The Autonomic Nervous System
Right now, your heart is beating. Your gut is working on breakfast. Your pupils are adjusting to the light. Your blood pressure moves up and down many times a minute. You do not have to think about any of it. All of this quiet work belongs to the autonomic nervous system. It is the part of you that runs your organs while your mind is busy with other things.
In this chapter you will meet its two parts. The sympathetic system is the "fight or flight" side. The parasympathetic system is the "rest and digest" side. You will see how they pull most organs in opposite ways to keep the body in tune. You will learn the two-nerve-cell wiring that sets this system apart. You will meet the two chemicals that carry its messages. And you will see the clever reflex that keeps you from fainting each time you stand up.
By the end of this chapter you can
- Define the autonomic nervous system. Explain how it differs from the somatic (voluntary) motor system.
- Compare the sympathetic and parasympathetic divisions. Look at their main job, where they leave the brain and spinal cord, and what they do to organs.
- Describe the two-neuron autonomic pathway. Find the ganglia of each division.
- Explain dual innervation and autonomic tone. Predict what happens when one division takes the lead.
- Follow the fight-or-flight response through the body, organ by organ. Describe how the parasympathetic side brings you back to calm.
- Match the autonomic neurotransmitters (acetylcholine, norepinephrine) to their receptors. Predict the effect at each target.
- Draw the baroreceptor reflex as a negative feedback loop that guards blood pressure.
Section 1Two divisions, opposite jobs
A motor system for the organs you never think about
Your nervous system sends out two kinds of motor commands. The somatic system carries the commands you choose to make. It goes to skeletal muscle, the muscle you use to lift a cup. The autonomic nervous system (ANS) does everything else. It is the involuntary motor system for your organs. It controls smooth muscle, heart muscle, and glands.
The ANS sets your heart rate. It sets how wide your pupils and airways are. It sets the pace of digestion and how tight your blood vessels are. It also runs your sweat glands and the glands that make spit. All of this happens on its own, below your awareness. You only notice it now and then. You might feel your heart pound before a test. Or your mouth may go dry when you are nervous.
Two divisions that pull opposite ways
The ANS has two divisions. They usually work against each other. The sympathetic division is the body's gas pedal. It is the "fight or flight" system. It revs you up to face a threat. The parasympathetic division is the brake. It is the "rest and digest" system. It runs the body when you are calm. It takes care of the everyday jobs of digesting food, getting rid of waste, and storing energy.
Most organs get nerves from both divisions. This setup is called dual innervation, which means "two nerve supplies." The two supplies push the organ in opposite ways. The heart is the clearest example. Sympathetic signals speed it up. Parasympathetic signals slow it down. Your heart rate at any moment is the balance between the two.
Where each division leaves the central nervous system
The two divisions leave the brain and spinal cord (the CNS) from different places. This is the easiest way to tell them apart. All sympathetic fibers come out of the thoracolumbar region. That is the chest and lower-back part of the spinal cord. They leave from the side horns of spinal segments T1 through L2. So the sympathetic division is often called the thoracolumbar outflow.
Parasympathetic fibers come out of the two ends of the CNS. Some leave the brainstem in four cranial nerves. These are III, VII, IX, and the long, far-reaching vagus nerve, X. The rest leave the sacral spinal cord, at segments S2–S4, near the tailbone. This top-and-bottom pattern gives the parasympathetic division its nickname, the craniosacral outflow.
| Target organ | Sympathetic ("fight or flight") | Parasympathetic ("rest and digest") |
|---|---|---|
| Heart | Beats faster and stronger | Beats slower |
| Pupil | Dilates (gets wider) | Constricts (gets smaller) |
| Airways (bronchi) | Dilate, letting in more air | Constrict |
| Digestive tract | Slows down; blood is sent elsewhere | Speeds up; juices flow |
| Salivary glands | A little thick spit (dry mouth) | Lots of watery spit |
| Urinary bladder | Relaxes (holds urine) | Contracts (empties) |
| Liver | Releases sugar (glucose) into the blood | Stores sugar (glucose) |
Tap any organ in the interactive. You will see its two opposite nerve supplies side by side.
- The autonomic nervous system is the involuntary motor system. It runs smooth muscle, heart muscle, and glands.
- Its two divisions work against each other. The sympathetic side is fight or flight, the gas pedal. The parasympathetic side is rest and digest, the brake.
- Sympathetic fibers leave from the thoracolumbar cord (T1–L2). Parasympathetic fibers leave craniosacral spots (CN III, VII, IX, X and S2–S4).
