Lucas S M, Binder M D
J Neurophysiol. 1984 Jan;51(1):50-63. doi: 10.1152/jn.1984.51.1.50.
Experiments were performed to determine whether the topographic relationships between muscle spindles and their surrounding extrafusal fibers are preserved in the pattern of homonymous, monosynaptic connections from Ia-afferents to motoneurons. The medial gastrocnemius (MG) muscle of adult cats was chosen as a model system because previous studies have shown that its muscle nerve divides into several branches, each of which innervates a distinct muscle compartment (20, 27, 28, 38), and that the Ia-afferent fibers innervating muscle spindles within a compartment are found in the same nerve branch (27, 28). Thus, we could make intracellular recordings from MG motoneurons, determine which intramuscular compartment they innervated, and then compare the synaptic input they received from Ia-afferents innervating the same compartment with that which they received from Ia-afferents innervating different compartments. Our results indicate that homonymous Ia-afferent input is "topographically weighted" within the MG motor nucleus such that afferents innervating a given intramuscular compartment exert relatively greater synaptic effects in motoneurons that project to the same compartment than in other homonymous motoneurons. The degree of topographic weighting was quite variable in the different experiments but appeared more prominently in experiments in which a high proportion of the motoneurons studied were characterized by high-rheobase values and low input resistances. This suggests that topographic factors may exert more influence on the distribution of Ia-afferent input to large motoneurons than to small motoneurons. In the DISCUSSION, the extent of topographic weighting within the homonymous motoneuron pool is compared with weighting across synergist motoneuron pools, and alternative models of topographic weighting are proposed and evaluated.
进行了实验，以确定肌梭与其周围梭外肌纤维之间的局部解剖关系是否在从Ia类传入纤维到运动神经元的同名单突触连接模式中得以保留。选择成年猫的内侧腓肠肌（MG）作为模型系统，因为先前的研究表明，其肌神经分成几个分支，每个分支支配一个不同的肌间隔（20、27、28、38），并且在同一神经分支中发现支配一个肌间隔内肌梭的Ia类传入纤维（27、28）。因此，我们可以从MG运动神经元进行细胞内记录，确定它们支配哪个肌内间隔，然后将它们从支配同一间隔的Ia类传入纤维接收的突触输入与从支配不同间隔的Ia类传入纤维接收的突触输入进行比较。我们的结果表明，在MG运动核内，同名Ia类传入输入是“局部加权的”，使得支配给定肌内间隔的传入纤维在投射到同一间隔的运动神经元中比在其他同名运动神经元中发挥相对更大的突触作用。在不同的实验中，局部加权程度变化很大，但在高比例被研究的运动神经元具有高基强度值和低输入电阻特征的实验中更为明显。这表明局部因素对Ia类传入输入在大运动神经元中的分布可能比在小运动神经元中施加更大的影响。在讨论部分，将同名运动神经元池内的局部加权程度与协同运动神经元池之间的加权进行了比较，并提出和评估了局部加权的替代模型。
