Figure 1. Basic effects of the dendritic PIC on effective synaptic currents in spinal motoneurones.

A, during voltage clamp, a brief (1.5 s) period of excitatory, synaptic input was delivered to the cell via activation of muscle spindle Ia afferents. When the cell was held hyperpolarized, this produced an effective synaptic current with a sharp onset and offset matching input duration. At a depolarized holding potential at approximately spike voltage threshold in unclamped conditions, a repetition of the very same input had a much different result. Current during the input was much larger (amplification) and a prolonged tail current was generated, both due to activation of the dendritic (PIC). As this input does not act via either NMDA receptors or metabotropic glutamate receptors, the amplification can be attributed to the PIC (Lee & Heckman, 2000). Inward (depolarizing) currents are downward, the usual voltage clamp convention. B, the interaction between amplification of effective synaptic current and level of descending neuromodulatory input (labelled as weak, medium and strong). In the weak state, there is no significant PIC and synaptic current steadily decreases as holding potential is slowly depolarized. At medium and high levels, activation of the PIC causes a large increase in current, at a voltage level where spikes would be generated in unclamped conditions. As the cell is depolarized further, current sharply declines. This saturation is likely to be due both to loss of driving force in depolarized dendritic regions and to activation of dendritic voltage-sensitive outward currents. Inward (depolarizing) currents are downward. Based on data from Lee & Heckman (2000).