Study topic · about 9 minutes

The autonomic nervous system: sympathetic and parasympathetic

The autonomic nervous system runs the body's automatic jobs, like your heartbeat, digestion, pupils and sweating, without you having to think about them. Its sympathetic division readies the body for action (fight or flight), and its parasympathetic division calms and restores it (rest and digest). Both work all the time, kept in balance by the hypothalamus and the brainstem.

See the brainstem and its nerves in 3D

What does the autonomic nervous system do?

Right now your heart is beating, your pupils are adjusting to the light, and your gut is moving your last meal along. These jobs belong to the autonomic nervous system (ANS). Autonomic means self-governing.

The ANS is the part of the peripheral nervous system that controls heart muscle, smooth muscle and glands, such as sweat and salivary glands. Smooth muscle lines the blood vessels, airways, gut and bladder, and changes the size of the pupils.

Breathing is a special case. Brainstem centers set its rhythm on autopilot. But the breathing muscles are skeletal muscles, so you can also take over and hold your breath.

The ANS reaches its targets through a relay of two neurons. The first, the preganglionic neuron, runs from the brainstem or spinal cord to a ganglion. A ganglion is a cluster of nerve cell bodies outside the central nervous system. The second, the postganglionic neuron, carries the signal on to the organ. The somatic nervous system, by contrast, uses a single motor neuron to reach each skeletal muscle.

The ANS has two main divisions, sympathetic and parasympathetic, which usually push an organ in opposite directions. Many textbooks add a third, the enteric nervous system in the wall of the gut.

Two divisions, wired differently

The sympathetic division: fight or flight

Sympathetic preganglionic neurons sit in the thoracic and upper lumbar segments of the spinal cord (T1 to L2), so this is called the thoracolumbar division. Their short preganglionic fibers mostly end in the sympathetic chain, about 23 ganglia strung like beads down each side of the spine. Others end in collateral ganglia in front of the spine, such as the celiac ganglion, which serve the gut. From the ganglia, long postganglionic fibers run out to the organs.

Because the chain runs the length of the spine, sympathetic fibers reach the whole body, down to the sweat glands in your skin. One preganglionic neuron can also contact 10 to 20 targets. So the sympathetic division tends to act as a whole, changing many organs at once, which suits an emergency.

One set of sympathetic fibers skips the ganglia and runs straight to the adrenal medulla, the core of the adrenal gland above each kidney. Its cells act like postganglionic neurons without axons. They release adrenaline (epinephrine) and noradrenaline (norepinephrine) into the blood, about four parts adrenaline to one of noradrenaline. The blood spreads the fight-or-flight message and makes it last longer.

The parasympathetic division: rest and digest

Parasympathetic preganglionic neurons sit in the brainstem and in the sacral spinal cord (S2 to S4), so this is called the craniosacral division. Their long preganglionic fibers travel nearly all the way to the target, ending in small ganglia near the organs or inside their walls. The short postganglionic fibers then have only a short way to go. This wiring suits more targeted effects, like narrowing just the pupils. Four cranial nerves carry parasympathetic fibers:

• Oculomotor nerve (III): narrows the pupils and focuses the eyes on close objects.
• Facial nerve (VII): switches on the tear glands and two of the three main pairs of salivary glands.
• Glossopharyngeal nerve (IX): switches on the parotid gland, the largest salivary gland.
• Vagus nerve (X): slows the heart, narrows the airways and speeds up digestion along most of the gut.

The sacral fibers serve the bladder, the lower large intestine and the sex organs. The vagus alone carries about three quarters of all parasympathetic fibers. Its name is Latin for "wandering", because it roams from the medulla down through the neck into the chest and belly.

Organ by organ: what each division does

Most organs get fibers from both divisions, which is called dual innervation. Sweat glands, the adrenal medulla and most blood vessels get only sympathetic fibers. The body controls them by turning sympathetic activity up or down, like a dimmer switch.

The two divisions are never simply on or off. Both fire a steady background stream of signals. The resting balance between them is called autonomic tone, and the body changes course by shifting it. At rest, the heart is under vagal tone. Left to itself, its built-in pacemaker would beat about 100 times a minute, but the vagus slows it to about 60 to 80. During exercise the vagus eases off, and sympathetic fibers push the rate higher. Blood vessels, by contrast, are kept partly narrowed by steady sympathetic tone.

The chemical messengers and their receptors

The ANS uses two main chemical messengers. Acetylcholine carries the signal across every autonomic ganglion, in both divisions. At the organs, parasympathetic fibers release acetylcholine too, while most sympathetic fibers release noradrenaline, also called norepinephrine.

