What Triggers the Stress Response?
Any action of a stressor on the body initiates two parallel, closely interconnected processes:
- Mobilization of physiological mechanisms. They organize into new functional systems whose task is to provide specific adaptation to a particular stimulus. This can be a sharp drop in temperature (cold), lack of oxygen (hypoxia), heavy physical exertion, or exposure to toxins.
- Activation of nonspecific reactions. These are standard, universal processes that develop in response to absolutely any unusual exposure. This aggregate of nonspecific changes constitutes the true essence of the stress response.
Role of Stress and Possible Outcomes
The stress response plays a fundamental role in survival. It acts as an obligatory and inevitable link in the process of urgent adaptation to any extraordinary factor. Further adaptation is impossible without this stage: stress always precedes the formation of the sustained resistance stage and serves as its primary trigger mechanism.
The subsequent course of events depends on the success of adaptation and has two main outcomes:
- Favorable outcome: if the body manages to develop sustained resistance, all resulting homeostatic disturbances are successfully eliminated. At this point, the biological goal is achieved, and the stress response completely ceases.
- Unfavorable outcome: if resistance fails to form for any reason, deviations in the internal environment not only persist but continue to escalate. In this case, the stress state becomes prolonged.
Three Stages of the Adaptation Syndrome
The development of the general adaptation syndrome is a strictly programmed sequence of events. The stress response is conventionally divided into three consistent stages:
- Alarm reaction (Alarm reaction) — primary general response and mobilization of resources.
- Stage of resistance — period of maximal resistance to the damaging factor.
- Stage of exhaustion — failure of adaptation mechanisms under excessive exposure.
Alarm Reaction in Detail
The first phase of stress represents the stage of emergency adaptation. Its core essence lies in the lightning-fast mobilization of all the body's protective reserves to counter the extraordinary factor.
Trigger factors of the alarm stage:
- Direct impact of the extreme stimulus itself, which severely disrupts homeostasis (acute pain, critical cooling, hypoxia, barometric pressure changes — hypobaria or hyperbaria).
- Secondary deviation of key homeostatic parameters from the physiological norm. This includes a drop or rise in oxygen partial pressure (pO2), acid-base balance (pH) shifts, sharp blood pressure (BP) fluctuations, changes in circulating blood volume (CBV), and body temperature.
Mechanism of development:
- The factor's impact causes a sharp increase in the flow of afferent signals into the central nervous system.
- These signals radically alter the current activity of cortical and subcortical regulation centers.
- An urgent formation of a new efferent signaling program, adequate to the emerged threat, occurs in the nerve centers.
- The ready program is immediately implemented in the periphery with the active involvement of both neural and humoral regulation mechanisms.