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How does the sympathetic nervous system produce the fight-or-flight response?

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Diagram of a human figure showing the full nervous system, including the brain, spinal cord, and branching peripheral nerves throughout the body, with labels for each part. — human nervous system The conduction of electrochemical stimuli from sensory receptors in the human nervous system occurs via organized groups of specialized cells, consisting largely of neurons, various neural support cells, and tracts of nerve fibers.

The sympathetic nervous system produces the fight-or-flight response by activating an immediate, widespread physiological adjustment to perceived threats. Anatomically, this process begins when preganglionic neurons—originating in the lateral horns of the thoracic and lumbar segments of the spinal cord—are stimulated.12

These neurons send signals that trigger the release of large quantities of epinephrine and norepinephrine from the adrenal gland (specifically the adrenal medulla). These hormones bind to alpha-adrenergic and beta-adrenergic receptors on target organs, resulting in systemic changes that prepare the body for action:314

Cardiovascular: Increased heart rate and cardiac output.31
Respiratory: Bronchial dilation to increase airflow.32
Muscular/Vascular: Skeletal muscle vasodilation alongside cutaneous and gastrointestinal vasoconstriction.12
Ocular: Pupillary dilation.31
Metabolic: Increased blood glucose concentrations to provide energy.3

This response is a critical survival mechanism, though chronic activation can lead to long-term health consequences such as hypertension and hyperglycemia.1