[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/#s-nerve_muscle) › Smooth muscle contraction [Physiology](https://durgical.com/en/physiology/) · [Nerve and muscle](https://durgical.com/en/physiology/#s-nerve_muscle) # Smooth muscle contraction - Mechanism - Exam priority: medium Smooth muscle in vessels, airways and gut squeezes slowly and can hold tone for hours using very little energy. Updated 2026-11-01 Also called: Calmodulin–MLCK pathway, Latch state **See it in 3D:** [Trachea](https://durgical.com/en/anatomy/trachea/) · [Stomach](https://durgical.com/en/anatomy/stomach/) · [Urinary bladder](https://durgical.com/en/anatomy/urinary-bladder/) ## Key points - Ca²⁺–calmodulin activates MLCK → phosphorylated myosin light chain → cross-bridge cycling. No troponin. - Ca²⁺ sources: voltage-gated L-type channels, receptor-operated channels, and SR release through IP₃ receptors (Gq → PLC → IP₃). - Relaxation: myosin phosphatase (activated by NO → cGMP); β2-receptor cAMP lowers Ca²⁺ and MLCK activity. - Ca²⁺ sensitization: RhoA/Rho kinase inhibits myosin phosphatase, raising force without more Ca²⁺. - Latch state: dephosphorylated myosin stays attached and cycles slowly, holding tone with very little ATP. ## What it is Smooth muscle lines blood vessels, airways, the gut, bladder and uterus. Its contraction is switched on when Ca²⁺ binds calmodulin and activates myosin light-chain kinase (MLCK), which phosphorylates myosin. Myosin phosphatase reverses this, so tone reflects the balance between the two, set by nerves, hormones, stretch and local factors such as nitric oxide. ## How it works, step by step - A stimulus (α1 or M3 receptor agonist, stretch, depolarization) raises cytosolic Ca²⁺. - Gq-coupled receptors activate phospholipase C, producing IP₃ (SR Ca²⁺ release) and DAG. - Depolarization opens L-type Ca²⁺ channels, adding extracellular Ca²⁺. - Four Ca²⁺ ions bind calmodulin; the complex activates myosin light-chain kinase. - MLCK phosphorylates the regulatory light chain, enabling myosin to cycle on actin anchored to dense bodies. - Myosin phosphatase removes the phosphate; slowly detaching heads give the latch state and sustained tone. - Ca²⁺ is removed by SERCA, plasma membrane Ca²⁺-ATPase and Na⁺/Ca²⁺ exchange, and the muscle relaxes. ## Regulation - *+* **Norepinephrine on α1 receptors (Gq)**: IP₃ and Ca²⁺ rise: vasoconstriction - *+* **Acetylcholine on M3 receptors (Gq)**: Bronchoconstriction, gut and bladder contraction - *+* **Stretch (myogenic response)**: Arterioles constrict when pressure rises (autoregulation) - *−* **β2 agonists (Gs → cAMP)**: Lower Ca²⁺ and MLCK activity: bronchodilation - *−* **Nitric oxide (cGMP)**: Activates myosin phosphatase: vasodilation ## Clinical notes - Asthma: airway smooth muscle is hyperresponsive; inhaled β2 agonists relax it via cAMP, and anticholinergics block M3-driven constriction. - Arterial hypertension: increased vascular tone and remodeling raise peripheral resistance; Ca²⁺ channel blockers and α1 blockers lower tone. - Nitrates and phosphodiesterase-5 inhibitors act through the NO–cGMP pathway; combining them can drop blood pressure dangerously. ## Memory aid Calmodulin calls the kinase; the phosphatase pays the bill for relaxation. ## Test yourself Think of the answer, then tap the question to see it. ## How does a β2 agonist relax bronchial smooth muscle at the molecular level? β2 receptors activate Gs and adenylyl cyclase; cAMP activates PKA, which lowers cytosolic Ca²⁺ and phosphorylates MLCK so it is less active. Less myosin phosphorylation means relaxation. ## Why can smooth muscle hold tone for hours while using far less ATP than skeletal muscle? In the latch state dephosphorylated cross-bridges remain attached and detach very slowly, so force is kept with little cycling and little ATP use. ## Why are nitrates and phosphodiesterase-5 inhibitors dangerous together? Nitrates raise cGMP via NO and PDE-5 inhibitors stop cGMP breakdown. The combined vasodilation can cause severe hypotension. ## How can vascular tone rise without any increase in cytosolic Ca²⁺? Through Ca²⁺ sensitization: RhoA/Rho kinase inhibits myosin phosphatase, so myosin stays phosphorylated at the same Ca²⁺ level. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/smooth-muscle-contraction/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Trachea](https://durgical.com/en/anatomy/trachea/) - [Stomach](https://durgical.com/en/anatomy/stomach/) - [Urinary bladder](https://durgical.com/en/anatomy/urinary-bladder/) ## Related topics - [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/) - [Sliding filaments and the cross-bridge cycle](https://durgical.com/en/physiology/sliding-filaments-and-the-cross-bridge-cycle/) - [Sympathetic and parasympathetic effects on each organ](https://durgical.com/en/physiology/sympathetic-and-parasympathetic-effects-on-each-organ/) ## Related conditions - [Asthma](https://durgical.com/en/conditions/asthma/) - [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/) ## In Biochemistry - [IP₃, DAG and calcium pathway](https://durgical.com/en/biochemistry/topics/ip-dag-and-calcium-pathway/) - [G protein-coupled receptors and cAMP](https://durgical.com/en/biochemistry/topics/g-protein-coupled-receptors-and-camp/) - [Cyclic AMP (cAMP)](https://durgical.com/en/biochemistry/molecules/cyclic-amp/) [← 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: [10.8 Smooth Muscle](https://openstax.org/books/anatomy-and-physiology-2e/pages/10-8-smooth-muscle) (OpenStax (Rice University), 2022); [Asthma](https://www.ncbi.nlm.nih.gov/books/NBK430901/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 8). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016). ## **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/smooth-muscle-contraction/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/smooth-muscle-contraction/