It has to be small, and it has to be lipid soluble in order for it to pass through a cell membrane.

这些分子必须非常小,而且必须是脂溶性的,才能穿过细胞膜

耶鲁公开课 - 生物医学工程探索课程节选

They're small molecules that - whose principal function in the body is to carry signals from one cell in the nervous system to another.

它们都是小分子,在体内主要功能是在,神经系统内将信号,从一个细胞传导到另一个细胞

耶鲁公开课 - 生物医学工程探索课程节选