Nucleus accumbens

Location

The nucleus accumbens forms most of the ventral striatum. It is situated in the basal forebrain anterior to the anterior commissure. It lies just inferior to the anterior limb of the internal capsule, and is continuous with the putamen dorsolaterally and the head of the caudate nucleus dorsomedially. The nucleus accumbens also lies in close proximity to the olfactory tubercle of anterior perforated substance, fundus striati (the transitional zone between the caudate nucleus and the putamen in the ventral portion of the dorsal striatum) and to the Broca’s diagonal band and the septal nuclei. It is anatomically located in a unique way to represent a central structure between the amygdala, prefrontal cortex, basal ganglia and thalamus. That said, the nucleus accumbens has a modulatory role in processing of information from the amygdaloid complex to these regions and thus modulates the emotional and behavioral components of feelings.

Structure and connectivity

Morphologically, the nucleus accumbens has been shown to be longest along its anterior-posterior axis and shortest on the dorsal-ventral axis. It is divided into two functionally distinct subregions: a shell region which is related to the limbic system, and a central core region which is related to the extrapyramidal motor system. In a human brain, these regions are difficult to distinguish by gross inspection.

The nucleus accumbens receives afferent projections from the prefrontal cortex, amygdaloid complex (via the ventral amygdalofugal pathway), cells of the bed nucleus of the stria terminalis, amygdalofugal fibers that course through the stria terminalis, the hippocampal formation (via the precommissural fornix), thalamus, and from midbrain (mesencephalon) structures including the ventral tegmental area (VTA) via the medial forebrain bundle and substantia nigra. The nucleus accumbens in turn sends efferent projections to the midbrain, hypothalamus, amygdala, nuclei of the brainstem, and the globus pallidus. The nucleus accumbens, through these broad connections mediates several of its functions. For example, the VTA has been shown to play a key role in reward processing. Thalamic connections are involved in feeding and sexual behavior, while the prefrontal cortex plays a role in motivation.

The nucleus accumbens receives dopaminergic projections from the ventral tegmental area and substantia nigra of the midbrain. This connection forms part of a major dopaminergic pathway called the mesolimbic pathway (reward pathway), which connects the VTA with regions of the limbic system including the nucleus accumbens, hypothalamus, amygdala, hippocampus and the prefrontal cortex. Dopamine has been shown to be the major neurotransmitter of the nucleus accumbens and plays an important role in the experience of reward and pleasure. The nucleus accumbens is thus associated with the mesolimbic dopamine pathway and is said to mediate the psychomotor effects of stimulant drugs and addiction.

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Function

The nucleus accumbens functions as an important modulation center between the limbic and the motor systems because of its input from the limbic system and its output to motor nuclei of the basal ganglia. The nucleus accumbens is involved in various cognitive, emotional, and psychomotor functions. It serves as an important area for motivation, reward and pleasure, addiction, impulsivity and risk taking behaviours. It is also involved in the control of survival and reproductive behaviours.

The nucleus accumbens has also been implicated in several psychiatric disorders such as schizophrenia, depression, Parkinson’s disease and Alzheimer’s disease, and in recent years has become a target for psychosurgical interventions.