Nitric Oxide Molecule: Medical visualization of a nitric oxide molecule. Nitric oxide is an important signaling molecule, and is so small, that unlike other neurotransmitters, it can diffuse quite far and act on several nearby neurons, not just those within the synapse connection. Nitric oxide causes relaxation of smooth muscle and increases blood flow, thereby playing an important role in digestion as well as the development and maintenance of erection. Nitric oxide doesn't affect blood flow directly, but instead initiates a series of chemical reactions that allow the blood vessels to dilate. The most important milestones along this chemical pathway are two proteins known as cyclic GMP (cGMP) and PDE 5. cGMP is the protein that signals the muscles in the blood vessel walls to relax and fill with blood. PDE 5 breaks down cGMP to prevent overstimulation of the blood vessels and excessive relaxation. If not enough cGMP is made or if cGMP is broken down too quickly, the blood vessels won't remain relaxed long enough to fill with sufficient blood to allow the penis to become erect. Current erectile dysfunction drugs work by blocking the effect of PDE 5 and allowing cGMP to remain active longer. These drugs can have beneficial sexual effects for men-and, some studies have found, for women. They make it easier for men to both achieve and maintain an erection, and for women to experience the vaginal lubrication and the enhanced clitoral sensitivity associated with normal sexual arousal.
Nitric oxide (nitrogen oxide, nitrogen monoxide) is a molecular, chemical compound with chemical formula of NO that is a colorless gas under standard conditions. Nitric oxide is a free radical—i.e., its bonding structure includes an unpaired electron—and it is in the class of heteronuclear diatomic molecules that are of historic theoretical interest (for the insights they gave in formulating early modern theories of bonding). It is a practically important intermediate in the chemical industry and day-to-day life, for instance appearing as a by-product of incomplete combustion of fuels burned in fossil fuel power plants and automobile engines, and it is produced naturally during the electrical discharges of lightning in thunderstorms.
In mammals including humans, NO is an important cellular signaling molecule involved in many physiological and pathological processes. It is a powerful vasodilator with a short half-life of a few seconds in the blood. Long-known pharmaceuticals such as nitroglycerine and amyl nitrite were found to be precursors to nitric oxide more than a century after their first use in medicine. Low levels of nitric oxide production are important in protecting organs such as the liver from ischemic damage.
Despite being a simple molecule, NO is an important biological regulator and is therefore a fundamental component in the fields of neuroscience, physiology, and immunology. It was proclaimed “Molecule of the Year” in 1992. Research into its function led to the 1998 Nobel Prize for discovering the role of nitric oxide as a cardiovascular signalling molecule. Nitric oxide should not be confused with nitrous oxide (N2O), an anaesthetic, or with nitrogen dioxide (NO2), a brown toxic gas and a major air pollutant, the latter being a product to which nitric oxide is rapidly oxidised in air.
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