Eli5: How does the heart keep beating, and how did it start beating in the first place?


Eli5: How does the heart keep beating, and how did it start beating in the first place?

In: 1

Two step answer here.

1. Heart beat is AUTONOMOUS ELECTRICAL MUSCULAR activity, pretty much the building block of all nervous systems. Where did that start? Most likely with something super simple like the [Bacterial Flagellar Motor](https://www.youtube.com/watch?v=cwDRZGj2nnY)

2. But what about hearts themselves i.e something that pumps liquid across the body in steady intervals like a clock? We think Jellyfish are the earliest organisms to have evolved something like a heart about 500 million years ago. Jellyfish have a simple cardiovascular system consisting of a muscular sac that pumps fluid, which is believed to function similarly to a heart. The sac, called a gastrovascular cavity, circulates nutrients and oxygen throughout the jellyfish’s body and also helps to remove waste products. Perhaps not quite as steady as human hearts but the starting point nonetheless.

The heart has two regions which act as natural pace makers: the predominant one is the SA Node (sino-atrial) and the secondary one is the AV Node, atrial ventricular.

These regions kick off the contraction process for the entire heart. The SA Node fires a little more frequently than the AV Node, so when all is normal, the heart is paced by the SA Node.

If a small heart attack kills the SA Node, then the AV Node would take over.

*My training is going on 30 years old, so please correct any mistakes I have made.

Only some cells in the heart are capable of beating continuously. This is called the sino-atrial node
This is the heart’s own little “brain” you can say, because it does not need continuous electricity impulse from the brain for every time it contracts. It gives it’s own signal to itself (however the sino atrial node is under regulation from brain which tells it how fast or slow to contract via the sympathetic and parasympathetic nerves)

These SA node cells are only slightly different from other heart cells in that the sodium movement inside the cell is not voltage gated, meaning there is no need of electrical impulse for ions to move in (rest of the heart cells get their current from SA node, SA node itself doesn’t get any current from outside)

Let me know if you need further clarification