Pushing and Pulling
The heart is autogenic and depolarizes no less
But it can be influenced by the ANS.
Autogenic means leaky channels
And within the channel annals,
This means changes in membrane potential
And for the heart, are is essential.
The sympathetic speeds it up and the parasympathetic, it slows.
And both the sympathetic and parasympathetic talk to the nodes
But only the sympathetic to the myocardium goes.
A change in rate is chronotropic.
A change in force is inotropic.
A muscarinic receptor for acetylcholine
Opens potassium channels and that will mean
That as potassium leaves, it will oppose the leak
Of sodium entering and both will tweak
The membrane potential so that it’s slower
To reach the threshold and not from potassium, go lower.
So overall, to reach threshold, it takes longer
But the contraction is not stronger.
Beta 2 is the receptor for the sympathetic response
And norepinephrine it will ensconce.
Separate sodium channels will open and add to the leak
And faster get to the threshold peak.
But norepinephrine binds to the muscle
And in doing so, will make it hustle.
So overall, to reach threshold, it’s quicker
And contraction strength, a stronger ticker.
But epinephrine too, has the same effects
With the sympathetic flex.
Beta blockers which calm the heart
Antagonists that will slow the start
Bind to these receptors tightly
And make the heart beat more lightly.
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