Tracts的用法和样例：
例句
- 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。
