What is the main action of barbiturates?

The main action of barbiturates is to quiet nervous excitement. These drugs are central nervous system depressants that primarily work by enhancing the action of the neurotransmitter gamma-aminobutyric acid (GABA). By binding to GABA receptors, barbiturates increase the inhibitory effects in the brain, leading to sedation, decreased anxiety, and calming of hyperactive neural states. This action makes them effective in treating conditions like anxiety, insomnia, and seizure disorders.

The other options present actions that are not the primary effects of barbiturates. While some medications may increase muscle relaxation or affect cognitive function, such effects are not the primary mechanism associated with barbiturates. Additionally, they do not typically lead to an increase in blood pressure; rather, they can cause a reduction in blood pressure due to their depressant effects on the central nervous system. Thus, the most accurate description of barbiturates' main action is their ability to reduce nervous excitement.