Light micrograph (LM) of Nicotine crystals. Nicotine is an organic compound, an alkaloid found naturally throughout the tobacco plant, with a high concentration in the leaves. It constitutes 0.3 to 5% of the plant by dry weight. It is a potent nerve poison and is included in many insecticides. In lower concentrations, the substance is a stimulant and is one of the main factors leading to habit-forming qualities of tobacco smoking. As nicotine enters the body, it quickly gets distributed through the bloodstream and can cross the blood-brain barrier. It acts on the nicotinic acetylcholine receptors. The release of adrenaline causes an increase in heart rate, blood pressure and respiration, as well as higher glucose levels in the blood. Nicotine alkaloids are also found in the leaves of the coca plant. Magnification: x40 when shortest axis printed at 25 millimetres.