# Cardiac excitation-contraction coupling
**Cardiac excitation-contraction coupling** (EC coupling) is the sequence of mechanisms that links electrical depolarization of the heart muscle cell membrane to contraction of the cell's myofilaments, using calcium as the second messenger between the two[1](https://heart.bmj.com/content/89/4/371). Each heartbeat begins with an action potential at the sinoatrial node, spreads through the conduction system to the working myocardium, and ends with calcium-triggered contraction of ventricular and atrial cardiomyocytes[2](https://www.ncbi.nlm.nih.gov/books/NBK597451/). Because the heart must both contract and fully relax between beats, the process includes calcium removal during diastole as an integral part of the cycle[3](https://www.uptodate.com/contents/excitation-contraction-coupling-in-myocardium).
| Key fact | Detail |
|---|---|
| Trigger | Depolarization opens L-type Ca²⁺ channels (Ca\(_{\mathrm{V}}\)1.2) in the T-tubule membrane; the resulting small Ca²⁺ influx triggers a much larger release from the sarcoplasmic reticulum[4](https://www.sciencedirect.com/science/article/pii/S0022282812004270) |
| Release mechanism | Calcium-induced calcium release (CICR): Ca²⁺ entering through L-type channels activates type 2 ryanodine receptors (RyR2) on the junctional sarcoplasmic reticulum[4](https://www.sciencedirect.com/science/article/pii/S0022282812004270) |
| Dyad geometry | Ca²⁺-releasing units (dyads) pair T-tubules (diameter about 200 nm) with the SR across a cleft of roughly 15 nm; each dyad contains about 25 L-type channels and about 100 RyR2s, a roughly 1:4 ratio[5](https://academic.oup.com/ehjcvp/article/7/6/e91/6367047) |
| Contraction step | Ca²⁺ binds troponin, allowing thick and thin filaments to slide, the cell to shorten, and ventricular pressure to develop[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5497788/) |
| Relaxation | Cytosolic Ca²⁺ is resequestered into the SR by SERCA2a, regulated by phospholamban, and extruded by the Na⁺/Ca²⁺ exchanger (NCX1)[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC5607780/) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC10070045/pdf/nihms-1886435.pdf) |
| Efflux balance | In the steady state, Ca²⁺ influx equals efflux on every beat; roughly 30% of cytosolic Ca²⁺ leaves via NCX, the sarcolemmal Ca²⁺-ATPase carries only 2% to 3%, and a small portion enters mitochondria[9](https://link.springer.com/content/pdf/10.1007/s00018-025-05960-x.pdf) • [10](https://www.ahajournals.org/doi/pdf/10.1161/CIRCRESAHA.110.223578) |
| Elementary events | Ca²⁺ sparks, the elementary release events of heart muscle, were described by Cheng, Lederer, and Cannell in 1993[11](https://www.nature.com/articles/415198a) |
## The calcium transient
Contraction is initiated by a transient rise in intracellular free Ca²⁺ that rises and decays with each cardiac cycle[12](https://journals.physiology.org/doi/full/10.1152/ajpheart.2001.280.5.H1928). Normal function requires that intracellular Ca²⁺ be sufficiently high in systole and low in diastole, and force depends on the amount of Ca²⁺ bound to troponin[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5497788/). The transient is produced by a sequence of events: action-potential-activated initial Ca²⁺ entry, a massive release from the calcium-storing sarcoplasmic reticulum, and diastolic removal through SR reuptake and sarcolemmal extrusion[2](https://www.ncbi.nlm.nih.gov/books/NBK597451/).
## Calcium-induced calcium release
The accepted model of the cardiac Ca²⁺ transient is calcium-induced calcium release (CICR)[2](https://www.ncbi.nlm.nih.gov/books/NBK597451/). Arnold Fabiato established the mechanism in the 1970s using "skinned" cardiac cells whose sarcolemma had been removed: rapid application of Ca²⁺-containing solution triggered release from intracellular stores, and the release could be blocked by ryanodine, an agent that disrupts the SR release channels now called ryanodine receptors[12](https://journals.physiology.org/doi/full/10.1152/ajpheart.2001.280.5.H1928). Later experiments on intact cells, in which a small "trigger" Ca²⁺ rise was produced by photolysis of caged Ca²⁺, showed that CICR operates with the sarcolemma intact as well[12](https://journals.physiology.org/doi/full/10.1152/ajpheart.2001.280.5.H1928).
