The sympathetic nervous system is one of the two main divisions of the autonomic nervous system, along with the parasympathetic nervous system. It prepares the body for action in stressful or emergency situations—the so-called fight-or-flight response.

When activated, it increases heart rate and blood pressure, dilates the pupils and airways, redirects blood flow to skeletal muscles, releases glucose from the liver, and inhibits digestion and salivation. These effects are mediated by norepinephrine and epinephrine acting on adrenergic receptors.

Effects of activation

• Increased heart rate and blood pressure
• Dilated pupils and bronchi
• Release of glucose for energy
• Suppressed digestion and urination

Chronic activation of the sympathetic nervous system is associated with anxiety, hypertension, and stress-related illness.