Which neurotransmitter is released by sympathetic nerve fibers to cause vasoconstriction?

Norepinephrine is the neurotransmitter released by sympathetic nerve fibers that plays a crucial role in causing vasoconstriction. When the sympathetic nervous system is activated, such as during a fight-or-flight response, norepinephrine is released from the nerve endings and binds to adrenergic receptors on the smooth muscle cells of blood vessels. This binding leads to the contraction of these muscle cells, resulting in the narrowing of the blood vessels, or vasoconstriction. This physiological response helps redirect blood flow to essential organs and muscles, increasing blood pressure and supporting heightened physical activity.

Understanding the role of norepinephrine in this process highlights its significance in the body's ability to respond to various stressors. In contrast, other neurotransmitters mentioned, such as dopamine, acetylcholine, and serotonin, are involved in different physiological functions and pathways that do not primarily drive vasoconstriction through the sympathetic nervous system. For instance, acetylcholine is often associated with parasympathetic activities, like promoting relaxation and vasodilation rather than constriction. Therefore, norepinephrine is uniquely positioned as the primary neurotransmitter linked to the sympathetic-induced vasoconstriction response in the systemic circulation.