Drugs and the Nervous System

A psychoactive drug is a chemical that acts upon the central nervous system, altering brain function. This can result in temporary changes of a person's mood and how the person views certain things, as well as their judgment and behavior. Lysergic acid diethylamide (LSD), caffeine, and mescaline are examples of psychoactive drugs. Other psychoactive drugs are used to relieve pain or help with depression. The problem with using some of these drugs (especially the depressants and stimulants) is that they can be habit-forming, causing chemical dependency, and may lead to substance abuse.

To understand the effects of psychoactive drugs on the nervous system, it is first important to know about the mechanisms used by nerve cells to transmit messages. The synapse is the space found between two nerve cells, across which neurotransmitters travel. The synapse is the location of signal transmission from one neuron (the presynaptic neuron) to another receiving neuron (the postsynaptic neuron), muscle, or gland. Neurons communicate with one another using neurotransmitters, which flow directly between cells. However, most synapses are chemical, and electrical signals from the presynaptic neuron are converted to the release of a neurotransmitter, which is a chemical that binds to neuronal receptors and excites or inhibits postsynaptic cells. For example, gamma aminobutyric acid (GABA) is a neurotransmitter that works in neurons within the brain's cortex to influence motor control and vision and regulate the body's reaction to anxiety. During a chemical synapse, an action potential reaches the axon terminal of a presynaptic neuron, and neurotransmitters are released from a synaptic vesicle, which is a structure that holds neurotransmitters, into the gap between cells, or synaptic cleft. Neurotransmitters bind to receptors on the membrane of the next cell and make this cell either more or less likely to fire an action potential, depending on the nature of the neurotransmitter.

Neurotransmitters have an excitatory or inhibitory effect on postsynaptic neurons or glands. Common neurotransmitters include the following:

• glutamate—an amino acid used to activate synapses in the CNS
• serotonin—considered to play a critical role in depression; regulates many functions, including sleep, appetite, and mood
• acetylcholine—activates skeletal muscle and can inhibit or excite organs of the autonomic nervous system
• dopamine—neurotransmitter that regulates motor behavior and the brain's pleasure and reward centers
• epinephrine/norepinephrine—play a key role in the fight-or-flight response by allowing the body to respond to emergency situations

Psychoactive drugs produce their effects by binding to the receptors of the neurotransmitters. For example, NMDA, which is N-methyl-D-aspartate or C₅H₉NO₄, is an amino acid receptor that activates glutamate receptors and works as a nonselective ion channel allowing ionized calcium to flow into a cell. NMDA receptors are located in the membrane of a postsynaptic neuron and are associated with memory function and brain aging. Specific drugs, called agonists, bind to these receptors and may counteract the natural reactions occurring at NMDA receptor sites. NMDA receptor agonists work to block the receptors from full activity and are possible controls for Alzheimer's disease, Lou Gehrig's disease, and Parkinson's disease. As such, psychoactive drugs can alter the amount of each neurotransmitter that is stored in the vesicles of the terminal ends of the neuron. They can also block the neurotransmitter from attaching to the postsynaptic terminal altogether. This prevents the signals from reaching the next neuron. Some drugs alter the release or the deactivation of the neurotransmitter. In the case of stimulants, such as caffeine, the drug causes more and more of the neurotransmitter to be released, increasing the activity of the neuron. This is what produces the "rush" a person feels when drinking caffeinated beverages. Depressants, such as heroin, have the opposite effect, reducing the activity of the neurotransmitters and making a person feel relaxed and at ease.