- The 'fight or flight' reaction is primarily mediated by the sympathetic nervous system, which is a part of the autonomic nervous system responsible for preparing the body to respond to stress or danger.
- When activated, the sympathetic nervous system triggers a series of physiological changes:

- Increased heart rate and blood pressure to enhance blood flow to muscles
- Dilation of bronchioles to improve oxygen intake
- Release of glucose from energy stores to provide immediate energy
- Redistribution of blood flow away from non-essential organs to the skeletal muscles

In contrast, the parasympathetic nervous system (Option 1) primarily facilitates "rest and digest" activities, which are opposite to the 'fight or flight' response.

The somatic nervous system (Option 3) controls voluntary movements of skeletal muscles but does not mediate autonomic stress responses.

Finally, while the hypothalamic-pituitary-adrenal (HPA) axis (Option 4) contributes to the longer-term stress response by releasing cortisol, it is not the primary driver of the immediate physiological changes seen in the 'fight or flight' reaction.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 71: The Autonomic Nervous System and the Adrenal Medulla