The autonomic nervous system maintains homeostasis through complementary sympathetic (arousal, metabolic mobilization) and parasympathetic (rest, conservation) divisions. Sympathetic activation increases heart rate, dilates pupils, inhibits digestion, and mobilizes glucose via norepinephrine and epinephrine. Parasympathetic activation decreases heart rate, promotes digestion, and activates bladder via acetylcholine. Most organs receive dual innervation, allowing coordinated control tailored to physiological demands.
From your study of neural anatomy and synaptic transmission, you already know the basic wiring: neurons release neurotransmitters that bind receptors, causing target cells to depolarize or hyperpolarize. The autonomic nervous system applies this machinery to involuntary control of the body's internal organs. What makes the ANS distinctive is its two-neuron chain. Rather than a single neuron running from the spinal cord to the target organ, the ANS uses a preganglionic neuron that synapses in a peripheral ganglion, where a postganglionic neuron then projects to the organ. This relay architecture allows divergence — one preganglionic neuron can branch to synapse onto many postganglionic neurons, enabling coordinated, body-wide responses.
The two divisions differ in both anatomy and chemistry. The sympathetic division has short preganglionic fibers (synapsing in paravertebral ganglia near the spine) and long postganglionic fibers that release norepinephrine onto target organs. The parasympathetic division has long preganglionic fibers (traveling all the way to ganglia embedded in or near target organs) and short postganglionic fibers that release acetylcholine. Both divisions use acetylcholine at the preganglionic synapse — it is the postganglionic transmitter that differs. This distinction is pharmacologically critical: drugs targeting adrenergic receptors (norepinephrine) selectively affect sympathetic end-organ effects, while muscarinic blockers (blocking ACh receptors) selectively affect parasympathetic effects.
The simplest organizing framework is "fight-or-flight" versus "rest-and-digest." Sympathetic activation prepares the body for action: heart rate and contractility increase (↑ cardiac output), bronchioles dilate (↑ airflow), pupils dilate (↑ visual field), blood is redirected from the gut to skeletal muscle, and the liver mobilizes glucose. Parasympathetic activation reverses these priorities: heart rate decreases, digestion is promoted (↑ peristalsis, ↑ secretion), glands secrete, the bladder contracts, and the pupils constrict. The mnemonic SLUDD captures the parasympathetic end-organ effects: Salivation, Lacrimation, Urination, Defecation, Digestion.
The functional significance of dual innervation is that most organs receive both sympathetic and parasympathetic input with opposing effects, allowing fine-tuned regulation. Consider the heart: sympathetic stimulation increases heart rate via β₁ adrenergic receptors; parasympathetic stimulation decreases it via muscarinic receptors on the SA node. At rest, parasympathetic tone dominates — which is why athletes with high vagal tone have slow resting heart rates. Under stress, sympathetic tone overrides this. This reciprocal arrangement is also why both overactivation and underactivation of either division can cause pathology: excessive sympathetic tone raises blood pressure and increases cardiovascular risk; loss of parasympathetic innervation to the GI tract produces ileus (bowel paralysis). The ANS is not simply an on/off switch but a continuously modulated dial with two opposing hands.