Sherff C M, Mulloney B
Division of Biological Sciences, University of California, Davis 95616-8755, USA.
J Neurophysiol. 1997 Jul;78(1):92-102. doi: 10.1152/jn.1997.78.1.92.
Four different functional types of motor neurons innervate each swimmeret: return-stroke excitors (RSEs), power-stroke excitors (PSEs), return-stroke inhibitors (RSIs), and power-stroke inhibitors (PSIs). We studied the structures and passive electrical properties of these neurons, and tested the hypothesis that different types of motor neurons would have different passive properties that influenced generation of the swimmeret motor pattern. Cell bodies of neurons innervating one swimmeret were clustered in two anatomic groups in the same ganglion. The shapes of motor neurons in both groups were similar, despite the differences in locations of their cell bodies and in their functions. Diameters of their axons in the swimmeret nerve ranged from <2 to approximately 35 microm. Resting membrane potentials, input resistances, and membrane time constants were recorded with microelectrodes in the processes of swimmeret motor neurons in isolated abdominal nerve cord preparations. Membrane potentials had a median of -59 mV, with 25th and 75th percentiles of -66.0 and -53 mV. The median input resistance was 6.4 M omega, with 25th and 75th percentiles of 3.4 and 13.7 M omega. Membrane time constants had a median of 9.3 ms, with 25th and 75th percentiles of 5.7 and 15.0 ms. Excitatory and inhibitory motor neurons had similar passive properties. RSE motor neurons were typically more depolarized than the other types, but the passive properties of RSE, PSE, RSI, and PSI neurons were not significantly different. Membrane time constants measured from cell bodies were briefer than those measured from neuropil processes, but membrane potentials and input resistances were not significantly different. The relative sizes of different motor neurons were measured from the sizes of their impulses recorded extracellularly from the swimmeret nerve. Smaller motor neurons had lower membrane potentials and were more likely to be active in the motor pattern than were large motor neurons. Motor neurons of different sizes had similar input resistances and membrane time constants. Motor neurons that were either oscillating or oscillating and firing in phase with the swimmeret motor pattern had lower average membrane potentials and longer time constants than those that were not oscillating. When the state of the swimmeret system changed from quiescence to continuous production of the motor pattern, the resting potentials, input resistances, and membrane time constants of individual swimmeret motor neurons changed only slightly. On average, both input resistance and membrane time constant increased. These similarities are considered in light of the functional task each motor neuron performs, and a hypothesis is developed that links the brief time constants of these neurons and graded synaptic transmission by premotor interneurons to control of the swimmeret muscles and the performance of the swimmeret system.
回摆激发神经元（RSEs）、推摆激发神经元（PSEs）、回摆抑制神经元（RSIs）和推摆抑制神经元（PSIs）。我们研究了这些神经元的结构和被动电学特性，并检验了这样一个假设，即不同类型的运动神经元具有不同的被动特性，这些特性会影响游泳足运动模式的产生。支配一对游泳足的神经元的细胞体聚集在同一神经节的两个解剖学组中。尽管两组运动神经元的细胞体位置和功能存在差异，但它们的形状相似。它们在游泳足神经中的轴突直径范围从小于2微米到约35微米。在分离的腹神经索标本中，用微电极记录游泳足运动神经元的静息膜电位、输入电阻和膜时间常数。膜电位的中位数为-59 mV，第25和第75百分位数分别为-66.0 mV和-53 mV。中位数输入电阻为6.4 MΩ，第25和第75百分位数分别为3.4 MΩ和13.7 MΩ。膜时间常数的中位数为9.3 ms，第25和第75百分位数分别为5.7 ms和15.0 ms。兴奋性和抑制性运动神经元具有相似的被动特性。RSE运动神经元通常比其他类型的神经元更去极化，但RSE、PSE、RSI和PSI神经元的被动特性没有显著差异。从细胞体测量的膜时间常数比从神经纤维过程测量的要短，但膜电位和输入电阻没有显著差异。不同运动神经元的相对大小是根据从游泳足神经细胞外记录的冲动大小来测量的。较小的运动神经元具有较低的膜电位，并且比较大的运动神经元更有可能在运动模式中活跃。不同大小的运动神经元具有相似的输入电阻和膜时间常数。与游泳足运动模式同步振荡或振荡并放电的运动神经元比不振荡的运动神经元具有更低的平均膜电位和更长的时间常数。当游泳足系统的状态从静止转变为持续产生运动模式时，单个游泳足运动神经元的静息电位、输入电阻和膜时间常数仅略有变化。平均而言，输入电阻和膜时间常数都增加了。根据每个运动神经元执行的功能任务来考虑这些相似性，并提出了一个假设，将这些神经元的短时间常数和运动前中间神经元的分级突触传递与游泳足肌肉的控制和游泳足系统的性能联系起来。
