Sympathetic and parasympathetic divisions: Notes

Sympathetic vs parasympathetic anatomy comes down to three questions: where the neurons leave the central nervous system, where they stop to relay, and how far they reach. This page answers each one. You will follow the two-neuron chain that every autonomic signal travels, trace the sympathetic division from the thoracic and lumbar spinal cord through the sympathetic chain and the collateral ganglia, see why the adrenal medulla counts as part of that division, and then follow the parasympathetic division out of the brainstem and sacral cord to ganglia sitting on the organs themselves.

The two-neuron chain

Touch a hot pan and pull your hand back. One motor neuron runs from your spinal cord all the way to the muscle in your arm. Now think about the sweat that breaks out on your palm a moment later. That signal also starts in your spinal cord, but it changes neurons partway out.

That is the defining feature of the autonomic motor pathway: two neurons in series between the central nervous system (CNS) and the effector. They meet at an autonomic ganglion (ganglion = knot or swelling), a cluster of neuron cell bodies outside the CNS.

The preganglionic neuron (pre- = before) has its cell body in the brainstem or spinal cord. Its axon, the preganglionic fiber, leaves the CNS and ends in an autonomic ganglion.
The postganglionic neuron (post- = after) has its cell body in that ganglion. Its axon, the postganglionic fiber, runs on to the effector: cardiac muscle, smooth muscle or a gland.

Preganglionic fibers are thinly myelinated. Postganglionic fibers are unmyelinated, so they conduct more slowly. Figure 1 shows the chain for both autonomic divisions.

Figure 1. The two-neuron chain. Somatic motor output uses one neuron. Both autonomic divisions use two, and they differ in where the ganglion sits: near the spinal cord for the sympathetic division, on or inside the target organ for the parasympathetic division.

Somatic versus autonomic motor pathways

You met the somatic and autonomic nervous systems as the two branches of the motor division. Their wiring differs at every step, as Figure 2 shows side by side.

Two pathways from a spinal cord cross section. Top, the autonomic efferent pathway: a myelinated axon runs from the lateral horn to a ganglion, where it synapses on a second neuron whose unmyelinated axon runs on to a curved strip of smooth muscle; an enlarged inset shows a swelling along that axon releasing transmitter onto the muscle surface. Bottom, the somatic efferent pathway: one myelinated axon runs from the ventral horn directly to a bundle of skeletal muscle fibers, and an inset shows it ending in a single synaptic terminal. — **Figure 2.** Top: an autonomic efferent pathway, a myelinated preganglionic fiber (labeled "presynaptic axon") synapsing in a ganglion, then an unmyelinated postganglionic fiber (labeled "postsynaptic axon") reaching smooth muscle, where it releases transmitter from swellings along its length. Bottom: a somatic pathway, one myelinated motor neuron running from the spinal cord to skeletal muscle and ending at a single synaptic terminal. OpenStax Anatomy and Physiology 2e, Figure 15.6, openstax.org, CC BY 4.0.

Look at the top inset. The postganglionic axon does not end in one terminal. It runs across the smooth muscle and bulges every few micrometers into swellings packed with vesicles. Each swelling is a varicosity (varix = a swollen vein), and transmitter released from it spreads to many nearby muscle cells. A somatic motor neuron, in contrast, ends at a single neuromuscular junction on each muscle fiber.

	Somatic motor pathway	Autonomic motor pathway
Neurons from CNS to effector	One	Two: preganglionic and postganglionic, meeting in a ganglion
Where the CNS cell body sits	Ventral horn of the spinal cord, or a brainstem motor nucleus	Lateral horn of the spinal cord (T1–L2), sacral gray matter (S2–S4), or a brainstem nucleus
Effectors	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Myelin	Heavily myelinated, fast	Thinly myelinated preganglionic fiber, unmyelinated postganglionic fiber, slower
Ending on the effector	One neuromuscular junction per muscle fiber	Varicosities strung along the axon
Transmitter at the effector	Acetylcholine	Acetylcholine or norepinephrine (next topic)
Effect on the effector	Always excites	Excites some effectors and inhibits others
If the nerve is cut	The muscle is paralyzed and wastes away	The effector keeps working on its own; the heart keeps beating and the gut keeps moving
Conscious control	Mostly voluntary	Mostly involuntary

Sympathetic division: the thoracolumbar outflow

Every sympathetic preganglionic neuron has its cell body in the lateral horn of the spinal cord, in segments T1 to L2. No sympathetic fibers leave the brain, and none leave the cervical or sacral cord. That is why the sympathetic division is also called the thoracolumbar division (thorac/o = chest, lumb/o = loin).

