Neurons are concentrated in clusters with defined nerve tracts leading in and out. 神经元聚集成束，束定义为引进引出的神经束。
The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. 中脑核包括四条运动束，用以传送上位运动神经元的轴突经脊髓至下运动神经元。
