Seebach B S, Ziskind-Conhaim L
Department of Physiology, University of Wisconsin Medical School, Madison 53706.
J Neurosci. 1994 Jul;14(7):4520-8. doi: 10.1523/JNEUROSCI.14-07-04520.1994.
The specificity of the convergence of primary afferent projections from ankle muscles onto motoneurons that innervate these muscles was studied in lumbar spinal cords of embryonic and neonatal rats. The connectivity pattern was determined for each motoneuron by stimulating nerves from ankle flexor and extensor muscles and recording the synaptic potentials in identified motoneurons. In mature mammals, muscle spindle afferents make direct excitatory connections with motoneurons that innervate homonymous and synergistic muscles, and with interneurons that inhibit motoneurons innervating antagonistic muscles. Therefore, appropriate primary afferent-motoneuron connections were identified when stimulation of homonymous and synergistic muscle nerves evoked monosynaptic EPSPs. Two criteria were used for identification of EPSPs as monosynaptic potentials: (1) the monosynaptic potentials were evoked at the shortest latency, and (2) they were more resistant to fatigue by repetitive nerve stimulation than the longer-latency, polysynaptic potentials. Functionally inappropriate primary afferent-motoneuron contacts were identified when stimulation of an antagonistic muscle nerve produced monosynaptic EPSPs instead of polysynaptic IPSPs in homonymous motoneurons. At days 18-21 of gestation, about 30% of motoneurons were innervated by primary afferents of antagonist muscles. Such functionally inappropriate synapses persisted at birth, but their percentage was significantly reduced within 3-5 d after birth. The findings suggested that in the developing spinal cord of the rat, a significant percentage of motoneurons were initially innervated by inappropriate primary afferents of antagonistic muscles. The decrease in percentage of such inappropriate connections was correlated temporally with the increase in the frequency of spontaneous activity and the onset of myelination.
在胚胎期和新生大鼠的腰脊髓中，研究了来自踝关节肌肉的初级传入投射汇聚到支配这些肌肉的运动神经元上的特异性。通过刺激踝关节屈肌和伸肌的神经，并记录已识别运动神经元中的突触电位，来确定每个运动神经元的连接模式。在成熟哺乳动物中，肌梭传入纤维与支配同名和协同肌肉的运动神经元以及抑制支配拮抗肌的运动神经元的中间神经元形成直接的兴奋性连接。因此，当刺激同名和协同肌肉神经诱发单突触兴奋性突触后电位（EPSP）时，可识别出适当的初级传入-运动神经元连接。将EPSP识别为单突触电位使用了两个标准：（1）单突触电位在最短潜伏期诱发，（2）与潜伏期较长的多突触电位相比，它们对重复神经刺激的疲劳更具抵抗力。当刺激拮抗肌神经在同名运动神经元中产生单突触EPSP而不是多突触抑制性突触后电位（IPSP）时，可识别出功能上不适当的初级传入-运动神经元联系。在妊娠第18-21天，约30%的运动神经元由拮抗肌的初级传入纤维支配。这种功能上不适当的突触在出生时持续存在，但在出生后3-5天内其比例显著降低。这些发现表明，在大鼠发育中的脊髓中，相当比例的运动神经元最初由拮抗肌的不适当初级传入纤维支配。这种不适当连接比例的下降在时间上与自发活动频率的增加和髓鞘形成的开始相关。
