Effect of non-linear summation of synaptic currents on the input/output properties of motoneurons

A single spinal motoneuron receives tens of thousands of synapses. The neurotransmitters riving potential of the linear sum ct of this non-linearity on the current delivered to the soma during activation of predetermined numbers and distributions of excitatory and inhibitory synapses. To accomplish this goal we constructed compartmental models constrained by detailed measurements of the geometry of the dendritic riving potential was active ng of a balanced activation of excitatory and inhibitory synapses decreased the current delivered to the soma further, but reduced the non-linearity with respect to the total number of active excitatory synapses. Unexpectedly, simulations that mimicked experimental measures of non-linear inear summation. difficult to detect, despite the substantial ‘loss’ of current due to non-linear summation. The magnitude of this ‘loss’ appears to limit motoneuron activity, based solely on activation of iontotropic receptors, to levels that are inadequate to generate functionally meaningful muscle forces. released by many of these synapses act on iontotropic receptors and alter the d neighbouring synapses. This interaction introduces an intrinsic non-linearity in motoneuron input-output properties where the response to two simultaneous inputs is less than of the responses to each input alone. Our goal was to determine the impa trees of three feline motoneurons. The current ‘lost’ due to local changes in d substantial and resulted in a highly non-linear relationship between the number of synapses and the current reaching the soma. Background synaptic activity consisti summation, activation of two sets of excitatory synapses, resulted in nearly l This result suggests that non-linear summation can be