Activation of L-type Ca channels on the dendrites of spinal motoneurons provides a powerful means of amplifying synaptic currents that would otherwise cause marginal changes in motoneuron activity. Simulations using compartmental models constrained by anatomical and physiological data (i.e. changes in the threshold for plateau potentials, as recorded in the soma, due to tonic excitatory or inhibitory synaptic activity; Bennett et al., 1998), predict that L-type Ca channels are located in hot-spots, 150-400 µm, from the soma (Rose et al., SFN 2002). The goal of this study was to determine the impact of these hot spots on the current delivered to the soma in response to tonic activation of uniformly distributed excitatory and inhibitory synapses. In the absence of L-type Ca channels, the relationship between the current reaching the soma and the number of active synapses is non-linear. The current/synapse falls more than 2 fold at higher levels of synaptic activity due to non-linear summation. In contrast, w...