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View submission: ELI5:Why the heart doesn't get cramp ?
There are 3 types of muscle in the body. Skeletal, smooth, and cardiac.
Smooth muscles are muscles that are found in the digestive system to create peristalsis - or the movement of food/nutrition through the digestive system. Smooth muscles are also in the female reproductive system.
Cardiac muscles = the heart/heart cells and control heart rate/function.
Smooth and cardiac are working without conscious thought, and these group of muscles are constantly working to maintain homeostasis/normal function. Skeletal muscles are voluntary, meaning a person has control over them.
So, because they are different types of muscle/cells (made up differently), they act differently and therefore, react differently to different situations, such as stress/fatigue.
Cardiomyocytes have a high mitochondrial density, which allows them to produce adenosine triphosphate (ATP) quickly, making them highly resistant to fatigue.
The Sinoatrial Node (or SA node which is located in the right atrium of the heart) is the hearts dominate pacemaker. The generation of pacemaking (or normal, regular heart rate/rhythm, which is 60-100 beats per min) is known as "automaticity." If the SA node fails (failure of the SA node would imply abnormal rhythms, a heart attack, a block, etc. - so you can think of this as the **cramp**) your heart's backup generator would then would kick in - the atrioventricular node (AV node, located between the atra and ventricles) would begin to take over the hearts pumping action. The AV node has a slower heart rate capacity and produces around 40-60 beats/min.
THEN - even if your SA and AV Nodes should fail, you are still left with a 3rd back up system - the Purkinje Fibers (or Bundle of His) which can sustain a beat of about 20-30, which is not really enough to help for long - and therefor at this point, you will need a different source (medical device/implant) to help pace your heart.
Hope this helps.
EDIT: Another relevant point (perhaps even more relevant than all the other afore mentioned stuff), is that people who become deficient or overly "intoxicated" with certain electrolytes (such as Potassium, Magnesium, and Calcium) are prone to cramps and spasms. And for someone who is unable to correct this (say, someone who has kidney failure and isn't filtering these products properly, which causes them to build up, or someone who has had their thyroid removed) it can begin to mess with the heart.
There's nothing here!