The preganglionic axon leaves the cord in the ventral root, enters the spinal nerve, and within a centimeter or so turns off into a short connecting branch, the white ramus communicans (ramus = branch, communicans = connecting; plural white rami communicantes). It is white because its preganglionic fibers are myelinated. White rami exist only at T1–L2, because only those segments send preganglionic fibers out.

The sympathetic chain

The white ramus leads to the sympathetic chain (also called the sympathetic trunk): a string of ganglia joined by nerve fibers, one chain on each side of the vertebral column, running from the base of the skull to the coccyx. Its ganglia are the chain ganglia (also called paravertebral ganglia; para- = beside). There are about 22 to 23 on each side. Although fibers enter the chain only from T1–L2, the chain extends above and below, so every spinal nerve still gets sympathetic fibers. In the neck the cervical ganglia fuse into three, the largest being the superior cervical ganglion, which supplies the head.

A preganglionic fiber that enters the chain has three routes (Figure 3):

Synapse at the same level. It synapses on a postganglionic neuron in the chain ganglion it enters.
Travel up or down, then synapse. It runs up or down the chain and synapses in a ganglion at a higher or lower level. This is how the head and neck, and the sacral region, get sympathetic supply.
Pass straight through. It leaves the chain without synapsing, in a splanchnic nerve (splanchn- = the internal organs), and synapses in a collateral ganglion in the abdomen.

Three panels, each pairing a small schematic with a three-dimensional drawing of a spinal cord cross section, its spinal nerve and the neighboring sympathetic chain ganglion. In the top panel a red fiber leaves the lateral horn, enters the chain through the white ramus communicans and synapses in the ganglion at the same level. In the middle panel the fiber climbs the chain to synapse in a higher ganglion, and postganglionic fibers return to spinal nerves through gray rami communicantes. In the bottom panel the fiber passes through the chain without synapsing and runs in a splanchnic nerve to a prevertebral ganglion, where it synapses. — **Figure 3.** Three routes through the sympathetic chain. (a) The preganglionic fiber enters by the white ramus communicans and synapses in the chain ganglion at its own level. (b) It climbs or descends the chain and synapses at another level; postganglionic fibers return to the spinal nerve by a gray ramus communicans. (c) It passes through the chain without synapsing and runs in a splanchnic nerve to a prevertebral (collateral) ganglion. Two printed labels are off: in (b) the lower "Gray ramus communicans" pointer actually touches the white ramus, and in (c) "spinal ganglion" means the chain ganglion. OpenStax Anatomy and Physiology 2e, Figure 15.3, openstax.org, CC BY 4.0.

After a synapse in the chain, the postganglionic fiber has two ways out:

Back into a spinal nerve through a gray ramus communicans (plural gray rami communicantes). It is gray because its postganglionic fibers are unmyelinated. Every spinal nerve, from C1 to the coccyx, has a gray ramus. These fibers ride the spinal nerve out to the skin and limbs, where they supply sweat glands, the arrector pili muscles and the smooth muscle of blood vessel walls.
In separate sympathetic nerves that run straight from the chain to organs in the head and chest, such as the eye, the heart, the trachea and the lungs.

Here is the full route for the sweat on your palm: lateral horn cell body → ventral root → spinal nerve → white ramus → chain ganglion (synapse) → gray ramus → spinal nerve → sweat gland.

