[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Neurophysiology](https://durgical.com/en/physiology/areas/neuro/) › Autonomic tone and denervation supersensitivity

# Autonomic tone and denervation supersensitivity

- Regulation
- For completeness

Autonomic nerves fire slowly all the time; that steady tone lets them turn organs up or down, and cutting them makes organs oversensitive.

Updated Nov 13, 2026

Also called: Sympathetic tone, Parasympathetic tone, Pharmacological pupil testing

**See it in 3D:** [Sympathetic trunk](https://durgical.com/en/anatomy/sympathetic-trunk/) · [Vagus nerve](https://durgical.com/en/anatomy/vagus-nerve/)

## Key points

- About 1 impulse/s keeps a normal autonomic effect; full activation needs only about 10–20 impulses/s, far less than skeletal muscle.
- Sympathetic vascular tone keeps vessels partly constricted; raising or lowering it constricts or dilates them without a second nerve.
- Loss of tone: sympathectomy or spinal anesthesia gives immediate vasodilation and hypotension; intrinsic vessel tone recovers partly over days to weeks.
- Denervation supersensitivity: receptor up-regulation and, after postganglionic loss, absent reuptake make the organ overrespond to circulating transmitter.
- Horner pupil dilates with apraclonidine (supersensitive α1 receptors); an Adie tonic pupil constricts with dilute pilocarpine (supersensitive muscarinic receptors).

## What it is

Autonomic effectors respond fully at low firing rates, and the nerves discharge continuously at a basal rate called tone. Tone lets one division both increase and decrease an organ's activity, for example vessel diameter. When a nerve is cut, tone is lost at once; within days to weeks the organ becomes more sensitive to its transmitter (denervation supersensitivity), which is the basis of several pupil tests.

## How it works, step by step

- Autonomic nerves fire continuously at a low rate set by brainstem and hypothalamic centers.
- Because effectors respond to small changes in rate, centers can raise or lower activity smoothly from this baseline.
- Cutting the nerve removes tone at once, so the organ shifts toward the opposite state (vessels dilate, heart rate rises after vagotomy).
- Over days the denervated cells increase receptor number and lose the transmitter-clearing terminal.
- Tiny amounts of circulating or applied agonist then produce a large response: supersensitivity.

## Regulation

- *+* **Sympathetic tone (vessels)**: Keeps arterioles partly constricted; its loss drops peripheral resistance.
- *−* **Vagal tone (heart)**: Holds resting heart rate well below the intrinsic SA node rate of about 100/min.
- *+* **Adrenal medulla basal secretion**: Adds circulating epinephrine and norepinephrine that support tone even when nerves are quiet.
- *+* **Denervation (after days to weeks)**: More receptors and, for postganglionic loss, no reuptake: exaggerated response to transmitter or agonists.

## Clinical notes

- Hypotension after high spinal anesthesia or cervical cord injury reflects sudden loss of sympathetic vascular tone.
- In Horner syndrome, the cocaine test shows failure of the affected pupil to dilate, since there is no norepinephrine release to prolong; apraclonidine dilates it through supersensitivity.

## Memory aid

Tone is the idling engine: you can accelerate or brake. Cut the wire and the parts start listening harder.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does a Horner pupil dilate with apraclonidine while a normal pupil hardly changes?

The denervated iris dilator has up-regulated α1 receptors, so apraclonidine's weak α1 action becomes strong (denervation supersensitivity).

## How can the sympathetic system dilate a blood vessel without any vasodilator nerve?

By lowering its tonic firing below the resting rate, which reduces constriction.

## Why does resting heart rate rise after both vagi are blocked?

Resting vagal tone normally slows the SA node below its intrinsic rate; removing it lets the node fire near 100/min.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/autonomic-tone-and-denervation-supersensitivity/)

[More Physiology questions →](https://durgical.com/en/practice/?f=physiology)

## Connections

## See it in 3D 2

The structures of Durgical's 3D anatomy where it happens.

- [Sympathetic trunk](https://durgical.com/en/anatomy/sympathetic-trunk/)
- [Vagus nerve](https://durgical.com/en/anatomy/vagus-nerve/)

## Related topics 5

- [The autonomic nervous system: organization](https://durgical.com/en/physiology/the-autonomic-nervous-system-organization/)
- [Adrenergic and cholinergic receptors](https://durgical.com/en/physiology/adrenergic-and-cholinergic-receptors/)
- [Nervous control of the circulation and the CNS ischemic response](https://durgical.com/en/physiology/nervous-control-of-the-circulation-and-the-cns-ischemic-response/)
- [The visual pathway, pupillary reflexes and eye movements](https://durgical.com/en/physiology/the-visual-pathway-pupillary-reflexes-and-eye-movements/)
- [The autonomic nerve terminal: release, reuptake and drug targets](https://durgical.com/en/physiology/the-autonomic-nerve-terminal-release-reuptake-and-drug-targets/)

## Related conditions 2

- [Horner syndrome](https://durgical.com/en/conditions/horner-syndrome/)
- [Spinal cord injury](https://durgical.com/en/conditions/spinal-cord-injury/)

## In Pharmacology 2

- [Pilocarpine](https://durgical.com/en/pharmacology/pilocarpine/)
- [Apraclonidine](https://durgical.com/en/pharmacology/apraclonidine/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Horner syndrome: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000708.htm) (MedlinePlus, US National Library of Medicine, 2025); [Neuroanatomy, Cranial Nerve 10 (Vagus Nerve)](https://www.ncbi.nlm.nih.gov/books/NBK537171/) (StatPearls (NCBI Bookshelf), 2025); [Spinal Cord Injuries](https://www.ncbi.nlm.nih.gov/books/NBK560721/) (StatPearls (NCBI Bookshelf), 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/physiology/autonomic-tone-and-denervation-supersensitivity/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/autonomic-tone-and-denervation-supersensitivity/