Davies L, Wiegner A W, Young R R
Spinal Cord Injury Service, Brockton/West Roxbury Veterans Administration Medical Center, MA 02132.
Brain Res. 1993 Jan 29;602(1):104-10. doi: 10.1016/0006-8993(93)90248-l.
The activation of motoneurons in a muscle pool is said to proceed as an ordered array in response to both dorsal root stimulation and voluntary activation, with small motoneurons being recruited before larger ones. We have examined 19 voluntarily recruited soleus motor units in 5 normal subjects and found that in 18 cases, the lowest threshold motor unit recruited by slowly increasing 'tonic' voluntary activity was different from the lowest threshold unit recruited by electrical stimulation of the tibial nerve in the popliteal fossa (phasic Hoffman reflex). The initially recruited voluntary units were, however, part of the pool influenced by the stimulated afferents because, during tonic activation, the timing of their discharge could be shown to be altered by electrical stimulation at a lower intensity than that required for H recruitment at rest. These findings suggest that the pool of soleus motoneurons responding to the voluntary command "tonically plantar flex your ankle" differs somewhat, in order of activation, from the pool responding to phasic stimulation of the largest diameter fibers in the tibial nerve, perhaps because of inhomogeneities in the distribution of descending or segmental inputs to the soleus motoneuron pool. Whether this partitioning is functional or a reflection of minor, random variations in synaptic density remains to be determined.
据说，肌肉运动神经元池的激活是按照有序的序列进行的，以响应背根刺激和自主激活，小运动神经元比大运动神经元更早被募集。我们检查了5名正常受试者中19个自主募集的比目鱼肌运动单位，发现在18例中，通过缓慢增加“紧张性”自主活动募集的最低阈值运动单位，与通过刺激腘窝处胫神经（相位性霍夫曼反射）募集的最低阈值单位不同。然而，最初募集的自主单位是受刺激传入神经影响的运动神经元池的一部分，因为在紧张性激活期间，其放电时间可以通过低于静息时H反射募集所需强度的电刺激而改变。这些发现表明，响应“紧张性地跖屈踝关节”自主指令的比目鱼肌运动神经元池，在激活顺序上与响应胫神经最大直径纤维相位性刺激的运动神经元池有所不同，这可能是由于比目鱼肌运动神经元池的下行或节段性输入分布不均匀所致。这种划分是功能性的，还是突触密度微小随机变化的反映，仍有待确定。
