Spark

by John J. Ratey

The amygdala connects to many parts of the brain and thus receives a wide array of input—some of it routed through the high-level processing center of the prefrontal cortex, and some of it wired indirectly, bypassing the cortex, which explains how even a subconscious perception or memory can trigger a stress response. Within ten milliseconds of sounding the alarm, the amygdala fires off messages that cause the adrenal gland to release different hormones at different stages. First, norepinephrine triggers lightning-fast electrical impulses that travel through the sympathetic nervous system activate the adrenal gland to dump the hormone epinephrine, or adrenaline, into the bloodstream. Heart rate, blood pressure, and breathing increase, contributing to the physical agitation we feel under stress. At the same time, signals carried by norepinephrine and corticotropin-releasing factor (CRF) travel from the amygdala to the hypothalamus, where they are handed off to messengers that take the slow train through the bloodstream. These messengers prompt the pituitary gland to activate another part of the adrenal gland, which releases the second major hormone of the stress response: cortisol. This relay from the hypothalamus to the pituitary to the adrenal gland is known as the HPA axis, and its role in summoning cortisol and in turning off the response makes it a key player in the story of stress. Meanwhile, the amygdala has signaled the hippocampus to start recording memories and an...