In the intact heart, the trigger is the Ca²⁺ entering through L-type voltage-gated channels (Ca\(_{\mathrm{V}}\)1.2, also called dihydropyridine receptors) during the action potential plateau[4](https://www.sciencedirect.com/science/article/pii/S0022282812004270) • [5](https://academic.oup.com/ehjcvp/article/7/6/e91/6367047). The discovery in the early 1960s that stimulation of the heart increases calcium influx across the plasma membrane was key to understanding the role of extracellular calcium[13](https://academic.oup.com/cardiovascres/article-pdf/52/1/8/844991/52-1-8.pdf).
## Structural organization: the dyad
Efficient coupling depends on geometry. L-type channels and RyR2s are concentrated at dyadic junctions between the T-tubules, invaginations of the sarcolemma, and the sarcoplasmic reticulum[5](https://academic.oup.com/ehjcvp/article/7/6/e91/6367047) • [14](https://link.springer.com/article/10.1007/s00421-026-06413-4). The dyad brings Ca²⁺ entry into close proximity with release sites, so that a small local Ca²⁺ signal can activate the much larger SR release without requiring a whole-cell rise in Ca²⁺[15](https://www.nature.com/articles/s41569-026-01286-8). These spatial microdomains localize the molecular players that orchestrate cardiac function[11](https://www.nature.com/articles/415198a).
## Termination and relaxation
During diastole, L-type channels close, terminating the Ca²⁺ influx that drives RyR2 opening, and excess cytosolic Ca²⁺ is removed mainly by SERCA2a reuptake into the SR[9](https://link.springer.com/content/pdf/10.1007/s00018-025-05960-x.pdf). SERCA2a is regulated by phospholamban, and the Na⁺/Ca²⁺ exchanger extrudes Ca²⁺ to the extracellular space[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC5607780/) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC10070045/pdf/nihms-1886435.pdf). Functional coupling requires that the fluxes across all membranes balance, so that in the steady state Ca²⁺ influx equals Ca²⁺ efflux on every beat[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5497788/).
## Alternative and proposed mechanisms
A second mechanism, the voltage-sensitive release mechanism (VSRM), has been proposed alongside CICR. Under this hypothesis, SR Ca²⁺ release is graded by membrane potential rather than by the magnitude of the L-type Ca²⁺ current, and its electrophysiological and pharmacological characteristics differ markedly from CICR. Both mechanisms may operate in the same cell, and the VSRM is reported to be depressed in heart failure[12](https://journals.physiology.org/doi/full/10.1152/ajpheart.2001.280.5.H1928).
## Comparison with skeletal muscle
The term "excitation-contraction coupling" was introduced by Alexander Sandow in 1952 for skeletal muscle, as "the entire sequence of reactions, excitation, inward acting link, and activation of contraction"[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC9396671/). The cardiac version differs fundamentally in its trigger. Skeletal muscle EC coupling is mediated by a voltage-dependent mechanical coupling between the dihydropyridine receptor acting as a voltage sensor and RyR1, whereas cardiac coupling depends on the diffusion of Ca²⁺ ions from Ca\(_{\mathrm{V}}\)1.2 to RyR2[13](https://academic.oup.com/cardiovascres/article-pdf/52/1/8/844991/52-1-8.pdf) • [14](https://link.springer.com/article/10.1007/s00421-026-06413-4). Cardiac junctions are dyads between T-tubule and SR membranes, in contrast to the triadic junctions of skeletal muscle[14](https://link.springer.com/article/10.1007/s00421-026-06413-4).
## Calcium cycling in disease
Disturbances of EC coupling underlie major cardiac disease. In heart failure, reduced SERCA2a protein suppresses SR Ca²⁺ load and transient amplitude, decreasing contractility, while enhanced diastolic SR Ca²⁺ leak through dysfunctional RyR2 channels further depletes SR content[9](https://link.springer.com/content/pdf/10.1007/s00018-025-05960-x.pdf). Diastolic leak can produce frequent Ca²⁺ sparks and Ca²⁺ waves, generating excess inward electrogenic NCX current (3 Na⁺ in, 1 Ca²⁺ out), which can produce delayed afterdepolarizations and can initiate arrhythmias[9](https://link.springer.com/content/pdf/10.1007/s00018-025-05960-x.pdf) • [18](https://pmc.ncbi.nlm.nih.gov/articles/PMC4167289/) • [19](https://pmc.ncbi.nlm.nih.gov/articles/PMC3085083/). RyR2 mutations are associated with catecholaminergic polymorphic ventricular tachycardia and arrhythmogenic right ventricular dysplasia type 2[9](https://link.springer.com/content/pdf/10.1007/s00018-025-05960-x.pdf). Altered EC coupling is also recognized as a feature of heart failure more broadly, alongside energy deficits and oxidative stress[17](https://www.nature.com/articles/s41569-025-01167-6). Because of this, calcium-handling proteins, including RyR2, LTCC, SERCA2a, and their regulators such as CaMKII and PKA, are the main targets in the search for drugs against heart failure and arrhythmias[5](https://academic.oup.com/ehjcvp/article/7/6/e91/6367047) • [9](https://link.springer.com/content/pdf/10.1007/s00018-025-05960-x.pdf).
