Important note: The PSM relates the dose–response curve for a single pulse (SP) to the dose–response curve for MPs, and this paper contains a great deal of discussion about dose–response curves and their shape. One very important property of a dose–response curve is its “sharpness,” i.e., the difference between a dose for which damage is virtually impossible and a dose for which damage is virtually guaranteed. Unfortunately, there are two quantities that are in common use to characterize this property, and both are often called “slope.” The first definition, which is used in this paper and denoted as SL, refers to the slope of the best fit line obtained with probit analysis. The second definition is the ratio of the $ED84$ over the $ED50$ ($s=ED84/ED50$). For our definition, a “large slope” corresponds to a steep dose–response curve. For example, in Fig. 1, the slope of the dose–response curves is increasing from right to left. For log-normal dose–response curves, the two are directly related and one can be obtained from the other $SL=1log(ED84/ED50)$.^{4}