Lucas S M, Cope T C, Binder M D
J Neurophysiol. 1984 Jan;51(1):64-74. doi: 10.1152/jn.1984.51.1.64.
The spike-triggered averaging technique (26) was used to determine whether the synaptic input from medial gastrocnemius (MG) Ia-afferent fibers to homonymous motoneurons is "topographically weighted" (22) by means of differences in projection frequency, excitatory postsynaptic potential (EPSP) amplitude, or a combination of both factors. Motoneurons were classified as either "same branch" or "other branch," depending on whether a Ia-afferent fiber and motor axon were contained in the same or different intramuscular nerve branches. No difference was found in the projection frequency of Ia-afferents to the same branch and other branch motoneurons (95 versus 94%, respectively). The mean EPSP amplitude was larger in the same branch group of motoneurons (92 +/- 8 (SE) microV; n = V; n = 97) than in the other branch group (77 +/- 7 microV; n = 79). This difference was most striking in high-rheobase (greater than or equal to 10 nA) motoneurons, for which the mean EPSP amplitude in the same branch group was 82 +/- 12 microV (n = 48), whereas that in the other branch group was 52 +/- 5 microV (n = 37). In 60 cases it was possible to compare the EPSPs produced by a same branch afferent and an other branch afferent in the same motoneuron. The same branch afferent produced the larger EPSP in 73% (44/60) of the cases. Moreover, the mean ratio of the same branch to the other branch EPSP amplitudes was 1.7, which was both statistically significant and consistent with analogous results from our preceding study of aggregate EPSPs (22). Mean rise times and half-widths of EPSPs in the same branch group were not significantly different from those in the other branch group. Furthermore, no significant differences in rise times or half-widths between the two groups were evident when motoneurons were segregated according to their rheobase values. This suggests that the segregation of Ia-afferent and motor axons across the intramuscular nerve branches is not reflected in the locations of Ia terminals on the motoneuron somadendritic surface and that other factors must account for observed EPSP amplitude differences. Our data suggest that the topographic weighting of homonymous Ia-afferent input to cat MG motoneurons is mediated by a gradient of EPSP amplitude rather than by a gradient of Ia connectivity and also suggest that the effect is most prominent in high-rheobase motoneurons.
采用锋电位触发平均技术（26）来确定来自内侧腓肠肌（MG）Ia传入纤维到同名运动神经元的突触输入是否通过投射频率、兴奋性突触后电位（EPSP）幅度的差异或两者因素的组合进行“地形加权”（22）。运动神经元根据Ia传入纤维和运动轴突是否包含在同一或不同的肌内神经分支中，分为“同一分支”或“其他分支”。未发现Ia传入纤维到同一分支和其他分支运动神经元的投射频率有差异（分别为95%和94%）。同一分支组运动神经元的平均EPSP幅度（92±8（SE）μV；n = 97）大于其他分支组（77±7μV；n = 79）。这种差异在高基强度（≥10 nA）运动神经元中最为显著，同一分支组的平均EPSP幅度为82±12μV（n = 48），而其他分支组为52±5μV（n = 37）。在60例中，可以比较同一运动神经元中由同一分支传入纤维和其他分支传入纤维产生的EPSP。在73%（44/60）的情况下，同一分支传入纤维产生的EPSP更大。此外，同一分支与其他分支EPSP幅度的平均比值为1.7，这在统计学上具有显著意义，并且与我们之前对复合EPSP的研究（22）的类似结果一致。同一分支组EPSP的平均上升时间和半宽度与其他分支组没有显著差异。此外，当根据运动神经元的基强度值进行分类时，两组之间的上升时间或半宽度也没有明显差异。这表明肌内神经分支上Ia传入纤维和运动轴突的分离并未反映在运动神经元胞体树突表面Ia终末的位置上，其他因素必定可以解释观察到的EPSP幅度差异。我们的数据表明，猫MG运动神经元同名Ia传入输入的地形加权是由EPSP幅度梯度介导的，而不是由Ia连接性梯度介导的，并且还表明这种效应在高基强度运动神经元中最为突出。
