"Involuntary actions always occur in response to stimuli picked up by receptors"- To which stimulus does the heart respond to keep beating/pumping blood through the body?

"Involuntary actions always occur in response to stimuli picked up by

1.	"Involuntary actions always occur in response to stimuli picked up by receptors"- To which stimulus does the heart respond to keep beating/pumping blood through the body?
Answer» "Involuntary actions always occur in response to stimuli picked up by receptors"- To which stimulus does the heart respond to keep beating/pumping blood through the body?