Now the thing to notice about this diagram is that we've superimposed the time of
the twitch, that is a single contraction from a single action potential and
that's what's shown here in green.
So this tension, that is our single twitch or contractile period,
is also diagrammed on the same time scale.
And if you notice that the twitch is very long and
so it covers about 250 milliseconds.
And so most of the time that the twitch is occurring,
then we are in a absolute refractory period so
that we can not stimulate the cell and get a second twitch to occur, and
that means that there is no summation of contractile events.
So, we can't summate The contractions as we could, we'll scale it to muscle.
Now, why is that important?
If you think about it for the heart,
that means that you could never sum it to tetanus within the heart.
So, you can't stimulate the heart so
rapidly that you will get a Charlie Horse within the heart or a cramp.
And that's really important because if you think about it, every beat the heart
has to fill with blood in order to expel the blood to the body.
And so for each beat, we have to relax and
then fill during relaxation, and then contract to expel the blood.
If the heart is constantly contracted, it would not be able to fill the heart and
so it would lose the pumping action from the heart.
And so that would be lethal, that would be death.