Controls involuntary functions (heart rate, digestion, pupils). Sympathetic: fight-or-flight (noradrenaline). Parasympathetic: rest-digest (acetylcholine). Opposing effects on most organs; coordinated by brainstem/hypothalamus.
Most of what your nervous system does, you never think about. Your heart beats, your pupils adjust to light, your gut churns through a meal, your blood vessels dilate and constrict — all without any conscious input. The autonomic nervous system (ANS) governs this involuntary physiology, and it does so through two complementary branches with opposing effects on almost every organ they innervate.
You are already familiar with synaptic transmission: a presynaptic neuron releases a neurotransmitter that binds receptors on a postsynaptic cell, producing either excitation or inhibition. The ANS uses this same machinery, but the circuit has two neurons in series rather than one. A preganglionic neuron in the brainstem or spinal cord synapses onto a postganglionic neuron in a ganglion (a cluster of neurons outside the central nervous system), which then projects to the target organ. Crucially, both the sympathetic and parasympathetic divisions use acetylcholine at this first (ganglionic) synapse. The divisions diverge at the second synapse — where the postganglionic neuron meets the target organ.
The sympathetic division prepares the body for action. Its postganglionic neurons release noradrenaline onto target tissues, activating receptors that increase heart rate and contractile force, dilate pupils and airways, constrict blood vessels in the gut (redirecting blood to muscle), and slow digestion. The sympathetic division is also the stimulus for the adrenal medulla, which releases adrenaline (epinephrine) directly into the bloodstream — amplifying and prolonging the fight-or-flight response beyond what local noradrenaline release alone could sustain. The cell bodies of preganglionic sympathetic neurons are located in the thoracic and lumbar spinal cord (the "thoracolumbar" outflow), and their ganglia lie in chains close to the spinal column, giving each preganglionic neuron access to many postganglionic targets simultaneously.
The parasympathetic division promotes the rest-and-digest state. Its postganglionic neurons release acetylcholine onto target organs, activating muscarinic receptors that slow heart rate, constrict pupils, stimulate digestion and glandular secretion, and relax airways. Parasympathetic preganglionic neurons originate in the brainstem (via cranial nerves, especially the vagus nerve, which reaches most thoracic and abdominal organs) and in the sacral spinal cord — the "craniosacral" outflow. Unlike sympathetic ganglia, parasympathetic ganglia are located close to or within the target organs, giving each preganglionic fiber a more targeted, less broadcast effect.
Both divisions are tonically active at rest, and most organs are under the simultaneous influence of both. This dual innervation allows continuous, graded control. Resting heart rate, for example, is largely held below the heart's intrinsic pacemaker rate by ongoing parasympathetic (vagal) tone — withdraw the vagal brake and heart rate rises even without increasing sympathetic input. The brainstem and hypothalamus integrate sensory information about the body's internal state and orchestrate the relative activity of both divisions to maintain homeostasis, adjusting the balance moment to moment in response to exercise, stress, digestion, and posture.