Introduction: Not long ago the concept of extraversion and introversion considered only in the context of psychology. However we know that the causes of human behavior depend on brain activity. This will provide a basis for psychotherapeutic and medical correction of social behavior disorders.

Aim: The aim of the study is to determine the features of the structure and functioning of the human brain, which determine the extroverted and introverted behavior of the individual.

Materials and methods: Analysis and systematization of neurobiological research results.

Results: The difference in behavi   patterns between introverts and extroverts lies in the different sensitivity of brain neuron receptors to dopamine and acetylcholine molecules.  Dopamine is a neurotransmitter that plays a major role in the functioning of the reward system. Extrovert’s dopamine receptors are less sensitive, so a certain dose will give a weaker chemical effect than for introverts. To achieve their “level of happiness”, such people must resort to external stimulation of the reward system. That is why extroverts spend more time on social contacts, looking for novelty and tend to take risks more often. Introverts have too much dopamine to cause excessive stimulation, so they try to refrain from situations that cause it. Scientists have confirmed this hypothesis in an experiment. The volunteers used a methylphenidate stimulant that increases the amount of dopamine, and then the researchers checked the speed of the reaction, As a result, methylphenidate did not affect the dopamine system of introverts. This confirms that the connection with the inner world is more important than the external reward. Acetylcholine, a neurotransmitter involved in internal rewards. It is easier to get it in a quiet environment, so introverts tend to be alone. In addition to different sensitivity to chemicals, there are differences in the length of the dopamine and acetylcholine pathways. The first passes through areas that process visual, auditory and gustatory information. And the second is much longer: it extends through the right and left frontal lobe, hippocampus, the right front insular. The considerable length of this neural pathway explains why introverts tend to overthinking.

Conclusion: Differences in the response of nerve cells to specific chemicals and length nerve pathways define the concepts of extraversion and introversion.