What physiological response is initiated first in reaction to stress?

The physiological response that is initiated first in reaction to stress is the nervous response. When an individual perceives a stressor, the body reacts almost instantaneously through the activation of the autonomic nervous system, particularly the sympathetic branch. This response leads to the "fight or flight" reaction, where various physiological changes occur, including increased heart rate, heightened alertness, and the release of adrenaline.

This immediate response prepares the body to either confront or flee from the threat, making it the fastest mechanism for dealing with stress. While hormonal and other responses also play significant roles in the overall stress response, they typically follow the initial nervous response. Behavioral and cognitive responses occur as the individual processes the stressor and decides how to act, but they are not the first physiological reactions to take place.