In addition to the effects of calcium and cortisol, the effects of norepinephrine (NE), a neurotransmitter essential to SNS activation, contribute to the apparent ubiquity of PTSD alterations. Hence, every time re-experiencing or reminders occur, the SNS system fires up and the startle response is strengthened. The PTSD startle is so strong and sensitive that it frequently does not habituate (become tolerable) (Garrick, Morrow, Eth, Marciano, & Shalev, 1997; Orr, 1997). This lack of habituation, however, does not quite account for the incidences in which intrusive recollections and arousal symptoms on the one hand and the avoidant symptoms on the other have been observed to coexist and to alternate. Antelman et al. (1997) suggest that the seeming paradox of concurrent and alternating intrusive and avoidant symptoms can be explained through the concept of a bi-directional (and oscillating) sensitization mechanism. They argue that the multiple neurobiological systems involved in PTSD become sensitized by stimuli at different rates (yielding arousal symptoms) and then, each system having reached its threshold for arousal tolerance, oscillate or cycle off (yielding avoidance symptoms).