The receptors are named after drugs that first told them apart: nicotinic receptors respond to nicotine, and muscarinic receptors to muscarine, a mushroom poison. Receptors for noradrenaline and adrenaline, called adrenergic receptors, come in alpha and beta types. For example, beta-1 receptors speed up the heart, beta-2 receptors relax the airways, and alpha-1 receptors narrow blood vessels and widen the pupils.

The receptor decides what a messenger does. Acetylcholine excites the next neuron in a ganglion through nicotinic receptors, yet it slows the heart through muscarinic ones.

Who is in charge? Control centers in the brain

At the top sits the hypothalamus, an almond-sized area at the base of the brain. The physiologist Charles Sherrington called it the "head ganglion" of the autonomic system. It tracks body temperature, water and energy, and sends orders down to the brainstem and spinal cord. For example, it makes you sweat when you are hot, and narrows skin blood vessels to save heat when you are cold. It also runs the hormone system through the pituitary gland (see hormones and the brain).

The medulla, at the bottom of the brainstem, does the minute-to-minute work. Its cardiovascular center speeds up or slows the heart, and narrows or widens blood vessels to keep blood pressure steady. Nearby breathing centers set the rhythm of each breath, fine-tuned by centers in the pons.

Stand up quickly, and blood pools in your legs and blood pressure dips. Stretch sensors called baroreceptors, in the aorta and in the carotid arteries of the neck, report the drop. Within seconds, the medulla turns vagal signals down and sympathetic signals up, so the heart speeds up and blood vessels narrow. When this baroreceptor reflex lags, you feel dizzy for a moment.

Emotions can take over too. When the amygdala detects a threat, its output side, the central nucleus, signals the hypothalamus and brainstem, and your heart starts to pound. Read more in emotion and stress.

The gut even has its own control center. The enteric nervous system is a mesh of about 100 million to 600 million neurons (estimates vary) in the gut wall. It can run the squeezing waves and digestive juices of the gut on its own, earning it the nickname "second brain". Sympathetic signals slow it down, and parasympathetic signals speed it up.

Take the brainstem lesson in 3D

When the balance tips: fainting and medicines

The most common kind of fainting is vasovagal syncope, named after the blood vessels (vaso) and the vagus. A trigger such as the sight of blood, sudden pain, fear or standing still for a long time sets off an overreaction. The vagus suddenly slows the heart, while sympathetic signals to the blood vessels fall away, so the vessels widen. Blood pressure drops, too little blood reaches the brain, and the person passes out, usually for less than a couple of minutes. Lying flat lets blood reach the brain again, so most people wake up quickly.

Not every faint is vasovagal. Heart rhythm problems and other conditions can also cause fainting.

Many medicines act on autonomic receptors. Anticholinergic drugs block muscarinic receptors, cutting off parasympathetic signals. The salivary glands make less saliva, so the mouth feels dry, and the heart loses its vagal brake, so it beats faster. The pupils may widen, the gut slows, urinating gets harder and sweating drops. Some medicines for motion sickness or an overactive bladder work this way, and some medicines for depression share these side effects.

Other medicines copy or block sympathetic signals. Asthma reliever inhalers such as albuterol (salbutamol in the UK) open the airways by stimulating beta-2 receptors. They can also cause a fast heartbeat and shaky hands. Beta blockers do the opposite at the heart: they block its beta receptors, so it beats more slowly and less forcefully. Eye doctors widen your pupils for an exam with drops that block muscarinic receptors or stimulate alpha receptors.

A tumor at the top of a lung or an injury in the neck can cut the sympathetic supply to one side of the face. The result is Horner syndrome: on that side, a small pupil, a slightly drooping upper eyelid and less sweating.

Memory tricks

In these study guides

• MCATNeurons, the nervous system and hormones
• AP Psychology1.2 Overview of the nervous system
• Intro psychologyNeurons and the nervous system (sections 3.2 and 3.3)
• Anatomy and physiologyThe autonomic nervous system (chapter 15)
• NursingNeuro basics
• A-level PsychologyThe divisions of the nervous system

Take it further in 3D

• The brainstem and cranial nervesLife support and the nerves of the head. A short guided lesson in the 3D app.

Sources

1. Anatomy and Physiology 2e, 15.1 Divisions of the Autonomic Nervous System, OpenStax
2. Anatomy and Physiology 2e, 15.2 Autonomic Reflexes and Homeostasis, OpenStax
3. Anatomy and Physiology 2e, 15.3 Central Control, OpenStax

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