- Most organs have dual innervation. The body steers them by shifting the balance of steady autonomic tone.
Section 2The two-neuron pathway
Two neurons in a relay, not one
One simple fact sets the whole autonomic system apart. It takes two neurons (nerve cells) to carry a signal from the CNS to an organ. They meet at a relay station called a ganglion. A ganglion is a small cluster of nerve cell bodies outside the CNS.
The first nerve cell is the preganglionic neuron. "Pre" means "before," so it comes before the ganglion. Its cell body sits inside the CNS. Its long fiber, the axon, runs out to the ganglion and connects there. The second nerve cell is the postganglionic neuron. "Post" means "after." Its cell body sits in the ganglion. Its axon goes the rest of the way to the organ.
Compare this to the somatic system. There, a single motor neuron runs all the way from the spinal cord to the muscle. It has no relay in between. The extra connection in the ganglion is where the two autonomic divisions differ the most.
Sympathetic wiring: short first, long second, and it spreads
In the sympathetic division, the ganglia sit close to the spinal cord. Most of them form the sympathetic chain ganglia, also called paravertebral ganglia. These make a beaded double column that runs along each side of the spine. You can see them beside the cord in the figure. A few others sit in front of the aorta, the body's main artery. These are the prevertebral (collateral) ganglia, such as the celiac and mesenteric ganglia. The splanchnic nerves reach them.
The relay is near the cord. So sympathetic preganglionic fibers are short, and postganglionic fibers are long. Here is the key point. Each preganglionic fiber connects to many postganglionic neurons. This fanning out is called divergence. It is why a sympathetic response is not local. It sweeps through the whole body at once.
Parasympathetic wiring: long first, short second, and it stays local
The parasympathetic division flips this layout. Its ganglia sit in or right next to the target organ. These are called terminal or intramural ganglia. So preganglionic fibers are long, and postganglionic fibers are very short.
There is little fanning out. So a parasympathetic signal has a small, local effect on one organ. It does not cause a body-wide rush. You can slow the heart without also cramping the gut. This precise control fits the calm, one-job-at-a-time style of rest and digest.
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Where it leaves the CNS | Thoracolumbar (T1–L2) | Craniosacral (brainstem + S2–S4) |
| Where the ganglia are | Near the spinal cord (chain & prevertebral) | Near or in the target organ |
| Preganglionic fiber | Short | Long |
| Postganglionic fiber | Long | Short |
| Divergence (fanning out) | High → body-wide response | Low → local response |
Switch between the two divisions in the interactive. Watch the ganglion move from beside the cord to beside the organ.
- Every autonomic path uses two neurons, a preganglionic one and a postganglionic one. They meet at a ganglion.
- Sympathetic: ganglia near the cord (chain & prevertebral). Short first fiber, long second fiber. High divergence → body-wide response.
- Parasympathetic: ganglia at the organ. Long first fiber, short second fiber. Low divergence → local response.
- The adrenal medulla is a changed sympathetic ganglion. It releases epinephrine into the blood as a hormone.
Section 3Fight or flight, rest and digest
One switch, and the whole body answers
Sympathetic fibers fan out widely. The adrenal medulla adds a hormone boost. So the sympathetic division works as one team that acts all at once. Picture yourself stepping off a curb as a car horn blasts. Before you can think, your heart pounds. Your breathing deepens. Your eyes go wide. Your palms sweat, and your stomach seems to drop.
That is the fight-or-flight response. The sympathetic division is shifting the whole body toward one goal. That goal is a sudden burst of hard physical effort, so you can escape a threat or face it. Every change has a purpose. Each one moves resources away from long-term upkeep and toward instant muscle power.
The sympathetic surge, organ by organ
Follow the surge through the body. Every change makes sense. The heart beats faster and harder to move oxygen quickly. The airways widen so more air reaches the lungs. The pupils dilate to take in more of the scene. The liver pours stored sugar, or glucose, into the blood to fuel muscle.
Blood vessels constrict (tighten) in the gut and skin. But they dilate (widen) in skeletal muscle. This sends blood where it is needed. Digestion and urination pause, since there is no time for either. Sweat glands turn on to cool the body before hard work. It all fits together. Everything that helps you run or fight gets turned up. Everything that can wait gets turned down.
| Fight-or-flight change | How it helps you survive |
|---|---|
| Heart beats faster and harder | Gets oxygen and fuel to muscle faster |
| Bronchioles (small airways) dilate | More air flows in, so you take in more oxygen |
| Pupils dilate | You see the threat more sharply and widely |
| Liver releases glucose | Ready fuel for a burst of effort |
| Blood shifts to skeletal muscle | Power goes to muscles, not the gut |
| Digestion and urination stop | Saves energy for the emergency |
| Sweating increases | Cools the body before hard work |
Rest and digest: paying the debt back
Once the threat is gone, the parasympathetic division takes over. It brings the body back to normal. Then it gets back to the quiet work the emergency cut short. The heart slows. The airways and pupils go back to normal. Blood flows back to the gut. Digestion, taking in nutrients, and getting rid of waste start up again.
