The basal ganglia we have been discussing handle movement. But there is a part of the same anatomical family that handles something different: reward and motivation. That part is the nucleus accumbens, also called the ventral striatum.
The nucleus accumbens sits in the ventral part of the striatum, near the bottom of the brain. Like the rest of the striatum, it receives cortical input — but the input pattern is different. Where dorsal striatum receives from motor and somatosensory cortex, ventral striatum receives heavily from prefrontal cortex, amygdala, and hippocampus. The signals reaching the accumbens are about value, prediction, and emotion, not about movement plans.
The accumbens receives its dopamine not from the substantia nigra, but from the ventral tegmental area (VTA), which we met in Stage 2. The VTA-to-accumbens projection is the mesolimbic pathway, and it is the cellular substrate of reward and craving.
Here is the critical clinical fact. Every drug of abuse — cocaine, methamphetamine, opioids, nicotine, alcohol, cannabis, MDMA — ends up making the VTA fire dopamine into the nucleus accumbens. They reach this end point through different molecular routes: cocaine blocks dopamine reuptake at the accumbens itself; opioids disinhibit VTA neurons by suppressing GABAergic interneurons; nicotine directly excites VTA neurons through nicotinic receptors; alcohol works through several mechanisms including indirect dopamine elevation. The biochemistry is different. The destination is the same.
Over time, repeated dopaminergic surges to the accumbens reorganize the circuit. The system down-regulates in the absence of the drug — synaptic dopamine drops, postsynaptic receptors normalize, and the natural rewards (food, sex, social interaction, work, family) feel less rewarding than before. This is part of why people with substance use disorders describe a flattening of normal pleasure: the system has been recalibrated to the supraphysiological surge of the drug, and ordinary reward no longer reaches the threshold the system expects.
Meanwhile, cues that predict the drug — the corner, the smell, the time of day, the bar, the people — gain salience. The cortex and amygdala learn the association, and the accumbens responds to the cue with anticipatory dopamine release, generating the felt experience of craving. We will see this in detail in the craving clinical scenario in Stage 11.
Hold this geography. The dorsal striatum gates movement. The ventral striatum (nucleus accumbens) gates motivation and reward. Both are basal ganglia. Both run on dopamine. But the consequences of dysregulation in each are profoundly different — Parkinson's disease vs addiction.