downward 怎么理解？

Pharmacodynamics of drugs, that is their binding affinities to receptors, channels, or other sites of action and subsequent downward cascades are much more difficult to investigate than pharmacokinetics. Changes occurring at the levels of receptors, channels, and functional connectivity have been outlined by Akhtar [5 &&]. Briefly, aging is accompanied by a decreased number of synapses in the brain and their binding function and  downward  receptor signaling. This explains, in part, for example, the higher sensitivity to benzodiazepines, propofol, and opioids. As early as 2001, Eger showed a clear correlation between age and the minimum alveolar concentration of volatile anesthetics [23]. This fact may be explained by progressive, age-associated changes in the functional connectivity of the brain [5 &&]. As the functional connectivity of the aging brain seems to be particularly sensitive, the influence of deep anesthesia on the increased risk for postoperative delirium may be associated with this effect [5 &&].

药效学（即药物与受体、通道或其他作用位点的结合能力以及随后的向下级联反应）比药代动力学更难研究。Akhtar[5]概述了受体、通道和功能连接水平的变化。简而言之，衰老伴随着大脑中突触数量及其结合功能和向下的受体信号的减少。这在一定程度上解释了老年患者对苯二氮卓类、丙泊酚和阿片类药物较高的敏感性。早在2001年，Eger就证明了年龄与吸入麻醉药的最小肺泡浓度之间的明确相关性[23]。这一变化可以通过大脑功能连接的进行性、年龄相关的改变来解释[5]。由于衰老大脑的功能连接似乎特别敏感，深度麻醉对术后谵妄风险增加的影响可能与该效应相关[5]。

downward  是不是就是 decreased？谢谢