atropine

atropine is an alkaloid derived from the solanaceous plants Atropa belladonna (deadly nightshade), Hyoscyamus niger (black henbane), and Datura stramonium (thornapple). These plants contain a mixture of two closely related alkaloids, hyoscyamine and hyoscine; atropine is a mixture of two isomers of hyoscyamine. In 1867, von Bezold found that atropine blocked the slowing of the heart caused by vagal stimulation. We now know that atropine blocks the action of the neurotransmitter acetylcholine at all the nerve endings where the membrane receptors are of the so-called muscarinic type. This includes those of the parasympathetic nervous system in the heart, glandular tissue, and smooth muscle. Thus atropine causes a rise in heart rate and inhibits secretions (for example of saliva, causing a dry mouth, and of the digestive enzymes). It also relaxes smooth muscle in the gastrointestinal tract, the urinary bladder, and the bronchial trees, by preventing the effects of the normal background discharge of parasympathetic neurons to these organs.

The central nervous system also contains muscarinic receptors. Blockade of these by atropine leads to restlessness and mental excitement, and can improve the rigidity and tremor characteristic of Parkinson's disease. Large doses of atropine can cause hallucination.

Long-lasting pupillary dilation results if atropine drops are placed in the eye. The iris has both circular and radial muscles, and the balance between the tonic activities of these two muscle groups controls the pupil diameter. The circular muscle is under parasympathetic control, so when the transmitter, acetylcholine, is blocked with atropine, the pupil will dilate. It is told that Spanish ladies put atropine drops in their eyes for the allure given by large, black pupils: hence the name belladonna — ‘fine lady’.

Alan W. Cuthbert

See also autonomic nervous system; neurotransmitter; membrane receptors.