Cushing S, Bui T, Rose P K
Canadian Institutes of Health Research Group in Sensory-Motor Systems, Department of Physiology, Center for Neuroscience, Queen's University, Kingston, Ontario, Canada.
J Neurophysiol. 2005 Nov;94(5):3465-78. doi: 10.1152/jn.00439.2005. Epub 2005 Aug 3.
A single spinal motoneuron receives tens of thousands of synapses. The neurotransmitters released by many of these synapses act on iontotropic receptors and alter the driving potential of neighboring synapses. This interaction introduces an intrinsic nonlinearity in motoneuron input-output properties where the response to two simultaneous inputs is less than the linear sum of the responses to each input alone. Our goal was to determine the impact of this nonlinearity 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 trees of three feline motoneurons. The current "lost" as a result of local changes in driving potential was substantial and resulted in a highly nonlinear relationship between the number of active synapses and the current reaching the soma. Background synaptic activity consisting of a balanced activation of excitatory and inhibitory synapses further decreased the current delivered to the soma, but reduced the nonlinearity with respect to the total number of active excitatory synapses. Unexpectedly, simulations that mimicked experimental measures of nonlinear summation, activation of two sets of excitatory synapses, resulted in nearly linear summation. This result suggests that nonlinear summation can be difficult to detect, despite the substantial "loss" of current arising from nonlinear 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.
单个脊髓运动神经元会接收数以万计的突触。这些突触中许多释放的神经递质作用于离子otropic受体，并改变相邻突触的驱动电位。这种相互作用在运动神经元的输入-输出特性中引入了一种内在的非线性，即对两个同时输入的反应小于对每个单独输入的反应的线性总和。我们的目标是确定这种非线性对在激活预定数量和分布的兴奋性和抑制性突触时传递到胞体的电流的影响。为了实现这一目标，我们构建了受三只猫运动神经元树突树几何形状详细测量约束的房室模型。由于驱动电位的局部变化而“损失”的电流相当大，并导致活跃突触数量与到达胞体的电流之间存在高度非线性关系。由兴奋性和抑制性突触的平衡激活组成的背景突触活动进一步降低了传递到胞体的电流，但相对于活跃兴奋性突触的总数降低了非线性。出乎意料的是，模拟非线性总和的实验测量，即两组兴奋性突触的激活，导致了近乎线性的总和。这一结果表明，尽管非线性总和会导致大量电流“损失”，但非线性总和可能很难检测到。仅基于离子otropic受体的激活，这种“损失”的程度似乎将运动神经元的活动限制在不足以产生功能上有意义的肌肉力量的水平。
