What effect do amphetamines have on the body?

Amphetamines are classified as stimulants, which means they increase the activity of certain neurotransmitters in the brain, particularly dopamine and norepinephrine. By enhancing these neurotransmitter levels, amphetamines lead to increased brain activity, resulting in heightened alertness, increased concentration, and a surge of energy. This stimulation of the central nervous system can also raise heart rate and blood pressure, contributing to their stimulating effects on the body.

Considering the other choices, slowing down brain activity and the central nervous system, dilating blood vessels, or stiffening muscles does not align with the known pharmacological effects of amphetamines. These substances are not known to induce relaxation or muscle rigidity; rather, they create a stimulating effect, making choice D the correct characterization of their influence on the body.