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== Heart == | == Action Potential of the Heart == | ||
The action potential in the myocardium of the heart is similar to that in skeletal muscle. However, there are some fundamental differences. Like skeletal myocytes, a myocyte has a negative membrane potential when at rest. A notable difference between skeletal and cardiac myocytes is how each increases the calcium ion concentration in the cytoplasm to induce contraction. When skeletal muscle is stimulated by motor axons, influx of Na+ quickly depolarizes the skeletal myocyte and triggers calcium release from the sarcoplasmic reticulum. In cardiac myocytes, the release of Ca2+ from the sarcoplasmic reticulum is induced by an influx of the calcium ions into the cell through voltage-gated calcium channels on the sarcolemma. This is called calcium-induced calcium release and increases the myoplasmic free Ca2+ concentration causing muscle contraction. In both muscle types, after the absolute refractory period, potassium channels reopen and the resulting flow of K+ out of the cell causes repolarization to the resting state. The voltage-gated calcium channels in the cardiac sarcolemma are generally triggered by an influx in sodium during the resting phase of the action potential | |||
In the heart we can distinguish both physiologically and anatomically several types of action potentials in the: | |||
* SAN | |||
* Atria | |||
* AV node | |||
* Purkyně fibres | |||
* Ventricles | |||
=== Sinoatrial Node === | |||
The SAN has an autoregulated action potential in that it does not require an external stimulus and can self-excitate. One of the reasons that allows this self-excitation is that it is has a resting membrane potential of -55 to -60mV as opposed to -85 to -90mV in the ventricles. Therefore it is more easily depolarised as the threshold potential is only -40mV. | |||
However, the main reason is due to sodium funny channels. These channels make the cells in the SAN more leaky to sodium and calcium ions, so there is a constant influx of these ions into the cell which initiates the action potential. These channels are always open allowing a fast depolarisation, and fast repolarisation. The sinoatrial node does not have a resting stage. | |||
=== Atria === | |||
The atria action potential has four stages: | |||
# resting membrane potential from voltage gated sodium channels | |||
# rapid depolarisation of the membrane from the opening of fast sodium and calcium channels | |||
# initial repolarisation from potassium channels | |||
# plateau phase where there is a balance between influx of calcium ions and efflux of potassium ions | |||
# repolarisation from the sodium-potassium pump | |||
=== Atrioventricular node === | |||
The AVN has the same principle as the SAN but does not have as many sodium funny channels. This means that the cycle of its autorhythmicity is longer. It also has resting phase and its repolarisation is more rapid. | |||
=== Purkyně Fibres === | |||
These fibres transmit the action potential from the SAN to the ventricles. It is similar to that of the atria except that the plateau phase is much longer which allows a stronger contraction and prevents a tetanic contraction. | |||
=== Ventricles === | |||
The ventricles also have a similar action potential curve to that of the atria and purkyně fibres. Its plateau phase is longer than in the atria but not as long as in the purkyně network. | |||
{{Cite | |||
| type = book | |||
| surname1 = Guyton | |||
| name1 = Arthur | |||
| surname2 = Hall | |||
| name2 = John | |||
| title = Textbook of Medical Physiology | |||
| edition = 12th | |||
| year = 2011 | |||
| isbn = 9781416054528 | |||
}} | |||
== Links == | == Links == | ||
http://en.wikipedia.org/wiki/ | http://en.wikipedia.org/wiki/Cardiac_action_potential | ||
=== Related Articles === | === Related Articles === | ||
=== Bibliography === | === Bibliography === | ||
=== References === | === References === | ||
<references /> | |||
</noinclude> | </noinclude> | ||
Revision as of 16:40, 26 November 2012
Action Potential of the Heart
The action potential in the myocardium of the heart is similar to that in skeletal muscle. However, there are some fundamental differences. Like skeletal myocytes, a myocyte has a negative membrane potential when at rest. A notable difference between skeletal and cardiac myocytes is how each increases the calcium ion concentration in the cytoplasm to induce contraction. When skeletal muscle is stimulated by motor axons, influx of Na+ quickly depolarizes the skeletal myocyte and triggers calcium release from the sarcoplasmic reticulum. In cardiac myocytes, the release of Ca2+ from the sarcoplasmic reticulum is induced by an influx of the calcium ions into the cell through voltage-gated calcium channels on the sarcolemma. This is called calcium-induced calcium release and increases the myoplasmic free Ca2+ concentration causing muscle contraction. In both muscle types, after the absolute refractory period, potassium channels reopen and the resulting flow of K+ out of the cell causes repolarization to the resting state. The voltage-gated calcium channels in the cardiac sarcolemma are generally triggered by an influx in sodium during the resting phase of the action potential In the heart we can distinguish both physiologically and anatomically several types of action potentials in the:
- SAN
- Atria
- AV node
- Purkyně fibres
- Ventricles
Sinoatrial Node
The SAN has an autoregulated action potential in that it does not require an external stimulus and can self-excitate. One of the reasons that allows this self-excitation is that it is has a resting membrane potential of -55 to -60mV as opposed to -85 to -90mV in the ventricles. Therefore it is more easily depolarised as the threshold potential is only -40mV. However, the main reason is due to sodium funny channels. These channels make the cells in the SAN more leaky to sodium and calcium ions, so there is a constant influx of these ions into the cell which initiates the action potential. These channels are always open allowing a fast depolarisation, and fast repolarisation. The sinoatrial node does not have a resting stage.
Atria
The atria action potential has four stages:
- resting membrane potential from voltage gated sodium channels
- rapid depolarisation of the membrane from the opening of fast sodium and calcium channels
- initial repolarisation from potassium channels
- plateau phase where there is a balance between influx of calcium ions and efflux of potassium ions
- repolarisation from the sodium-potassium pump
Atrioventricular node
The AVN has the same principle as the SAN but does not have as many sodium funny channels. This means that the cycle of its autorhythmicity is longer. It also has resting phase and its repolarisation is more rapid.
Purkyně Fibres
These fibres transmit the action potential from the SAN to the ventricles. It is similar to that of the atria except that the plateau phase is much longer which allows a stronger contraction and prevents a tetanic contraction.
Ventricles
The ventricles also have a similar action potential curve to that of the atria and purkyně fibres. Its plateau phase is longer than in the atria but not as long as in the purkyně network.
GUYTON, Arthur – HALL, John. Textbook of Medical Physiology. 12th edition. 2011. ISBN 9781416054528.
Links
http://en.wikipedia.org/wiki/Cardiac_action_potential
Related Articles
Bibliography
References
