The cardiovascular system, or the heart and blood vessels, delivers oxygen and nutrients to all body tissues, and is crucial for survival. Smooth muscles play a significant role in the normal anatomy and physiology of blood vessels. A vascular smooth muscle (VSM) is a type of smooth muscle found in the walls of blood vessels. Mural cell is a term that refers to the combination of connective tissue cells called pericytes and vascular smooth muscle cells in blood vessels. The presence of vascular smooth muscles in the blood vessels allows the latter to respond to changes in blood volume and pressure, thereby increasing supply to tissues that need blood and decreasing supply to tissues with less demand.
As a muscle, a VSM can contract, resulting in the narrowing or constriction of the blood vessels. In general, arteries have a greater amount of these muscles than veins. This is explained by the fact that arteries are subjected to higher pressures due to the pumping action of the heart. Therefore, gross and microscopic examination of blood vessels would reveal that arteries have thicker walls than veins.
Like a skeletal muscle, the contraction of a vascular smooth muscle is controlled by the nervous system. Unlike skeletal muscles, which are voluntarily controlled, vascular smooth muscles are innervated by the autonomic nervous system, particularly the sympathetic nervous system. Like a cardiac muscle, a vascular smooth muscle contains actin and myosin. It does not have the protein called troponin, which is found in cardiac muscle. Additionally, while a cardiac muscle contracts rapidly and quickly every few hundred milliseconds, a vascular smooth muscle contracts slowly in a sustained and tonic manner.
A vascular smooth muscle has several receptors that allow it to respond to signals from the sympathetic nervous system or to sympathetic stimuli. These receptors are alpha-1, alpha-2, and beta-2 receptors. When the neurotransmitter norepinephrine binds to the alpha-1 receptors, contraction of the vascular smooth muscle occurs, resulting in a decrease in blood vessel diameter or vasoconstriction. As the alpha-2 receptors are activated, vasoconstriction also occurs. When the beta-2 receptors are activated vasodilation, or increase in diameter of the blood vessel, occurs.
Vascular smooth muscle receptors are important in the control of blood pressure. For instance, people who are having a severe allergic reaction called anaphylactic shock usually have very low blood pressures. In order to restore blood pressure to normal, alpha-1 receptor-stimulating medications like epinephrine are given. Alternatively, among people who have high blood pressures, drugs like prazosin or doxazosin, which antagonize or oppose the effect of alpha-1 receptor stimulation, are given to vasodilate the blood vessels and decrease the blood pressure.
