firing rate as the PIC activates. Lasts ~1–2 s.	Gradual voltage-dependent activation of INap and ICaP. The relatively slow acceleration may reflect increasing CDF of ICaP.	Amplification, with the level proportional to monoaminergic drive. Potentially provides gain control.
Saturation	Decreased sensitivity to increases in input currents.	Activation of Cav1.2 and 1.3s channels that depolarize the dendritic tree to near the reversal potential for excitatory inputs.	Unknown; perhaps a necessary consequence of amplification. The potency of inhibitory input is increased during saturation.
Hysteresis	De-recruitment occurs at a lower input level than recruitment.	Mainly due to ICaP and membrane bistability resulting from an N-shaped I-V relation. Requires a strong PIC that is maintained by CDF.	Self-sustained firing appears to be the foundation of steady output for posture and stabilization tasks.
Facilitation	Increased firing rate in response to repeated inputs.	CDF and cooperative gating of Cav1.2 and 1.3s channels.	Unknown.

PIC, persistent inward current; CDF, calcium-dependent facilitation; I_Nap, peristent Na current; I_CaP, PIC carried by Ca²⁺ ions.