Electrophysiology for the Heart. Luckily, all we need to know for the MCAT is you have an SA node that is the major pacemaker of the heart. You also have AV node, which has pacemaking activity. But the pace of the SA is faster, so it gets to dictate pace. So SA then conducts down to the AV node, both are in very close proximity to the atria of the heart. Read full transcript
And then through there you, this goes to bundle of his/left and right bundle branches. Which are just bundles that pass between the left and right ventricle. So this is our heart and this is right atrium, right ventricle, left atrium, left ventricle. You have SA and AV, you come down here and then you have the bundle branches that head down between both ventricles and then fanning out on either side are our Purkinje fibers.
And it's Purkinje fibers that contract and cause these ventricles to contract. And the important thing here, again the order is incredibly important, has shown up on recent MCATs. In fact, I remember it on my one of my MCATs as well. And other thing to understand there's pacemaker activity in all of these cells, really, but it's most predominant in SA node and it overrides all the other pacemaking activity.
Heart innervation, so I've alluded to this before. The heart does not need innervation to beat because it's myogenic. Nevertheless, it has some innervation from the Parasympathetic and Sympathetic nervous system. The parasympathetic nervous system slows it down, the sympathetic nervous system speeds it up.
But they don't control heart rate. And I just want you to be aware, you already know that it's acetylcholine that acts at the effector organs here, and norepinephrine, epinephrine here, specifically norepinephrine at the heart. I'm just gonna add it's the vagus nerve in the parasympathetic nervous system, that it's the actual nerve that's slowing down the heart.
So you have something called vagal slowing. Other thing I wanna add here, is sympathetic not only does it increase heart rate but it increases contractility. So, it increases the rate with which the heart beats and it increases the force with which the heart beats, which is called contractility