You spend most of your life in this state. Your body is building and storing, not spending. The two divisions are not enemies. They are a well-matched pair. The sympathetic side is the sprint. The parasympathetic side is the recovery. Good health depends on moving smoothly between them.
Flip the switch in the interactive to set off the sympathetic surge. Then let the parasympathetic division calm the body back down.
- The fight-or-flight response is a body-wide sympathetic reaction. All its parts work together to prepare for hard physical effort.
- It raises heart rate, airflow, pupil size, and blood sugar. It shifts blood to muscle. It pauses digestion and urination.
- The parasympathetic division brings the body back to normal. It restarts the everyday rest-and-digest jobs.
- Epinephrine in the blood lasts longer than the nerve signals. That is why the rush fades slowly after a scare.
Section 4The chemistry: acetylcholine and norepinephrine
Only two chemicals do all the talking
The ANS controls a lot. Yet it uses a very small set of words. Almost every autonomic message is carried by just two neurotransmitters. A neurotransmitter is a chemical that a nerve cell uses to pass on a signal. A neuron that releases acetylcholine (ACh) is called cholinergic. A neuron that releases norepinephrine (NE) is called adrenergic.
Which chemical is used where follows a simple pattern. All preganglionic neurons in both divisions are cholinergic. They release ACh in the ganglion. Then the postganglionic neurons split up. Parasympathetic postganglionic fibers release ACh onto the organ. Sympathetic postganglionic fibers release NE. There are two neat exceptions. Sympathetic fibers to sweat glands use ACh. And the adrenal medulla releases epinephrine into the blood.
The receptor decides the effect, not the chemical
This next idea is the one you most need to get. The same chemical can speed up one organ and slow down another. The result depends on the receptor it lands on. A receptor is a protein on the target cell that the chemical fits into, like a key in a lock.
ACh (cholinergic) receptors come in two kinds. Nicotinic receptors are in all autonomic ganglia. They are also where nerves meet skeletal muscle. Muscarinic receptors sit on the organs that parasympathetic fibers control. Adrenergic receptors also come in groups, called alpha and beta types.
Here is an example. When this family of chemicals acts on alpha-1 receptors, blood vessels tighten. When the same family acts on beta-2 receptors, the airways relax and open. Norepinephrine does most of the alpha-1 work. It binds beta-2 only weakly. So the job of opening the airways falls to epinephrine in the blood, which comes from the adrenal medulla. Beta-2 drugs such as albuterol do this job too. One chemical family can have opposite effects. The target cell's receptor decides what happens.
| Fiber | Neurotransmitter | Receptor on target |
|---|---|---|
| Preganglionic (both divisions) | Acetylcholine | Nicotinic |
| Parasympathetic postganglionic | Acetylcholine | Muscarinic |
| Sympathetic postganglionic (most) | Norepinephrine | Adrenergic (α or β) |
| Sympathetic fibers to sweat glands | Acetylcholine | Muscarinic |
| Adrenal medulla → blood | Epinephrine (a hormone) | Adrenergic (α or β) |
A loose spray, not a tight synapse
Look at the figure. The postganglionic axon does not end in one neat tip. Instead, it is dotted with beads called varicosities. These beads spray the chemical loosely over a wide patch of smooth muscle. So one autonomic fiber can affect a broad sheet of tissue at once. This works well for setting the tone of a whole blood vessel wall or a stretch of gut. It is not meant to command just one cell.
Tap each receptor in the interactive. See how the same two chemicals cause different effects.
- All preganglionic fibers and all parasympathetic postganglionic fibers release acetylcholine (cholinergic). Most sympathetic postganglionic fibers release norepinephrine (adrenergic).
- The receptor decides if a target is sped up or slowed down. It may be nicotinic or muscarinic, alpha or beta.
- Postganglionic axons release their chemical from varicosities. They spray a whole patch of tissue.
- Receptor types are the reason autonomic drugs can be so precise. Examples are beta-blockers, albuterol, and atropine.
