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The autonomic nervous system divides into sympathetic and parasympathetic divisions that regulate opposing functions. The sympathetic division prepares the body for fight-or-flight responses by elevating heart rate, metabolism, and mental alertness while suppressing digestion. The parasympathetic division promotes rest-and-digest activities, stimulating digestion and urination while decreasing heart rate and metabolism.

The sympathetic division triggers the fight-or-flight response during stress or danger. It increases heart rate, dilates blood vessels, and releases stress hormones like adrenaline and norepinephrine. These changes enhance mental alertness and physical readiness while temporarily suppressing digestive and urinary functions to redirect resources toward survival.

The parasympathetic division promotes rest-and-digest responses that help the body recover and conserve energy. It slows heart rate, decreases respiratory rates, and enhances digestive processes. This division counterbalances sympathetic activity, restoring the body to a calm state and maintaining homeostasis after stress subsides.

The ANS employs preganglionic and postganglionic neurons to transmit signals. The lightly myelinated preganglionic neuron originates in the brainstem or spinal cord and carries impulses to autonomic ganglia or the adrenal medulla. The unmyelinated postganglionic neuron then carries impulses from the ganglion to target muscles or glands.

Autonomic tone refers to the prevailing influence of both sympathetic and parasympathetic divisions on organ systems. This balance is crucial for optimal organ function and homeostasis. The hypothalamus regulates autonomic tone by integrating signals and modulating the balance between sympathetic and parasympathetic outputs to meet changing physiological demands.

During high-stress situations, the sympathetic division dilates pupils for better vision, increases heart rate, widens airways, and suppresses digestion. Once the crisis subsides, the parasympathetic division restores normalcy by constricting pupils, slowing heart rate, normalizing respiratory rates, and promoting digestive activities to return the body to baseline.

Norepinephrine is the primary neurotransmitter in sympathetic activation, functioning both at synapses and as a hormone in the bloodstream. This dual role ensures a comprehensive and sustained fight-or-flight response during stress. Norepinephrine works alongside adrenaline to elevate heart rate, increase metabolism, and enhance mental alertness.