Nucleus accumbens

The nucleus accumbens is the predominant component of the ventral striatum and therefore an important input region of the ventral basal ganglia.

Anatomical boundaries

The nucleus accumbens is bounded rostrally and laterally by the external capsule and medially by the lateral ventricle and septum. Its other boundaries are less distinct: caudally, it merges with the bed nucleus of the stria terminalis, while ventrally it transitions into the olfactory tubercle. Its dorsal boundary with the remainder of the striatum is also not sharply defined.

The 'core' and 'shell' components of the nucleus accumbens

The nucleus accumbens has two major subdivisions: the core and shell. The core consists of relatively densely packed neurons surrounding the anterior commissure and is bordered medially and ventrally by the shell. The shell surrounds these aspects of the core and can be further divided functionally into medial and lateral regions. The core and shell are not simply anatomical subdivisions; they differ in their cellular morphology, neurochemical characteristics, connections and functional organization.

Incoming neural afferents of the nucleus accumbens

The nucleus accumbens receives converging information from the hippocampal formation, amygdala and prefrontal cortex, together with important dopaminergic input from the midbrain. In broad functional terms, hippocampal inputs provide information about spatial and environmental context, amygdalar inputs contribute information about the significance or value of stimuli, and prefrontal inputs provide information related to goals, strategies and environmental contingencies. Dopamine has an important modulatory role in learning and in determining how these inputs influence nucleus accumbens activity.

Outgoing neural efferents of the nucleus accumbens

The core and shell participate in partly different basal ganglia circuits:

• Core circuitry resembles that of the dorsal striatum and communicates with the ventral pallidum and substantia nigra pars reticulata, ultimately influencing thalamic and brainstem targets.

• Shell circuitry is more distinctive: its medial and lateral regions communicate with corresponding parts of the ventral pallidum and with dopaminergic regions of the ventral tegmental area and substantia nigra. The medial shell additionally has direct projections toward hypothalamic and some brainstem regions.

Functional significance of nucleus accumbens

The nucleus accumbens is best understood as an integration and behavioural-selection region rather than simply a “reward centre.” By combining information about environmental context, stimulus value, goals and learning signals, it contributes to the learning and selection of flexible, goal-directed, exploratory and foraging behaviours. This integrative role explains why the nucleus accumbens occupies an important interface between limbic information and basal ganglia mechanisms involved in selecting behaviour.