Department of Biomedical Engineering, Physical Medicine and Rehabilitation, Northwestern University, Feinberg School of Medicine, Chicago, IL 60611, USA.
J Neurophysiol. 2010 Oct;104(4):2158-68. doi: 10.1152/jn.00477.2010. Epub 2010 Jul 21.
In intact adult vertebrates, muscles can be activated with a high degree of specificity, so that even within a single traditionally defined muscle, groups of motor units can be differentially activated. Such differential activation might reflect detailed control by descending systems, potentially resulting from postnatal experience such that activation of motor units is precisely tailored to their mechanical actions. Here we examine the degree to which such specific activation can be seen in the rhythmic patterns produced by isolated spinal motor systems in neonates. We examined motor output produced by the in vitro neonatal rat spinal cord with hindlimb attached. We recorded the activity of different regions within the posterior portion of biceps femoris (BFp; i.e., excluding the anterior/vertebral head). We found that in the rhythms evoked by bath application of serotonin/N-methyl-d-aspartate (5-HT/NMDA), all regions of BFp were active during extension. However, the regions of BFp were activated in a specific sequence, with the activation of rostral regions consistently preceding those of more caudal regions in both afferented and deafferented preparations. In the rhythms evoked by cauda equina (CE) stimulation, rostral and middle regions of BFp remained active in extension, but the caudal region of BFp was usually active in flexion. Stimulation of L5 and S2 dorsal roots typically evoked rhythms with all regions of BFp active during extension; although the same rostral to caudal sequence of activation observed in 5-HT/NMDA evoked rhythms could also be observed in these rhythms, we also observed cases with reversed sequences, with activity proceeding from caudal to rostral. S2 dorsal root stimulation occasionally evoked rhythms with flexor activity in caudal BFp, similar to CE-evoked rhythms. Taken together, these results suggest a high degree of individuated control of muscles by spinal pattern generating networks, even at birth.
在完整的成年脊椎动物中，肌肉可以高度特异性地被激活，以至于即使在单个传统定义的肌肉中，运动单位群也可以被不同地激活。这种差异激活可能反映了来自于后天经验的下行系统的精细控制，使得运动单位的激活被精确地调整到它们的机械作用。在这里，我们研究了在新生动物离体脊髓运动系统产生的节律模式中，这种特定激活可以达到何种程度。我们检查了附着后肢的新生大鼠脊髓体外产生的运动输出。我们记录了二头肌股骨（BFp）后段（即不包括前/椎体头）不同区域的活动。我们发现，在 5-羟色胺/N-甲基-D-天冬氨酸（5-HT/NMDA）浴施加诱发的节律中，BFp 的所有区域在伸展时均活跃。然而，BFp 的区域以特定的顺序被激活，在传入和传出的制剂中，头部区域的激活始终先于更尾部区域的激活。在尾骨（CE）刺激诱发的节律中，BFp 的头部和中部区域在伸展时仍然活跃，但 BFp 的尾部区域通常在弯曲时活跃。刺激 L5 和 S2 背根通常会诱发所有 BFp 区域在伸展时活跃的节律；尽管在 5-HT/NMDA 诱发的节律中也可以观察到相同的从头部到尾部的激活顺序，但我们也观察到了相反顺序的情况，即从尾部到头部的激活。偶尔刺激 S2 背根会诱发尾部 BFp 有屈肌活动的节律，类似于 CE 诱发的节律。总的来说，这些结果表明，即使在出生时，脊髓模式生成网络也能对肌肉进行高度个体化的控制。
