|MadSci Network: Cell Biology|
The question is: Why do heart cells have a refractory period after electical stimulation? Well, compared with neurons, cardiac cells have a much longer action potential consisting of 4 discrete phases. First, a rapid rising phase mediated by the opening of voltage-gated sodium channels. These channels will be rapidly inactivated to prevent further influx of sodium into the cell. Inactivation involves a set of states which is distinct from a normal closed channel. Basically, the channel is still open but becomes “plugged” from the inside mouth- thus no sodium can pass through until the plug is removed. Next, a fast repolarization phase which is mediated by voltage- gated potassium channels which allow potassium to flow out of the cell. Next, the activation of voltage-gated calcium channels leads to the prolonged plateau phase of the classic cardiac action potential. These channels allow calcium influx into the cell. Finally, a number of slowly activating, delayed rectifying potassium channels complete the repolarization phase resetting the membrane voltage to the resting potential where it is maintained by another family of potassium channels called inward rectifiers. Once the cell is again at rest the inactivated sodium channels can relax back into their closed state and can then be reopened by depolarization, leading to another action potential. (When the cell is repolarized, the plug is removed from the inner mouth of the sodium channel thus relieving inactivation and allowing the channel to go back to its resting closed state.) Therefore, the basic answer to the question is that the absolute refractory period is the time required for sodium channels to physically recover from inactivation. In a sense, this mechanism evolved to prevent premature beats from occuring in the heart which can lead to cardiac arrythmias- a condition where the synchronized firing of action potentials and thus muscle contraction is lost. A heartbeat “starts” in a small region called the sinoatrial node. It spreads in widening circuits to the atria and ventricles in a carefully controlled manner assuring the synchronized contraction of the four chambers.
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