[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Nerve and muscle](https://durgical.com/en/physiology/areas/nerve-muscle/) › Smooth muscle innervation and electrical activity

# Smooth muscle innervation and electrical activity

- Mechanism
- Exam priority: medium

Smooth muscle is switched on by nerves that release transmitter from beads along the axon, by hormones, by stretch and by its own rhythm.

Updated Nov 13, 2026

Also called: Slow waves, Varicosities, Junction potentials, Myogenic response

**See it in 3D:** [Stomach](https://durgical.com/en/anatomy/stomach/) · [Esophagus](https://durgical.com/en/anatomy/esophagus-abdominal-part/) · [Urinary bladder](https://durgical.com/en/anatomy/urinary-bladder/) · [Trachea](https://durgical.com/en/anatomy/trachea/)

## Key points

- Resting potential ≈ −50 to −60 mV, about 30 mV less negative than skeletal muscle; spike threshold in visceral muscle ≈ −35 mV.
- The upstroke is carried mostly by Ca²⁺ through L-type channels (few fast Na⁺ channels), so it is slow, and the same Ca²⁺ helps trigger contraction.
- Slow waves are rhythmic oscillations of membrane potential that do not contract the muscle themselves; spikes fire on their crests and those cause contraction (gut pacemaker).
- Stretch depolarizes single-unit smooth muscle and can trigger spikes: hollow organs contract when overfilled; arterioles constrict when pressure rises (myogenic response).
- Multi-unit muscle (iris, ciliary muscle, piloerector, vas deferens) is driven by nerves via graded junction potentials, often with no action potential at all.
- Hormones and drugs act through receptors: some open channels and change voltage; others (Gq → IP₃) release SR Ca²⁺ with little or no voltage change.

## What it is

Autonomic nerves do not form end plates on smooth muscle; they release transmitter from swellings along the axon that diffuses to many cells. The resting potential is less negative than in skeletal muscle, and action potentials are carried mainly by Ca²⁺, which also feeds contraction. Many smooth muscles are also excited without nerves: by stretch, by rhythmic slow waves, or by hormones acting with or without a change in voltage.

## How it works, step by step

- An autonomic axon runs over the muscle sheet and releases ACh or norepinephrine (or a cotransmitter) from many varicosities.
- At diffuse junctions the transmitter travels nanometres to micrometres and reaches many cells slowly; at contact junctions (≈ 20–30 nm, multi-unit muscle) the response is faster.
- Receptor activation opens cation or Ca²⁺ channels (depolarization) or K⁺ channels (hyperpolarization), giving a graded junction potential.
- In single-unit muscle, when depolarization reaches threshold, L-type Ca²⁺ channels open and a spike spreads through gap junctions to the whole sheet.
- Some tissues (ureter, uterus near term, some vessels) hold the depolarization for up to ≈ 1 s (plateau), which prolongs contraction.
- Pacemaker cells (interstitial cells of Cajal in the gut) generate slow waves; when a crest crosses threshold, a burst of spikes produces a rhythmic contraction.
- Stretch opens mechanosensitive cation channels; the depolarization adds to slow waves and fires spikes, so the organ contracts against the load.
- Ca²⁺ entering with the spike plus Ca²⁺ released from SR (IP₃ or Ca²⁺-induced release) activates calmodulin and MLCK, and the muscle contracts.

## Regulation

- *+* **Stretch**: Opens mechanosensitive channels: depolarization and spikes (myogenic tone, emptying of overfilled hollow organs).
- *+* **Parasympathetic ACh (M3, gut, bladder, airways)**: Depolarizes and raises IP₃: more spikes and more SR Ca²⁺ release.
- *+* **Norepinephrine on α1 receptors (arterioles, sphincters)**: Gq → IP₃: Ca²⁺ release and contraction, often with only a small voltage change.
- *+* **Oxytocin (uterus near term)**: Raises Ca²⁺ and spike frequency; receptor number rises at the end of pregnancy.
- *−* **β2 agonists, epinephrine (airways, uterus)**: cAMP opens K⁺ channels and speeds Ca²⁺ removal: hyperpolarization and relaxation.
- *−* **NO and other inhibitory transmitters (enteric nerves, endothelium)**: cGMP lowers Ca²⁺ and Ca²⁺ sensitivity: relaxation, e.g. sphincter opening and vasodilation.

## Electrical patterns in smooth muscle

| Pattern | What it looks like | Where | Consequence |
| --- | --- | --- | --- |
| Spike | Brief Ca²⁺ action potential (≈ 10–50 ms) | Most single-unit muscle (gut, bladder) | Twitch-like contraction; trains summate into tone |
| Plateau | Upstroke then delayed repolarization (hundreds of ms to ≈ 1 s) | Ureter, uterus, some arteries | Prolonged, strong contraction |
| Slow waves | Rhythmic oscillation, several per minute, no contraction alone | Stomach, intestine | Sets the maximum rate of peristaltic contractions |
| Junction potential | Graded depolarization, usually no spike | Multi-unit muscle (iris, vas deferens) | Contraction graded by nerve firing |
| Pharmacomechanical coupling | Little or no voltage change | Airways, vessels (Gq agonists) | SR Ca²⁺ release and Ca²⁺ sensitization |

## Clinical notes

- Achalasia: loss of inhibitory (NO-releasing) enteric neurons leaves the lower esophageal sphincter unable to relax, and peristalsis in the body of the esophagus fails.
- Paralytic ileus after surgery: sympathetic inhibition and inflammation suppress slow-wave-driven spikes, so the gut stops propelling content.
- Asthma: airway smooth muscle constricts to ACh, histamine and leukotrienes (Gq); β2 agonists relax it through cAMP.
- L-type Ca²⁺ channel blockers relax arterial smooth muscle and lower blood pressure, because vascular spikes and tone depend on Ca²⁺ entry.
- Preterm labour: increased uterine excitability (more gap junctions and oxytocin receptors, stronger plateaus) before term; tocolytics reduce Ca²⁺ entry or raise cAMP.

