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DISCOVERING THE BRAIN
be inhibitory, depending on prevailing conditions at the receptor site. Because ACh acts briskly and is subject to prompt breakdown in the synaptic cleft, it is well suited as the transmitter for motor neurons. Acetylcholine also acts in the autonomic nervous system, where it is responsible for such functions as contracting the pupil of the eye, slowing heartbeat, and stimulating salivation and digestion.
Two poisons that are well known to readers of mystery novels work their deadly effects by blocking the action of ACh. Curare, a plant extract used by South American Indians to treat their arrows for hunting, rapidly causes paralysis; botulin, a toxin produced by bacteria in improperly canned foods, paralyzes the muscles that control breathing and thereby causes suffocation. In clinical practice, drugs that block the action of ACh are useful in many ways. Short-lived ACh inhibitors are given in eye drops to dilate the pupil for ophthalmic examination. More enduring forms, such as atropine, reduce the secretion of saliva and bronchial fluid, which is helpful for anesthesia; hyoscine, or scopolamine, another related compound, is sometimes used as a sedative but does have the side effect of causing a very dry mouth. Conversely, drugs that inhibit the chemical breakdown of ACh and thus extend its action in the synaptic cleft are at least temporarily effective against myasthenia gravis; this is a crippling disease in which the body's own immune system attacks the receptor sites for ACh on the skeletal muscles. The muscles gradually weaken as they receive fewer synaptic transmissions, but drugs such as eserine can effectively increase the amount of ACh available to the remaining receptor sites.
In the cerebral cortex, ACh is thought to play a role in storing short-term memories. The hippocampus, for example, has dense areas of receptor sites for ACh, and their degeneration is one of the biological signs of Alzheimer's disease (see Chapter 4 ). Thus far, attempts to reverse symptoms by giving drugs that mimic or enhance the action of ACh have not been successful.
Along with ACh, another transmitter that is widely encountered throughout the nervous system is norepinephrine (also known as noradrenaline). In the central nervous system, the function of norepinephrine usually complements that of ACh; thus nor-