Collateral ganglia and splanchnic nerves

The abdominal and pelvic organs get sympathetic supply through a second set of ganglia. Collateral ganglia (also called prevertebral ganglia; pre- = in front of) are ganglia outside the chain that sit in front of the vertebral column, around the roots of the large arteries that branch from the body's main artery in the abdomen. They are named for those arteries:

The celiac ganglion (celi- = belly; often a pair, one on each side of its artery) supplies the stomach, liver, pancreas and spleen.
The superior mesenteric ganglion (mes- = middle, enter- = intestine) supplies most of the intestines.
The inferior mesenteric ganglion supplies the last part of the colon, the rectum, the urinary bladder and the reproductive organs.

Preganglionic fibers reach them through the splanchnic nerves. The greater splanchnic nerve (from about T5–T9) runs mainly to the celiac ganglion, and the lesser splanchnic nerve (about T10–T11) to the superior mesenteric ganglion. Lumbar splanchnic nerves reach the inferior mesenteric ganglion. Figure 4 puts the whole division together.

A drawing of the brainstem and spinal cord from the front, with the segments labeled from the pons down to S5 and a column of ganglia beside the cord on each side. Solid lines leave the cord only between T1 and L2. Some run to the chain of ganglia beside the cord, and some run past it as the greater and lesser splanchnic nerves to three round ganglia in front of the spine: the celiac, superior mesenteric and inferior mesenteric ganglia. Dashed lines run from the ganglia to a column of organs on the right, from the eye, tear gland and saliva glands at the top through the heart, airways, stomach, liver, pancreas, adrenal gland and intestines to the bladder and genitals at the bottom. A patch of skin on the left is supplied from the chain. — **Figure 4.** The sympathetic division. Preganglionic fibers (solid lines) leave the spinal cord only from T1 to L2. They synapse in the chain ganglia beside the vertebral column or pass through to the celiac, superior mesenteric and inferior mesenteric ganglia in front of it. Postganglionic fibers (dashed lines) reach organs from the eye to the external genitalia. Chain fibers also reach the skin at every level. OpenStax Anatomy and Physiology 2e, Figure 15.2, openstax.org, CC BY 4.0.

Two features make sympathetic output widespread. First, the ganglia sit near the spinal cord, so preganglionic fibers are short and postganglionic fibers are long. Second, one preganglionic neuron branches to synapse on many postganglionic neurons, often ten or more, spread over several chain levels. This branching is called divergence. A single burst from the lateral horn therefore reaches many organs at once.

The adrenal medulla

A small group of preganglionic fibers takes a route with no second neuron at all. These fibers run through the chain and a splanchnic nerve, pass through the celiac ganglion without synapsing, and end directly on cells in the center of the adrenal gland.

The adrenal medulla (ad- = near, ren- = kidney; medulla = inner part) is the inner core of each adrenal gland, which sits on top of the kidney. Its secreting cells are chromaffin cells (chrom- = color, affin- = attracted to; they stain brown with chromium salts). Chromaffin cells develop from the same embryonic tissue as postganglionic neurons, but they have no axons. When preganglionic fibers stimulate them, they release their product into the blood instead of onto a target. The adrenal medulla is an endocrine gland that behaves like a sympathetic ganglion.

About 80 percent of what chromaffin cells release is epinephrine (epi- = upon, nephr- = kidney; also called adrenaline, from the Latin for the same idea), and about 20 percent is norepinephrine. Epinephrine is a hormone: carried in the blood, it reaches cells all over the body, including cells no sympathetic fiber touches, and it stays in the blood for a few minutes after the nerve signal stops. The path runs: lateral horn → ventral root → white ramus → through the chain → splanchnic nerve → through the celiac ganglion → chromaffin cells → epinephrine into the blood.

Parasympathetic division: the craniosacral outflow

Parasympathetic preganglionic neurons sit at the two ends of the CNS, which is why this is the craniosacral division (crani/o = skull, sacr/o = sacrum):

Cranial outflow: nuclei in the brainstem send preganglionic fibers out in four cranial nerves, III, VII, IX and X.
Sacral outflow: neurons in the gray matter of cord segments S2–S4 send preganglionic fibers out in the ventral roots and then in the pelvic splanchnic nerves.

