Necessity of oppositely directed subthreshold currents to explain excitability in spinal lamina I neurons.

<p>(A) Blocking a subthreshold Ca²⁺ current with Ni²⁺ converted tonic-spiking neurons to phasic-spiking (right). Blocking a subthreshold K⁺ current with 4-AP converted single-spiking neurons to phasic-spiking (left). Compare with naturally occurring phasic-spiking pattern (center). (B) Application of Ni²⁺ and 4-AP converted class 1 and 3 neurons, respectively, to class 2 neurons according to the <i>f–I</i> curves. Firing rate was determined from the reciprocal of first interspike interval. According to these data, a subthreshold inward current is necessary for class 1 excitability, a subthreshold outward current is necessary for class 3 excitability, and class 2 excitability occurs when neither current is present.