Alterations in cardiac demand due to increased metabolic rate in a cold environment result in increased NP levels. NP signals via PKG to increase p38 MAPK and thus induce thermogenic machinery. As this bypasses the membrane adrenergic receptors and PKA, modulation of acute activity by the SNS is still possible but operates in the context of any chronic changes to basal thermogenic activity and capacity. A comparison could be made with a stereo system, where the volume dial is the adrenergic axis and the speaker size relates to the amount of p38 activation by other molecules such as NP. If the size of the speakers is increased tenfold, the stereo has a much higher maximum volume, but it is not necessary to turn the volume dial 10 more times to reach it.