Norepinephrine acts on the heart by
a. causing treshold to be reachad more quichiy
b. blocking the action of calcium e
c. causing a decrease in stroke volume
d. decreasing heart contractility

The correct option is : a. causing threshold to be reached more quickly      

Explanation:

Norepinephrine, also known as noradrenalin, is an organic compound, which belongs to the catecholamine family. It acts as a hormone and a neurotransmitter in the brain and the body and also impacts the heart.

It is one of the hormones secreted by the adrenal medulla. Norepinephrine is a part of the fight-or-flight mechanism of the body. It acts by binding to the β-1 receptors and opens the chemical or ligand gated calcium and sodium ion channels. Therefore, it causes the threshold to be reached faster by increasing the rate of depolarization and thus causes heart arrhythmia.

Therefore, Norepinephrine acts on the heart by causing threshold to be reached faster.