Content Reviewers:Yifan Xiao, MD, Viviana Popa, MD, Rishi Desai, MD, MPH, Darren Miller, Robyn Hughes, MScBMC
It does so by contracting around 70 times per minute.
In order for cardiomyocytes to contract, they first need to depolarize.
Depolarization is when ions move across the membrane of a cell, and the membrane potential becomes less negative or even slightly positive.
Think of a really pessimistic negative cell throwing his hands up and enjoying a moment of joy.
When one cell depolarizes enough - it can cause some ions like calcium to flow into neighboring cells and trigger them to depolarize as well.
If one cell after another depolarizes, then there’s a depolarization wave which you can imagine would look like a wave moving through a crowd at a football stadium.
So if depolarization waves are going through about once per second, that means that your heart beats once per second, or sixty times in a minute.
Now let’s zoom in on a cardiomyocyte.
When ions like calcium move from that cell into a neighboring cell, this triggers depolarization, and cardiomyocytes depolarize one after another.
Another feature of cardiomyocytes are passageways called transverse tubules, or T-tubules.
One last important element to depolarization and contraction is the sarcoplasmic reticulum, which is an organelle that stores the intracellular calcium.
When a depolarization wavefront hits a cardiomyocyte, a few calcium ions flow through gap junctions,
If there’s depolarization, then calcium and sodium ions start to move across the cell membrane and into the cell.
That’s where the T-tubules play a key role, by bringing calcium deep into the cell.
Once this extracellular calcium gets inside, it binds to the ryanodine receptors on the sarcoplasmic reticulum, which releases even more calcium into the cell - a process called calcium-induced calcium release.
This process helps to activate two contractile proteins, actin and myosin, which are called myofilaments.
Myosin is able to attach and pull actin with the help to adenosine-triphosphate or ATP to form cross-bridges that result in shortening of the muscle fiber.
Eventually, calcium ions are removed by ion transporters, that rely on ATP or concentration gradients.
Since calcium is stored in the sarcoplasmic reticulum, concentrations of calcium will vary with: how much calcium there is intracellularly and how much calcium is stored within the sarcoplasmic reticulum to be released.
One of the main methods intracellular calcium can be changed is with the autonomic nervous system.
Activation of the beta 1 receptors leads to downstream phosphorylation of proteins like sarcolemmal calcium channels on the sarcoplasmic reticulum membrane which increases the sarcoplasmic reticulum’s ability to release calcium.
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