Mitchelle Amer, Watson Charles
Faculty of Medicine, Notre Dame University, Perth, WA, Australia.
Faculty of Health Sciences, Curtin University, Perth, WA, Australia.
J Anat. 2016 Sep;229(3):394-405. doi: 10.1111/joa.12492. Epub 2016 May 12.
The motor neurons in the spinal cord of an echidna (Tachyglossus aculeatus) have been mapped in Nissl-stained sections from spinal cord segments defined by spinal nerve anatomy. A medial motor column of motor neurons is found at all spinal cord levels, and a hypaxial column is found at most levels. The organization of the motor neuron clusters in the lateral motor column of the brachial (C5 to T3) and crural (L2 to S3) limb enlargements is very similar to the pattern previously revealed by retrograde tracing in placental mammals, and the motor neuron clusters have been tentatively identified according to the muscle groups they are likely to supply. The region separating the two limb enlargements (T4 to L1) contains preganglionic motor neurons that appear to represent the spinal sympathetic outflow. Immediately caudal to the crural limb enlargement is a short column of preganglionic motor neurons (S3 to S4), which it is believed represents the pelvic parasympathetic outflow. The rostral and caudal ends of the spinal cord contain neither a lateral motor column nor a preganglionic column. Branchial motor neurons (which are believed to supply the sternomastoid and trapezius muscles) are present at the lateral margin of the ventral horn in rostral cervical segments (C2-C4). These same segments contain the phrenic nucleus, which belongs to the hypaxial column. The presence or absence of the main spinal motor neuron columns in the different regions echidna spinal cord (and also in that of other amniote vertebrates) provides a basis for dividing the spinal cord into six main regions - prebrachial, brachial, postbrachial, crural, postcrural and caudal. The considerable biological and functional significance of this subdivision pattern is supported by recent studies on spinal cord hox gene expression in chicks and mice. On the other hand, the familiar 'segments' of the spinal cord are defined only by the anatomy of adjacent vertebrae, and are not demarcated by intrinsic gene expression. The recognition of segments defined by vertebrae (somites) is obviously of great value in defining topography, but the emphasis on such segments obscures the underlying evolutionary reality of a spinal cord comprised of six genetically defined regions. The six-region system can be usefully applied to the spinal cord of any amniote (and probably most anurans), independent of the number of vertebral segments in each part of the spinal column.
已在针鼹（短吻针鼹）脊髓的尼氏染色切片中绘制出运动神经元图，这些切片来自根据脊神经解剖结构定义的脊髓节段。在脊髓的所有节段都发现了一个内侧运动神经元柱，并且在大多数节段发现了一个轴下柱。在臂部（C5至T3）和腿部（L2至S3）肢体膨大的外侧运动柱中，运动神经元簇的组织与先前在胎盘哺乳动物中通过逆行追踪揭示的模式非常相似，并且已根据它们可能支配的肌肉群初步确定了运动神经元簇。分隔两个肢体膨大的区域（T4至L1）包含节前运动神经元，这些神经元似乎代表脊髓交感神经输出。在腿部肢体膨大的紧尾端是一小列节前运动神经元（S3至S4），据信它代表盆腔副交感神经输出。脊髓的头端和尾端既不包含外侧运动柱也不包含节前柱。鳃运动神经元（据信支配胸锁乳突肌和斜方肌）存在于颈前部节段（C2 - C4）腹角的外侧边缘。这些相同的节段包含膈核，它属于轴下柱。针鼹脊髓不同区域（以及其他羊膜脊椎动物的脊髓）中主要脊髓运动神经元柱的存在与否为将脊髓分为六个主要区域——臂前区、臂区、臂后区、腿区、腿后区和尾区提供了基础。最近对鸡和小鼠脊髓hox基因表达的研究支持了这种细分模式具有相当大的生物学和功能意义。另一方面，脊髓熟悉的“节段”仅由相邻椎骨的解剖结构定义，而不由内在基因表达划分。由椎骨（体节）定义的节段的识别在定义地形方面显然具有很大价值，但对这些节段的强调掩盖了由六个基因定义区域组成的脊髓的潜在进化现实。六区域系统可有效地应用于任何羊膜动物（可能还有大多数无尾两栖动物）的脊髓，而与脊柱各部分椎骨节段的数量无关。
