Sympathetic Physiology

1. Thoracolumbar Origin and Ganglionic Anatomy

Sympathetic preganglionic cell bodies sit in the lateral horns of spinal cord segments T1–L2 — the thoracolumbar origin. Their short myelinated axons exit through ventral roots to two kinds of ganglia. Sympathetic chain ganglia (paravertebral) form a paired vertical ladder beside the vertebral column where most preganglionic fibers synapse. Prevertebral ganglia (celiac, superior mesenteric, inferior mesenteric) lie in front of the spine near major abdominal arteries and receive fibers that pass through the chain without synapsing — which is why some sympathetic fibers serve the body wall and limbs while others reach abdominal and pelvic organs.

2. The Two-Neuron Pathway and Its Chemistry

Every sympathetic pathway uses acetylcholine at the first synapse: the preganglionic neuron releases ACh onto nicotinic receptors on the postganglionic neuron. At the second synapse, most postganglionic neurons release norepinephrine onto adrenergic receptors — cholinergic at the ganglion, adrenergic at the effector. Two exceptions matter: sweat-gland fibers are sympathetic but postganglionically cholinergic (ACh onto muscarinic receptors), and the adrenal medulla has no separate postganglionic neuron at all.

3. The Adrenal Medulla

The adrenal medulla is a specialized sympathetic ganglion whose "postganglionic neurons" are modified endocrine cells called chromaffin cells. Sympathetic preganglionic fibers synapse directly onto these cells and, via ACh on nicotinic receptors, trigger secretion of the catecholamines epinephrine (~80%) and norepinephrine (~20%) into the blood. Because hormones travel everywhere, this converts a neural signal into a body-wide, longer-lasting hormonal signal — why an adrenaline surge outlasts the nerve impulse that caused it.

4. Adrenergic Receptors and Tissue-Specific Effects

Different tissues express different adrenergic receptor subtypes, so one signal can do opposite things in different organs:

• Alpha-1 (α1): contracts vascular smooth muscle (vasoconstriction, raising blood pressure), dilates the pupil, contracts sphincters.
• Alpha-2 (α2): presynaptically inhibits further norepinephrine release (a brake on the system); some postsynaptic α2 also constricts vessels.
• Beta-1 (β1): on the heart, increases heart rate and contractility.
• Beta-2 (β2): relaxes airway smooth muscle (bronchodilation) and dilates vessels in skeletal muscle and liver.
• Beta-3 (β3): stimulates lipolysis in adipose tissue and relaxes the bladder wall.

Norepinephrine binds all of these but has greatest affinity for alphas and beta-1; epinephrine strongly activates beta-2 and beta-3 as well, producing the classic bronchodilation and metabolic effects.

How it works

1. Preganglionic neurons fire from the thoracolumbar cord.
2. Acetylcholine excites postganglionic neurons at chain or prevertebral ganglia.
3. Postganglionic neurons release norepinephrine (sweat glands release ACh).
4. Norepinephrine binds the tissue's adrenergic receptor subtype.
5. The tissue responds per its receptor.
6. Adrenal epinephrine spreads and sustains the response.

Common confusions

Memory aids

"Beta-1 = one heart, Beta-2 = two lungs." For the alphas, remember "Alpha-1 = artery (constrict), Alpha-2 = auto-brake" — α1 tightens vessels and α2 puts the brakes on further release.

Quick review

Topic Recap

The sympathetic division is the thoracolumbar "fight-or-flight" system. Preganglionic neurons from T1–L2 release acetylcholine at chain or prevertebral ganglia; postganglionic neurons then release norepinephrine at effectors, while the adrenal medulla releases epinephrine and norepinephrine into the blood as circulating hormones. Five adrenergic receptor subtypes — α1 (vasoconstriction, pupil dilation), α2 (autoinhibition), β1 (cardiac stimulation), β2 (bronchodilation, vessel dilation), and β3 (lipolysis, bladder relaxation) — distribute the signal into tissue-specific effects, all running on a baseline of autonomic tone.

Knowledge Check

1. From which spinal cord segments do sympathetic preganglionic neurons originate?
2. Which neurotransmitter is used at the sympathetic ganglion, and which at most effector organs?
3. What is the adrenal medulla, and what does it secrete?
4. Which receptor subtype increases heart rate and contractility, and which dilates airways?
5. Why can a single sympathetic signal constrict skin vessels while dilating skeletal-muscle vessels?

Answers and Rationales

1. T1 through L2 — the thoracolumbar origin, matching the lateral-horn cell bodies.
2. Acetylcholine at the ganglion (nicotinic) and norepinephrine at most effectors (adrenergic), with sweat glands as the cholinergic exception.
3. The adrenal medulla is a modified sympathetic ganglion whose chromaffin cells secrete epinephrine (~80%) and norepinephrine (~20%) into the blood when stimulated by preganglionic ACh.
4. Beta-1 increases heart rate and contractility; beta-2 relaxes airway smooth muscle (bronchodilation).
5. Because the tissues express different receptor subtypes — α1 on skin vessels causes constriction, β2 on skeletal-muscle vessels causes dilation — despite the same norepinephrine/epinephrine reaching both.