Dean P
Department of Psychology, University of Sheffield, United Kingdom.
J Neurophysiol. 1997 Sep;78(3):1531-49. doi: 10.1152/jn.1997.78.3.1531.
Ocular motoneuron firing rate is linearly related to conjugate eye position with slope K above recruitment threshold theta. Within the population of ocular motoneurons K increases as theta increases. These differences in firing rate between motoneurons might be determined either by the intrinsic properties of the motoneurons, or by differences in synaptic input to them, or by a combination of the two. This question was investigated by simulating the input signal to medial rectus motoneurons (MR-MNs) from internuclear neurons of the abducens nucleus (INNs). INNs were represented as input nodes in a two-layer neural net, each with weighted connections to every output node representing an MR-MN. Individual simulated MR-MNs were assigned parameters corresponding to an intrinsic current threshold I(R) and an intrinsic frequency-current (f-I) slope gamma. Their firing rates were calculated from these parameters, together with the effective synaptic current produced by their synaptically weighted INN inputs, with the use of assumptions employed in computer simulations of spinal motoneuron pools. The experimentally observed firing rates of MR-MNs served as training data for the net. The following two training conditions were used: 1) synaptic weights were fixed and the intrinsic parameters of the MR-MNs were allowed to vary, corresponding to the situation in which each MR-MN receives a common synaptic drive and 2) intrinsic MR-MN properties were fixed and synaptic weights were allowed to vary. In each case, the varying quantities were trained with a form of gradient descent error reduction. The simulations revealed the following three problems with the common-drive model: 1) the recruitment of INNs produced nonlinear responses in MR-MNs with low thetas; 2) the range of I(R)s required to reproduce the observed range of theta were generally larger than those measured experimentally for cat ocular motoneurons; and 3) the intrinsic f-I slope gamma increased with I(R). Experimental data from cat indicate that gamma decreases with I(R). When synaptic weights were allowed to vary, all three problems with the common-drive model were overcome. This required MR-MNs receiving selective input from INNs with similar firing rate thresholds. These results suggest that the differences in firing rate properties among MR-MNs in relation to steady-state eye position cannot be derived from their intrinsic properties alone but result at least partly from differences in their synaptic inputs. An MR-MN's individual set of synaptic inputs constitutes, in effect, a premotor receptive field.
眼球运动神经元的放电频率与共轭眼位呈线性关系，斜率(K)高于募集阈值(\theta)。在眼球运动神经元群体中，(K)随(\theta)的增加而增加。运动神经元之间放电频率的这些差异可能由运动神经元的内在特性决定，或者由它们突触输入的差异决定，或者由两者的组合决定。通过模拟外展核的核间神经元（INNs）向内直肌运动神经元（MR-MNs）的输入信号来研究这个问题。在一个两层神经网络中，INNs被表示为输入节点，每个输入节点与代表MR-MN的每个输出节点都有加权连接。为单个模拟的MR-MN分配对应于内在电流阈值(I(R))和内在频率-电流（(f-I)）斜率(\gamma)的参数。根据这些参数，以及它们通过突触加权的INN输入产生的有效突触电流，并利用脊髓运动神经元池计算机模拟中采用的假设，计算它们的放电频率。MR-MNs的实验观察到的放电频率用作网络的训练数据。使用了以下两种训练条件：1）突触权重固定，允许MR-MNs的内在参数变化，这对应于每个MR-MN接受共同突触驱动的情况；2）MR-MN的内在特性固定，允许突触权重变化。在每种情况下，通过一种梯度下降误差减少形式对变化量进行训练。模拟揭示了共同驱动模型存在的以下三个问题：1）INNs的募集在低(\theta)的MR-MNs中产生非线性反应；2）再现观察到的(\theta)范围所需的(I(R))范围通常大于猫眼球运动神经元实验测量的范围；3）内在(f-I)斜率(\gamma)随(I(R))增加。来自猫的实验数据表明(\gamma)随(I(R))降低。当允许突触权重变化时，共同驱动模型的所有三个问题都得到了克服。这要求MR-MNs从具有相似放电频率阈值的INNs接收选择性输入。这些结果表明，与稳态眼位相关的MR-MNs之间放电频率特性的差异不能仅从它们的内在特性推导出来，而是至少部分地由它们突触输入的差异导致。实际上，MR-MN的一组单独的突触输入构成了一个运动前感受野。
