It predicts that the best timing is when the conditioned stimulus, which is the signal, comes before the unconditioned stimulus, which is what you have to prepare for.

它预测，最佳的时机,莫过于作为信号的条件刺激，出现在需要你做准备的，非条件刺激之前。

耶鲁公开课 - 心理学导论课程节选

Now, how they carry signals is that these neurotransmitters act as ligands.

它们传递信号的原理如下,神经递质作为一种配体

耶鲁公开课 - 生物医学工程探索课程节选