Myosin heads??

I was in a defensive mood last night and I dislike when I get like this. It's usually the result of when I let someone else get into my head with advice: "you deserve x!" or " don't put up with y!" Then I get all attitude-y but the truth is it doesn't suit me. I don't actually have the kind of energy it takes to hold onto negative feelings.

Last night was just a date (albeit a nice one) and whichever dudes are already in my heart, well, I care and can't shut the valve off just like that. So I'm not really all about the "fuckit." Although it was fun to pretend I was for 5 minutes.

Today I spent the day reading about the action potential of muscles -- damn that is some complicated stuff.

As I understand it:

Muscle contraction starts as an electrical signal in the brain which travels down a motor nerve called an axon. The axon connects the neuron and the muscle fiber. There are usually capillaries parallel to the nerve providing a blood supply.

So this is the part I don't get:

"This electrical impulse causes calcium ions to be released, which cause acetylcholine to be released into the tiny gap between the end of the nerve and the beginning of the muscle (the synapse). The acetylcholine activates nicotinic acetylcholine receptors on the muscle which opens sodium ion channels, leading to an "action potential" which then causes stores of calcium to be released from the sarcoplasmic reticulum."

(What the fuck is a sarcoplasmic reticulum? Is it the same as an endoplasmic reticulum but just for muscle cells?)

Anyway, almost there:

"The calcium ions expose the binding sites on the actin allowing myosin heads to bind to it, shortening the sarcomere. The sarcomere contraction continues until the bond between the myosin head and the actin filament is uncoupled by ATP. ATP also acts to reabsorb calcium back into the sarcoplasmic reticulum, reducing the stimulus for contraction."

What are myosin heads? I need to look this up. But I want to know because I'm trying to understand what is happening when a muscle knots up.

Happy Sunday!

(Sent from my phone)

Followup:
YouTube, you are awesome!