Department of Physiology, College of Medicine, PO Box 210093, University of Arizona, Tucson, AZ 85721-0093, USA.
J Neurophysiol. 2011 Sep;106(3):1467-79. doi: 10.1152/jn.00419.2011. Epub 2011 Jun 22.
Motor neurons are often assumed to generate spikes in proportion to the excitatory synaptic input received. There are, however, many intrinsic properties of motor neurons that might affect this relationship, such as persistent inward currents (PICs), spike-threshold accommodation, or spike-frequency adaptation. These nonlinear properties have been investigated in reduced animal preparation but have not been well studied during natural motor behaviors because of the difficulty in characterizing synaptic input in intact animals. Therefore, we studied the influence of each of these intrinsic properties on spiking responses and muscle force using a population model of motor units that simulates voluntary contractions in human subjects. In particular, we focused on the difference in firing rate of low-threshold motor units when higher threshold motor units were recruited and subsequently derecruited, referred to as ΔF. Others have used ΔF to evaluate the extent of PIC activation during voluntary behavior. Our results showed that positive ΔF values could arise when any one of these nonlinear properties was included in the simulations. Therefore, a positive ΔF should not be considered as exclusive evidence for PIC activation. Furthermore, by systematically varying contraction duration and speed in our simulations, we identified a means that might be used experimentally to distinguish among PICs, accommodation, and adaptation as contributors to ΔF.
运动神经元通常被认为其产生的冲动与所接收的兴奋性突触输入成比例。然而，运动神经元有许多内在特性可能会影响这种关系，例如持续内向电流（PICs）、冲动阈值适应或冲动频率适应。这些非线性特性已在简化的动物模型中进行了研究，但由于难以在完整动物中描述突触输入，因此在自然运动行为中尚未得到很好的研究。因此，我们使用模拟人类受试者自主收缩的运动单位群体模型，研究了这些内在特性中的每一种对冲动反应和肌肉力量的影响。特别是，我们关注了当较高阈值的运动神经元被招募并随后去招募时，低阈值运动神经元的放电率差异，称为ΔF。其他人使用ΔF 来评估在自主行为过程中 PIC 激活的程度。我们的结果表明，当在模拟中包含任何一种非线性特性时，都可能会出现正的ΔF 值。因此，正的ΔF 不应被视为 PIC 激活的唯一证据。此外，通过在我们的模拟中系统地改变收缩持续时间和速度，我们确定了一种可能在实验中用于区分 PICs、适应和适应作为ΔF 贡献的方法。
