Compensatory activation of alternative effectors, such as the adrenomedullary hormonal system, arginine vasopressin system, and renin-angiotensin-aldosterone system explains why destruction of the sympathetic nervous sytem produces only relatively small influence on blood pressure. This misled Cannon into thinking that the sympathetic nervous system was unimportant in the regulation of blood pressure in organisms at rest...
In the 1980s, however, several reports showed that sympathectomy compensatorily activates other effectors...and compensatory activation of these effectors maintains blood pressure at approximately normal levels. In the setting of sympathectomy, interference with any of the other effectors evokes immediate, precipitous declines in blood pressure...
It also took decades after Cannon's death before researchers began to demonstrate effects on 'nonemergency' stressors, such as mental challenge and exercise, on sympathetic outflow to the heart. One index of this outflow is the rate of entry of norepinephrine, the main neurotransmitter released by the sympathetic nerves, into the veins draining the heart (norepinephrine spillover).
David S. Goldstein: Adrenaline and the Inner World: An Introduction to Scientific Integrative Medicine.