Was wondering if you guys could clarify this for me. Are there basic differences between Cardiac potentials and Neuromuscular potentials. I mean the Purkinje fibers, is the potential in those based on Ca ions or is it still Na ion dependent. Basically there was a question in an old Kap exam which mentioned Cardiac potential and Na ions and I was a bit surprised as I always thought that Cardiac potential is more Ca dependent as opposed to Na. So anyways just trying to clarify some concepts. Is there also a difference between Cardiac Potential generated by the SA v/s those generated in Purkinje fibers? Thanks for your help.
Cardiac potential v/s Neuromuscular Potential...
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In both skeletal and heart muscle, the initial action potential is generated by influx of Na+. The big difference is that in heart muscle, this depolarization causes a huge influx of Ca++ into the cytosol. The purpose of this is to extend the depolarization period, thereby preventing action potentials from "adding up" to create one huge contraction. Think about what would happen if calcium wasn't involved -- action potentials add up, the heart contracts long and hard, stops beating, and you die.
Both the SA node and the Purkinje fibers operate on this basic principle, except the SA node is more rapidly depolarized/repolarized, resulting in a faster frequency. Purkinje fibers are a little slower, and they control the ventricles. This results in the lub-DUB sound -- the two-part beat is a result of the purkinje fibers being a little delayed, compared to the atrial pacemaker cells.
Both the SA node and the Purkinje fibers operate on this basic principle, except the SA node is more rapidly depolarized/repolarized, resulting in a faster frequency. Purkinje fibers are a little slower, and they control the ventricles. This results in the lub-DUB sound -- the two-part beat is a result of the purkinje fibers being a little delayed, compared to the atrial pacemaker cells.
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Visualizing it should also help.
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To be a little clearer, both skeletal and cardiac muscle contraction are driven by intracellular calcium released from the sarcoplasmic reticulum, but in cardiac muscle cells this release is caused by an influx of extracellular calcium as the membrane depolarizes (calcium induced calcium release) while in skeletal muscle it is caused by a direct physical interaction between the depolarized cell membrane and the sarcoplasmic reticulum.
The delay/separation of heart beats comes from the AV node rather than the Purkinje fibers. Though Purkinje fibers do have a slower rate of spontaneous depolarization, assuming the pacemaker cells in the heart are competent this doesn't matter since the Purkinje fibers will merely be transmitting potentials generated elsewhere. It's important to separate the rate of spontaneous depolarization from the rate of CONDUCTION of a depolarization.
