The connections and monosynaptic projections of muscle spindle afferents of individual heads of the longissimus lumborum have been studied in cats by natural stimulation, by electrical stimulation and by spike-triggered averaging from single identified afferents. The spindle afferents were classified by sensitivity to vibration and by the effect of succinylcholine on their response to ramp-and-hold muscle stretches. Axonal conduction and synaptic effects were recorded as field potentials and focal synaptic potentials during systematic exploration of the spinal cord in segments L1 to L4 with extracellular metal microelectrodes, singly and in linear arrays. Ascending branches of afferent axons within the cord had a significantly higher mean conduction velocity (CV: 56.5 m s−1) than descending branches (40.8 m s−1). The CV of ascending branches was significantly positively correlated with a measure of the strength of intrafusal bag2 muscle fibre contacts, but not to a measure of bag1 contacts. Two sites of monosynaptic excitatory projection in the cord were identified, namely to the intermediate region (laminae V, VI and VII) and to ventral horn region (laminae VIII and IX). In tests of 154 single afferents, signs of central projection were detected for 60, providing 122 regions of maximum negative focal synaptic potentials (FSPs) of mean amplitude 7.51 μV. Their longitudinal spacing indicated that axons gave off descending collaterals at intervals of 1.5–3.5 mm. Based on the amplitude of FSPs, the projection of secondary afferents is stronger than that of primaries in the intermediate region and possibly also in the ventral horn region. Evidence is also presented that spindle afferent input from different heads of the longissimus converges into any given spinal segment and that input in one spinal root projects to adjacent segments. It is concluded that the organization of the longissimus monosynaptic spindle input favours relatively tonic and diffuse stretch reflexes.
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机译：在猫中，通过自然刺激，电刺激以及通过从单个识别出的传入信号进行尖峰触发平均，研究了腰最长肌单个头部的肌肉纺锤传入神经的连接和单突触投射。根据对振动的敏感性和琥珀酰胆碱对其对斜向并按住的肌肉伸展的反应的影响，对纺锤传入的细胞进行分类。在利用细胞外金属微电极对L1至L4节段的脊髓进行系统性探索时，将轴突传导和突触效应分别记录为场电势和局灶性突触电势，并以线性阵列方式记录。脐带内传入轴突的上升分支的平均传导速度（CV：56.5 m s -1 ）明显高于下降分支（40.8 m s -1 ）。升支的CV与融合内bag2肌纤维接触强度的测量值呈显着正相关，但与bag1接触量的测量值则无显着正相关。脐带中有两个单突触兴奋性投射部位，分别是中间区域（椎板V，VI和VII）和腹角区域（椎板VIII和IX）。在154个单传入的测试中，检测到了60个中心投射的信号，提供了122个最大振幅为平均振幅7.51μV的负焦突触电位（FSP）区域。它们的纵向间隔表明，轴突以1.5-3.5 mm的间隔释放下降的侧支。基于FSP的振幅，在中间区域甚至在腹角区域，次要传入的投影要比主要传入的投影更强。也有证据表明，来自最长肌不同头的纺锤体传入输入会收敛到任何给定的脊柱节段，并且一个脊椎根部的输入会投射到相邻节段。结论是，最长的单突触主轴输入的组织有利于相对强直和弥散性的伸展反射。
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