For example, we can will ourselves breathe/blink faster, or choose to hold our breath. But at the same time, we can’t will a faster or slower heart rate or digestion when it might be advantageous to do so. What is the difference in the muscles involved or brain regions associated with these automated functions?
By the way, if you are lying in bed and want your heart to beat faster, take a sudden deep breath and hold it for a few seconds while noticing your pulse.
The diaphragm muscle moves down causing a vacuum in your chest.
That vacuum causes air to enter your lungs faster, but it ALSO sucks more blood back into your heart through the veins in your chest (increases “venous return”) (so now the walls of your heart stretch farther apart like a super inflated balloon.
The extra blood and stretch in your heart tricks the heart into needing to beat faster (“Starlings law”), so … surprise… you can voluntarily sort of control your heart rate that way if you really want to.
(Read about physiology; it’s awesome!)
You can slow it down by thinking about ice bath (James Bond movie style).
There are other ways of making your heart beat faster while lying in bed, but that’s beyond the scope of this ELI5 post. 🙂