Powers R K, Binder M D
J Neurophysiol. 1985 Feb;53(2):518-29. doi: 10.1152/jn.1985.53.2.518.
In the experiments described in the preceding paper electrical stimulation of the quadriceps (QUAD), medial tibial (MTIB), and flexor digitorum and hallucis longus (FDHL) muscle nerves was used to evoke oligosynaptic group I postsynaptic potentials (PSPs) in medial gastrocnemius (MG) motoneurons. In the present study, we attempted to specify the types of afferent fibers which mediate that oligosynaptic activity (FDHL to MG only). In one series of experiments, isolated single flexor digitorum longus (FDL) and flexor hallucis longus (FHL) afferents were identified as Ia, Ib, or group II fibers according to their conduction velocities, responses to muscle contraction, and mechanical thresholds to small amplitude triangular stretches applied to the parent muscles. We also determined the electrical thresholds of the identified afferent fibers by applying graded electrical stimulation to their muscle nerve. These results were used as criteria to define the types of afferents that mediated the electrically and stretch-evoked FDHL oligosynaptic PSPs recorded in MG motoneurons during a second series of experiments. The amplitudes of the oligosynaptic PSPs evoked in MG motoneurons increased as the strength of the electrical stimuli applied to the FDHL muscle nerves was raised to activate greater numbers of Ia- and Ib-fibers, but showed little or no additional increase when the stimulus intensity was raised further to include the majority of group II fibers. On this basis, a significant contribution by group II fibers to these oligosynaptic PSPs was considered unlikely. Simultaneous electrical activation of both Ia- and Ib-fibers produced distinct oligosynaptic PSPs in MG motoneurons, but these were likely due primarily to Ib-afferent activity, since selective activation of Ia-afferents (by stretch) rarely produced oligosynaptic PSPs in the same motoneurons. There was, however, evidence for some Ia contribution to these oligosynaptic PSPs. This is consistent with the demonstration that Ia- and Ib-afferent fibers converge onto common interneurons and that selective activation of Ia-fibers can produce PSPs similar to those evoked by concurrent stimulation of Ia- and Ib-fibers. On the basis of the present results and those of several related studies it is argued that the oligosynaptic PSPs evoked in MG motoneurons by submaximal group I stimulation of the FDHL, MTIB, or QUAD muscle nerves can be ascribed predominantly to the activation of Ib-afferent fibers, with only minimal Ia and probably no group II contribution.
在前一篇论文所描述的实验中，通过电刺激股四头肌（QUAD）、胫骨内侧肌（MTIB）以及趾长屈肌和拇长屈肌（FDHL）的神经，来诱发腓肠肌内侧头（MG）运动神经元的寡突触I类突触后电位（PSP）。在本研究中，我们试图明确介导该寡突触活动（仅FDHL至MG）的传入纤维类型。在一系列实验中，根据其传导速度、对肌肉收缩的反应以及对施加于其所属肌肉的小幅度三角拉伸的机械阈值，将分离出的单个趾长屈肌（FDL）和拇长屈肌（FHL）传入纤维鉴定为Ia、Ib或II类纤维。我们还通过对其肌肉神经施加分级电刺激来确定已鉴定传入纤维的电阈值。在第二系列实验中，这些结果被用作标准来定义介导在MG运动神经元中记录到的电刺激和拉伸诱发的FDHL寡突触PSP的传入纤维类型。随着施加于FDHL肌肉神经的电刺激强度增加以激活更多的Ia和Ib纤维，MG运动神经元中诱发的寡突触PSP的幅度增大，但当刺激强度进一步提高以纳入大多数II类纤维时，幅度几乎没有增加或没有额外增加。基于此，认为II类纤维对这些寡突触PSP的显著贡献不太可能。同时电激活Ia和Ib纤维在MG运动神经元中产生了不同的寡突触PSP，但这些可能主要归因于Ib传入活动，因为选择性激活Ia传入纤维（通过拉伸）在同一运动神经元中很少产生寡突触PSP。然而，有证据表明Ia对这些寡突触PSP有一定贡献。这与以下证明一致：Ia和Ib传入纤维汇聚到共同的中间神经元上，并且选择性激活Ia纤维可以产生与同时刺激Ia和Ib纤维诱发的PSP相似的PSP。基于目前的结果以及几项相关研究的结果，有人认为通过对FDHL、MTIB或QUAD肌肉神经进行次最大强度的I类刺激在MG运动神经元中诱发的寡突触PSP主要可归因于Ib传入纤维的激活，Ia的贡献极小，II类纤维可能没有贡献。
