Action Potentials and SMO
When we record a neuron intracellularly while injecting different levels of current pulses, the current will drive the subthreshold membrane potential oscillations (SMOs) toward the threshold potential, evoking APs upon threshold crossing (Llinas et al., 1991). The larger the depolarizing current is, the more likely the membrane potential is to cross the threshold and generate APs. This is the mechanism by which the intensity of a sensory signal is converted to a firing rate code. Intriguingly, the level of input current in these experiments will not only affect the firing rate but also the phase of APs, as phases advance systematically with increasing depolarization, even after the firing rate has been saturated (Figure 1). Using the phase, neurons are endowed with a broader dynamic range for encoding information than they are with the firing rate. A similar sensory encoding scheme has been proposed and experimentally observed in the salamander retina (Gollisch and Meister, 2008). If neurons encode information using the phase of APs, how will that information be read out?
Figure 1  The scheme of intracellular current clamp recordings from a neuron being depolarized by different levels of current injection. As the level of depolarizing current increases (gray levels), the amplitude of subthreshold membrane potential oscillation increases with it. At the moments when the oscillations reach the threshold (dashed line), the neuron generates action potentials (vertical lines represent truncated action potentials). Near the threshold the action potential generation is probabilistic. The number of action potentials (0, 1, 3, 3) increases with the level of depolarization. At the same time, with the increasing current, the phases of action potentials relative to the membrane oscillations advance (left pointing arrows). The range of the phase change is bound to π. Note that while the number of action potentials saturates at 3, the phase still advances. (Scale bar is at bottom left.)