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

Small animals make distress calls.

小动物发出求救信号。

耶鲁公开课 - 心理学导论课程节选