The American Psychological Association, in Washington, ., is the largest scientific and professional organization representing psychology in the United States. APA's membership includes more than 115,700 researchers, educators, clinicians, consultants and students. Through its divisions in 54 subfields of psychology and affiliations with 60 state, territorial and Canadian provincial associations, APA works to advance the creation, communication and application of psychological knowledge to benefit society and improve people’s lives.
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In the brain the level of extracellular dopamine is modulated by two mechanisms: phasic and tonic transmission .  Phasic dopamine release, like most neurotransmitter release in the nervous system, is driven directly by action potentials in the dopamine-containing cells.  Tonic dopamine transmission occurs when small amounts of dopamine are released without being preceded by presynaptic action potentials.  Tonic transmission is regulated by a variety of factors, including the activity of other neurons and neurotransmitter reuptake.