Section 5The baroreceptor reflex
The body's fastest blood-pressure guard
The autonomic system does more than react to emergencies. It also runs steady, silent reflexes. These keep key body numbers stable. The most important one is the baroreceptor reflex. It guards your blood pressure second by second. It is a classic negative feedback loop. That means it always pushes a change back the other way. It has three parts that you met in Chapter 1: a sensor, a control center, and effectors.
Sense, compare, correct
The sensors are baroreceptors. These are stretch sensors packed into two places. One is the wall of the carotid sinus, where blood heads up to the brain. The other is the aortic arch, the curve of the body's main artery. When blood pressure rises, these walls stretch more. Then the sensors fire faster. When pressure falls, they fire less.
Their reports travel along two cranial nerves. These are IX (glossopharyngeal) and X (vagus). The signals reach the cardiovascular center in the medulla, the lowest part of the brainstem. This is the control center. It compares the signal to the pressure the body wants.
Then the medulla adjusts its two autonomic effectors. It turns parasympathetic (vagal) and sympathetic output up or down. This changes heart rate, how hard the heart squeezes, and how wide the vessels are. The response always works against the change. So pressure is pushed back toward its normal set point.
| Blood pressure | Baroreceptor firing | Autonomic response | Result |
|---|---|---|---|
| Goes too high | Goes up | Parasympathetic up, sympathetic down | Heart slows, vessels widen → BP falls |
| Goes too low | Goes down | Sympathetic up, parasympathetic down | Heart speeds up, vessels tighten → BP rises |
Standing up without fainting
You count on this reflex every time you stand. When you get up, gravity pulls blood down into your legs. For a moment, pressure at your head drops. Baroreceptors sense the drop right away. The medulla turns up the sympathetic drive. Your heart speeds up and your vessels tighten. Pressure comes back before your brain runs short of blood. All this takes just a heartbeat or two.
Sometimes this reflex is slow. Aging, losing too much body water, or some drugs can cause this. Then the fix lags behind. You feel lightheaded when you stand up. This is called orthostatic hypotension, which means "low blood pressure on standing."
Push the pressure off target in the interactive. Watch the reflex pull it back home.
- The baroreceptor reflex is a negative feedback loop. It keeps blood pressure steady from moment to moment.
- Baroreceptors in the carotid sinus and aortic arch sense stretch. They report through CN IX and X to the medulla.
- The medulla fixes pressure by shifting the sympathetic/parasympathetic balance. This changes heart rate, heart force, and vessel width.
- A slow reflex causes orthostatic hypotension, or feeling lightheaded when you stand.
Section 6Somatic versus autonomic
Two ways the brain moves the body
It helps to put the autonomic system next to its sibling, the somatic motor system. The contrast makes both clearer. The somatic system carries commands you choose to make. They go to skeletal muscle. It uses a single motor neuron that runs from the CNS straight to the muscle. That neuron releases acetylcholine onto nicotinic receptors.
Its effect is always the same: excitation. A somatic motor neuron can only tell a muscle to contract. It can never tell it to relax. To stop a movement, the CNS simply stops sending signals.
Where the autonomic system differs
The autonomic system is different in almost every way. It is involuntary. It targets smooth muscle, heart muscle, and glands. It uses a two-neuron chain that relays at a ganglion. It uses ACh or NE, depending on the fiber. And it can either excite or inhibit (speed up or slow down) its target.
For example, the parasympathetic vagus nerve can actively slow the heart. No somatic neuron can do that to a muscle. This power to both push and hold back is what lets the ANS fine-tune organs in both directions.
| Feature | Somatic | Autonomic |
|---|---|---|
| Control | Voluntary (you choose) | Involuntary (automatic) |
| Effector (target) | Skeletal muscle | Smooth & heart muscle, glands |
| Neurons to target | One | Two (relay at a ganglion) |
| Neurotransmitter | Acetylcholine only | Acetylcholine or norepinephrine |
| Effect on target | Always excites | Excites or inhibits |
A third player: the gut's own brain
The figure also names a quieter third part. It is the enteric nervous system (ENS). It is a huge web of neurons built into the wall of the digestive tract. It has so many neurons that people call it the "second brain." The ENS can run gut reflexes on its own. It controls the squeezing and the juices of the gut, even if its links to the CNS are cut.
The autonomic divisions still adjust it. Parasympathetic signals speed up digestion. Sympathetic signals slow it down. But the day-to-day churning of your gut mostly runs itself.
Switch between the somatic and autonomic paths in the interactive. Compare how they are wired and how they act.
- The somatic system is voluntary. It uses one neuron to skeletal muscle and releases only ACh. It is always excitatory.
