Re: Why do heart cells have a refractory period after electical stimulation?

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.