When an action potential reaches the terminals at an axon’s end, it triggers the release of chemical messengers, called neurotransmitters. The neurotransmitter molecules crossthe synaptic gap and bind to receptor sites on the receiving neuron. For an instant, the neurotransmitter unlocks tiny channels at the receivingsite, and electrically charged atoms flow in, exciting or inhibiting the receivingneuron’s readiness to fire. Then, in a process called reuptake, the sendingneuron reabsorbs the excess neurotransmitters. This is also how nerve cells communicate with other nerve cells.
A particular pathway inthe brain may use only one or two neurotransmitters, and particularneurotransmitters may have particular effects on behavior and emotions. Acetylcholine (ACh) is one of the best-understood neurotransmitters.In addition to its role in learning and memory, ACh is the messenger at every junctionbetween a motor neuron and skeletal muscle. When ACh is released to our muscle cellreceptors, the muscle contracts. If ACh transmission is blocked, as happens duringsome kinds of anesthesia, the muscles cannot contract and we are paralyzed.
A problem the brain occurs when flooded with opiate drugs such as heroin and morphine, is that thebrain may stop producing its own natural opiates. When the drug is withdrawn, the brain may then be deprived of any form of opiate, causing intense discomfort. For suppressing the body’s own neurotransmitter production, nature charges a price. Drugs and other chemicals affect brain chemistry at synapses, often by either amplifying or blocking a neurotransmitter’s activity. An agonist molecule may be similarenough to a neurotransmitter to mimic its effects orit may block the neurotransmitter’s reuptake. Some opiate drugs, for example, producea temporary “high” by amplifying normal sensations of arousal or pleasure.This could lead to violent muscle contractions, convulsions, and possible death.Antagonists block a neurotransmitter’s functioning. For example, botulin, a poison that can form in improperly canned food, causes paralysis by blocking ACh release. (Small injections of botulin—Botox—smooth wrinkles by paralyzing the underlying facial muscles.)Other antagonists are enough like the natural neurotransmitter to occupy its receptor site and block its effect, but are not similar enough to stimulate the receptor.