Final answer:
Tranquilizers in low or moderate doses primarily affect the limbic system. They increase the activity of the GABA neurotransmitter system in the brain, which has a quieting effect and is often used to treat anxiety and insomnia.
Step-by-step explanation:
In low or moderate doses, tranquilizers mostly affect the limbic system. Tranquilizers, which are a type of depressant, work by increasing the activity of the gamma-Aminobutyric acid (GABA) neurotransmitter system in the brain. This has a quieting effect on the brain and is often prescribed to treat anxiety and insomnia. While most depressants reduce activity in the central nervous system fairly uniformly, tranquilizers primarily target the limbic system.