- The autonomic system is involuntary. It uses a two-neuron chain to smooth muscle, heart muscle, and glands. It uses ACh or NE, and it can excite or inhibit.
- The enteric nervous system is a "second brain" in the gut wall that runs itself. The ANS adjusts it but does not command it.
The chapter in one breath
- The autonomic nervous system is the involuntary motor system. It runs smooth muscle, heart muscle, and glands.
- Its two divisions work against each other. The sympathetic side is fight or flight and leaves the thoracolumbar cord. The parasympathetic side is rest and digest and is craniosacral. Most organs have dual innervation and are steered by shifting tone.
- Every autonomic path is a two-neuron chain that relays at a ganglion. Sympathetic ganglia sit near the cord and fan out widely. Parasympathetic ganglia sit at the organ and stay local.
- The fight-or-flight response is a body-wide sympathetic surge. The parasympathetic division brings back calm. Adrenal epinephrine clears slowly, so the rush fades slowly.
- Just two neurotransmitters, acetylcholine and norepinephrine, run the system. The receptor decides if a target is sped up or slowed down.
- The baroreceptor reflex is a negative feedback loop. It guards blood pressure by balancing sympathetic and parasympathetic output through the medulla.
- The somatic system is voluntary, uses one neuron, uses ACh, and always excites. The autonomic system is involuntary, uses two neurons, and can both excite and inhibit. The enteric system runs the gut mostly on its own.
ReferenceWords worth knowing
Here is a quick list of the terms in this chapter. Skim it now, or come back whenever a word trips you up.
- Acetylcholine (ACh)
- The chemical messenger released by all autonomic preganglionic fibers and by parasympathetic postganglionic fibers. Neurons that use it are called cholinergic.
- Adrenal medulla
- The inner core of the adrenal gland. It is a changed sympathetic ganglion that releases epinephrine into the blood, so fight or flight has a longer-lasting hormone phase.
- Adrenergic
- Describes a neuron or receptor that uses norepinephrine (or epinephrine). Adrenergic receptors come in alpha and beta types.
- Autonomic nervous system
- The involuntary motor part of the nervous system. It controls smooth muscle, heart muscle, and glands.
- Autonomic tone
- The steady background firing of both divisions. The body controls an organ by shifting the balance between them, not by switching one on from off.
- Baroreceptor
- A stretch sensor in the carotid sinus and aortic arch. It senses blood pressure by how much the vessel wall stretches.
- Baroreceptor reflex
- The negative feedback loop that keeps blood pressure steady. It adjusts sympathetic and parasympathetic output to the heart and vessels.
- Cholinergic
- Describes a neuron or receptor that uses acetylcholine. Cholinergic receptors are either nicotinic or muscarinic.
- Craniosacral outflow
- Where parasympathetic fibers leave the CNS: the brainstem (cranial nerves III, VII, IX, X) and the sacral spinal cord (S2–S4).
- Dual innervation
- When one organ gets nerves from both the sympathetic and parasympathetic divisions. The two push it in opposite ways.
- Enteric nervous system
- The large web of neurons in the wall of the digestive tract. It can run gut reflexes without help from the brain and spinal cord.
- Fight-or-flight response
- The body-wide sympathetic reaction that gets the body ready for hard physical effort when facing a threat.
- Ganglion (autonomic)
- The relay station outside the CNS. Here a preganglionic neuron connects to a postganglionic neuron.
- Muscarinic receptor
- An ACh receptor on the organs served by parasympathetic (and sweat-gland) fibers. It can excite or inhibit, depending on the organ.
- Nicotinic receptor
- An ACh receptor found in all autonomic ganglia and where nerves meet skeletal muscle. It always excites.
- Norepinephrine (NE)
- The chemical messenger released by most sympathetic postganglionic fibers. It acts on adrenergic receptors.
- Parasympathetic division
- The "rest and digest" part of the ANS. It leaves the CNS in a craniosacral pattern. It saves energy and runs digestion and waste removal.
- Postganglionic neuron
- The second neuron in an autonomic path. Its cell body is in the ganglion, and its axon runs to the organ.
- Preganglionic neuron
- The first neuron in an autonomic path. Its cell body is in the CNS, and its axon runs to the ganglion.
- Sympathetic chain ganglia
- The paired columns of ganglia (paravertebral ganglia) along the spine. They hold most sympathetic postganglionic cell bodies.
- Sympathetic division
- The "fight or flight" part of the ANS. It leaves the CNS in a thoracolumbar pattern and gets the body ready for hard effort.
- Thoracolumbar outflow
- Where sympathetic fibers leave the CNS: the side horns of spinal segments T1 through L2.