Vasomotor tone is the amount of tension in the smooth muscle inside the walls of blood vessels, particularly in arteries. The tension is controlled by the sympathetic nervous system through the release of norepinephrine, a neurotransmitter that stimulates receptors inside the vessels' walls.
The amount of tension on the walls of the vessels constricts or dilates the vessels. Nerve fibers in the middle layer of the vessel, known as the tunica media, secrete neurotransmitters into the extracellular fluid surrounding the smooth muscle in the tunica media. The smooth-muscle cells have receptors for the neurotransmitters. When some of these receptors are activated, it causes constriction of the vessels, and when others are activated, it causes dilation.
Certain chemicals such as endothelin cause constriction of the smooth muscles in blood vessels. Other chemicals such as nitric oxide cause dilation. Acute increases in vasomotor tone are accompanied by similar increases in blood pressure and need to be treated immediately. These sudden increases are often caused by some other condition such as artery occlusion. Loss in the vasomotor tone of blood vessels because of brain injury or other trauma can result in extreme dilation of the vessels, a serious condition known as neurogenic shock.