## References
1. [Scoote M, Poole-Wilson PA, Williams AJ. The therapeutic potential of new insights into myocardial excitation-contraction coupling. Heart (2003)](https://heart.bmj.com/content/89/4/371)
2. [Organization of Ca2+ Signaling Microdomains in Cardiac Myocytes, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK597451/)
3. [Excitation-contraction coupling in myocardium, UpToDate](https://www.uptodate.com/contents/excitation-contraction-coupling-in-myocardium)
4. [Ca2+ channel and Na+/Ca2+ exchange localization in cardiac myocytes, Journal of Molecular and Cellular Cardiology](https://www.sciencedirect.com/science/article/pii/S0022282812004270)
5. [Advances in the understanding of excitation-contraction coupling: the pulsing quest for drugs against heart failure and arrhythmias, European Heart Journal - Cardiovascular Pharmacotherapy (2021)](https://academic.oup.com/ehjcvp/article/7/6/e91/6367047)
6. [Calcium and Excitation-Contraction Coupling in the Heart, Circulation Research (PMC5497788)](https://pmc.ncbi.nlm.nih.gov/articles/PMC5497788/)
7. [Calcium Signaling and Cardiac Arrhythmias (PMC5607780)](https://pmc.ncbi.nlm.nih.gov/articles/PMC5607780/)
8. [Mechanisms Underlying Pathological Ca2+ Handling in Diseases of the Heart (PMC10070045)](https://pmc.ncbi.nlm.nih.gov/articles/PMC10070045/pdf/nihms-1886435.pdf)
9. [Bare DJ, Ai X. Stress-Induced calcium mishandling in cardiac (Patho)physiology. Cellular and Molecular Life Sciences (2025) 82:437](https://link.springer.com/content/pdf/10.1007/s00018-025-05960-x.pdf)
10. [Integrative Systems Models of Cardiac Excitation-Contraction Coupling, Circulation Research](https://www.ahajournals.org/doi/pdf/10.1161/CIRCRESAHA.110.223578)
11. [Bers DM. Cardiac excitation-contraction coupling. Nature 415, 198-205 (2002)](https://www.nature.com/articles/415198a)
12. [Ferrier GR, Howlett SE. Cardiac excitation-contraction coupling: role of membrane potential in regulation of contraction. Am J Physiol Heart Circ Physiol 280:H1928 (2001)](https://journals.physiology.org/doi/full/10.1152/ajpheart.2001.280.5.H1928)
13. [Katz AM. Cardiovascular Research 52:8-13 (2001)](https://academic.oup.com/cardiovascres/article-pdf/52/1/8/844991/52-1-8.pdf)
14. [Dulhunty AF, Casarotto MG. The nuts and bolts of excitation-contraction coupling in skeletal muscle. European Journal of Applied Physiology (2026)](https://link.springer.com/article/10.1007/s00421-026-06413-4)
15. [Local calcium dynamics and signalling in cardiomyocytes, Nature Reviews Cardiology (2026)](https://www.nature.com/articles/s41569-026-01286-8)
16. [Dirksen RT, Eisner DA, Ríos E, Sipido KR. Excitation-contraction coupling in cardiac, skeletal, and smooth muscle. J Gen Physiol 154(9) (2022)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9396671/)
17. [Mechano-energetic uncoupling in heart failure, Nature Reviews Cardiology 22:773-797 (2025)](https://www.nature.com/articles/s41569-025-01167-6)
18. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC4167289/)
19. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC3085083/)
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*Topic: Encyclopedia › Life and health › Human health and medicine › Human structure and function › Cardiovascular and lymphatic systems › Heart › Cardiac physiology and hemodynamics › Cardiac cycle, output and contractility › Contractility and excitation–contraction coupling*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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