Autonomic Nervous System
Think about the last time a loud noise startled you. The heart began pounding, breathing quickened, and the palms went sweaty -- none of it consciously chosen. Now think about sitting down to a heavy meal: the stomach starts churning, the heart rate eases a little, and a feeling of relaxation sets in. Again, none of this was a conscious command to the intestines.
That is the autonomic nervous system (ANS) at work. It is the part of the nervous system that runs everything not normally thought about -- heart rate, digestion, sweating, pupil size, blood pressure, and bladder control. "Autonomic" reflects that this system largely operates with its own authority, outside conscious control.
The ANS is purely motor (efferent) -- it carries commands from the central nervous system to smooth muscle, cardiac muscle, and glands. It does not itself carry sensation, although sensory feedback from these organs influences its activity.
The Two Divisions: Opposing Forces, One Goal
The ANS has two anatomically and functionally distinct divisions that usually act in opposition:
| Feature | Sympathetic | Parasympathetic |
|---|
| Common name | "Fight or flight" | "Rest and digest" |
| Origin | Thoracolumbar region (thoracic and lumbar spinal cord) | Craniosacral region (brainstem and sacral spinal cord) |
| Ganglia location | Close to the spinal cord, in a sympathetic chain | Close to or inside the target organ |
| Preganglionic fibre | Short | Long |
| Postganglionic fibre | Long | Short |
| Main neurotransmitter at effector | Noradrenaline (norepinephrine) | Acetylcholine |
| Overall effect | Mobilises the body for activity | Conserves energy and supports maintenance |
Both divisions are active at some baseline level at all times -- it is the balance between them that shifts. The body does not switch one division off and the other on; it turns one up and the other down.
The Two-Neuron Chain
The wiring of the ANS is what sets it apart from the somatic (voluntary) motor system. In the somatic system, a single neuron runs all the way from the spinal cord to the muscle. In the ANS, there are always two neurons in series:
- Preganglionic neuron -- cell body inside the central nervous system, with its axon running out to a ganglion.
- Postganglionic neuron -- cell body inside the ganglion, with its axon running on to the target organ.
The ganglion is a relay point where the first neuron hands off to the second. The preganglionic neuron releases acetylcholine at the ganglion in both divisions. The postganglionic neuron then releases either acetylcholine (parasympathetic) or noradrenaline (sympathetic) at the target organ.
A useful memory aid: both divisions use acetylcholine at the ganglion. The real difference is at the effector organ -- sympathetic postganglionic fibres release noradrenaline, while parasympathetic postganglionic fibres release acetylcholine. Sweat glands are a notable exception: they are innervated by sympathetic fibres but respond to acetylcholine.
What Each Division Actually Does
Sympathetic -- "Fight or Flight"
Under stress, danger, or intense activity, the sympathetic division takes charge:
- Heart -- increases the rate and force of contraction
- Lungs -- widens the bronchioles, allowing more oxygen in
- Eyes -- widens the pupils, letting in more light
- Blood vessels -- narrows in the skin and gut, redirecting blood to the skeletal muscles
- Digestive system -- slows down, since digestion is not a priority during an emergency
- Liver -- releases stored glucose into the blood for quick energy
- Sweat glands -- becomes active, helping to cool the body
- Adrenal medulla -- stimulated to release adrenaline (epinephrine) directly into the bloodstream, a hormonal backup to the direct nerve signals -- the adrenal medulla is itself a modified sympathetic ganglion
Parasympathetic -- "Rest and Digest"
When the body is safe, fed, and relaxed, the parasympathetic division takes over:
- Heart -- slows down
- Lungs -- narrows the bronchioles back to normal breathing
- Eyes -- narrows the pupils, useful for close vision such as reading
- Digestive system -- increases peristalsis and the secretion of digestive juices
- Salivary glands -- increase saliva production
- Bladder -- promotes contraction for urination
- Genitals -- the parasympathetic division promotes erection, while ejaculation is actually a sympathetic response -- a reminder that the two divisions can act on the very same organ in different, sequential roles, not just as simple opposites …