Parasympathetic preganglionic fibers run all the way to terminal ganglia: ganglia that sit close to the target organ or inside its wall. Those inside the wall are called intramural ganglia (intra- = within, mur- = wall). So parasympathetic preganglionic fibers are long and postganglionic fibers are very short, the reverse of the sympathetic pattern. Each preganglionic neuron contacts only a few postganglionic neurons, so parasympathetic effects stay more local. There is no parasympathetic chain, and parasympathetic fibers do not travel in the gray rami, so they do not reach the limbs, the skin or the walls of most blood vessels.

Figure 5 maps the four cranial nerves and the sacral outflow.

The same brainstem, spinal cord and organ column as the sympathetic map, now showing parasympathetic fibers in blue. From four shaded brainstem nuclei, fibers run out in cranial nerves III, VII, IX and X. Nerves III, VII and IX end in small circled ganglia near the eye, the tear and nasal glands and the saliva glands. Nerve X sweeps down to the heart, airways, stomach, liver, pancreas and intestines. From the lowest sacral segments, a second group of blue fibers runs to the colon, rectum, bladder and genitals. The red sympathetic chain is drawn faintly beside the cord for comparison. — **Figure 5.** The parasympathetic division. Brainstem nuclei send preganglionic fibers out in cranial nerves III, VII, IX and X; sacral segments S2–S4 send them to the pelvic organs. Each fiber runs to a terminal ganglion beside or within its target. The figure spells one nucleus "Super salivatory"; it means superior. Its "Otic ganglion" label sits beside the ganglion that supplies the two saliva glands under the jaw and tongue; the otic ganglion is the lowest circle, on nerve IX, supplying the largest saliva gland, in front of the ear. OpenStax Anatomy and Physiology 2e, Figure 15.4, openstax.org, CC BY 4.0.

Nerve	Terminal ganglion	What it supplies
III, oculomotor	Ciliary ganglion, behind the eye	The ring of iris muscle that narrows the pupil; the ciliary body muscle that focuses for near vision
VII, facial	Pterygopalatine and submandibular ganglia	The lacrimal gland (tears), glands of the nose and palate, and two of the glands that make saliva
IX, glossopharyngeal	Otic ganglion, below the skull near the ear	The largest gland that makes saliva, in front of the ear
X, vagus	Terminal and intramural ganglia in the organ walls	Heart, lungs and airways, esophagus, stomach, liver, pancreas, and the intestines as far as about two thirds of the way along the part of the colon that crosses the abdomen
S2–S4, pelvic splanchnic nerves	Intramural ganglia in the pelvic organs	The rest of the colon (the last third of the part that crosses the abdomen, and all of the colon that runs down the left side), the rectum, the urinary bladder and the reproductive organs

The vagus nerve alone carries about three quarters of all parasympathetic preganglionic fibers.

Sympathetic vs parasympathetic: the anatomy side by side

	Sympathetic division	Parasympathetic division
Other name	Thoracolumbar division	Craniosacral division
Where preganglionic cell bodies sit	Lateral horn, T1–L2	Brainstem nuclei (with nerves III, VII, IX, X) and sacral gray matter, S2–S4
Where the ganglia sit	Near the spinal cord: chain ganglia and collateral ganglia	Near or inside the target organ: terminal and intramural ganglia
Preganglionic fiber	Short	Long
Postganglionic fiber	Long	Short
Branching (divergence)	High: one preganglionic neuron reaches many postganglionic neurons	Low: one preganglionic neuron reaches a few
Reach	Whole body, including skin, limbs and blood vessel walls	Head, and the organs of the chest, abdomen and pelvis only
Link to a hormone gland	Adrenal medulla, stimulated directly by preganglionic fibers	None
Resulting pattern of output	Widespread, many organs at once	Local, organ by organ

Both divisions reach most organs of the chest and abdomen. The next topic explains the transmitters and receptor proteins that let the same organ respond to each division differently.

Common mix-ups

"Sympathetic fibers come from the whole spinal cord." The fibers are distributed at every level, through the chain and the gray rami, but they leave the CNS only from T1–L2.
"The white ramus carries postganglionic fibers." White rami carry myelinated preganglionic fibers into the chain; gray rami carry unmyelinated postganglionic fibers back out.
"Parasympathetic fibers only travel in the vagus nerve." The vagus carries most of them, but nerves III, VII and IX and the sacral outflow carry the rest.