CNS stimulant
A CNS stimulant is the mirror image of a depressant: a drug that speeds up brain activity, raising alertness, attention, and wakefulness. In small doses it is the lift you feel from a cup of coffee; in stronger forms it is the basis of medicines for attention disorders and, on the illicit side, drugs of abuse.
Most stimulants act by increasing signalling of the monoamine neurotransmitters dopamine and norepinephrine. Amphetamines and methylphenidate boost their release and block their reuptake, raising synaptic levels; caffeine works differently, by blocking adenosine receptors that normally promote drowsiness. The result is a brain tilted toward excitation.
Clinically, stimulants treat attention-deficit/hyperactivity disorder (where they paradoxically improve focus), narcolepsy and other causes of excessive sleepiness, and occasionally appetite suppression. Caffeine and nicotine are the most widely used stimulants of all, though largely outside formal therapeutics.
Because they amplify dopamine reward signalling, many stimulants carry abuse and dependence potential, and high doses can cause anxiety, insomnia, raised heart rate and blood pressure, and in overdose, seizures or dangerous cardiovascular events.
Methylphenidate is prescribed for ADHD because, by raising dopamine and norepinephrine in attention circuits, it improves a child's ability to focus and sit still.
Therapeutic stimulant use targets specific attention circuits.
The paradox that stimulants calm hyperactive children with ADHD makes sense once you see that they strengthen the dopamine and norepinephrine circuits the prefrontal cortex uses to focus and inhibit impulses.