Sympathetic nerve fibers travel around the tunica media of the artery, secrete neurotransmitters such as norepinephrine into the extracellular fluid surrounding the smooth muscle (tunica media) from the terminal knob of the axon.
Endothelin, and angiotensin are the vasocontrictors of smooth muscles while nitric oxide and prostacyclin are vasodilators of the smooth muscles [REF].