All the major biological explanations related to drug abuse assume that these substances exert their effects by altering brain chemistry or neuronal (basic functional cell of the brain) activity'

The answer is psychoactive.

Psychoactive effects are caused by psychoactive drugs like  Alcohol,  caffeine, nicotine, canabis or amphetamines, that change the mental state of someone, altering brain chemistry or neuronal activity.

This drugs change perceptions, moods, and/or consciousness.

All the major biological explanations related to drug abuse assume that these substances exert their psychoactive effects by altering brain chemistry or neuronal (basic functional cell of the brain) activity. Psychoactive effects is how drug substances alter and affect the brain’s mental functions leading to temporary changes in perceptions, moods and consciousness. Examples of substances with psychoactive effects are alcohol, caffeine, cannabis and LSD.

It is called the medulla oblongata or people usually call it medulla. it is the lower half of the brain stem continuous with the spinal cord.