## Memory aid

Smooth muscle answers five callers: Nerves, Rhythm (slow waves), Stretch, Hormones, and Local factors. Only the nerve has to ring through a varicosity.

## Test yourself

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

## Why is the upstroke of a smooth muscle action potential slower than that of a skeletal muscle fiber?

It is carried mainly by Ca²⁺ through L-type channels, which open and close more slowly than the fast Na⁺ channels of skeletal muscle.

## A gut segment shows rhythmic slow waves but no contraction. What is missing?

Spikes. Slow waves only bring the membrane near threshold; contraction starts when a crest crosses threshold (≈ −35 mV) and Ca²⁺ spikes fire.

## Why does the iris constrict smoothly even though its smooth muscle rarely fires action potentials?

It is multi-unit muscle: each cell is driven by nearby nerve varicosities, and graded junction potentials and Ca²⁺ entry grade the contraction.

## How can a Gq-coupled agonist contract airway smooth muscle with almost no change in membrane potential?

Pharmacomechanical coupling: Gq → PLC → IP₃ releases Ca²⁺ from the SR, and Rho-kinase increases Ca²⁺ sensitivity, without needing a spike.

## What happens to an arteriole when the pressure inside it suddenly rises, and why?

It constricts (myogenic response): stretch opens mechanosensitive cation channels, depolarizes the muscle and opens L-type Ca²⁺ channels.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/physiology/smooth-muscle-innervation-and-electrical-activity/)

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

## Connections

## See it in 3D 4

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

- [Stomach](https://durgical.com/en/anatomy/stomach/)
- [Esophagus](https://durgical.com/en/anatomy/esophagus-abdominal-part/)
- [Urinary bladder](https://durgical.com/en/anatomy/urinary-bladder/)
- [Trachea](https://durgical.com/en/anatomy/trachea/)

## Related topics 8

- [Smooth muscle contraction](https://durgical.com/en/physiology/smooth-muscle-contraction/)
- [Skeletal, cardiac and smooth muscle compared](https://durgical.com/en/physiology/skeletal-cardiac-and-smooth-muscle-compared/)
- [Adrenergic and cholinergic receptors](https://durgical.com/en/physiology/adrenergic-and-cholinergic-receptors/)
- [Sympathetic and parasympathetic effects on each organ](https://durgical.com/en/physiology/sympathetic-and-parasympathetic-effects-on-each-organ/)
- [The enteric nervous system, slow waves and GI control](https://durgical.com/en/physiology/the-enteric-nervous-system-slow-waves-and-gi-control/)
- [Small bowel and colon motility](https://durgical.com/en/physiology/small-bowel-and-colon-motility/)
- [Local and humoral control of blood flow](https://durgical.com/en/physiology/local-and-humoral-control-of-blood-flow/)
- [Parturition: onset and uterine contraction](https://durgical.com/en/physiology/parturition-onset-and-uterine-contraction/)

## Related conditions 5

- [Asthma](https://durgical.com/en/conditions/asthma/)
- [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/)
- [Achalasia](https://durgical.com/en/conditions/achalasia/)
- [Paralytic ileus and colonic pseudo-obstruction (Ogilvie)](https://durgical.com/en/conditions/paralytic-ileus-and-colonic-pseudo-obstruction/)
- [Preterm labour and birth](https://durgical.com/en/conditions/preterm-labour-and-birth/)

## In Biochemistry 1

- [IP₃, DAG and calcium pathway](https://durgical.com/en/biochemistry/topics/ip-dag-and-calcium-pathway/)

## In Pharmacology 5

- [Calcium channel blockers](https://durgical.com/en/pharmacology/classes/calcium-channel-blockers/)
- [Nifedipine](https://durgical.com/en/pharmacology/nifedipine/)
- [Salbutamol](https://durgical.com/en/pharmacology/salbutamol/)
- [Oxytocin](https://durgical.com/en/pharmacology/oxytocin/)
- [Bethanechol](https://durgical.com/en/pharmacology/bethanechol/)

[← 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: [Achalasia](https://www.ncbi.nlm.nih.gov/books/NBK519515/) (StatPearls (NCBI Bookshelf), 2025); [Asthma](https://www.ncbi.nlm.nih.gov/books/NBK430901/) (StatPearls (NCBI Bookshelf), 2025); [Anatomy, Abdomen and Pelvis: Bladder](https://www.ncbi.nlm.nih.gov/books/NBK531465/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/physiology/smooth-muscle-innervation-and-electrical-activity/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/smooth-muscle-innervation-and-electrical-activity/