Striatum

The striatum is a subcortical part of the forebrain and is the largest component of the basal ganglia, which is a group of nuclei in the brain associated with controlling voluntary motor movements, procedural learning, habit learning, eye movements, cognition, and emotion, among other things^[1].

The striatum is typically discussed as being comprised of the caudate nucleus and the putamen together, however, the ventral (lower portion of the) striatum also contains the nucleus accumbens and the olfactory tubercle, and the striatal fundus, which sits between the putamen and caudate nucleus, has also been included in some descriptions of the striatum^[3].

The striatum is most known for its role as the main input nuclei of the basal ganglia^[4].

1. Cerebral Cortex: All areas of the cortex send projections to the striatum, including the motor, premotor, prefrontal, and sensory areas. These inputs are primarily excitatory and utilise glutamate as the neurotransmitter.

2. Thalamus: The striatum also receives input from the centromedian (CM) and parafascicular (Pf) nuclei of the thalamus. These inputs also use glutamate as a neurotransmitter.

3. Substantia Nigra pars Compacta (SNc) and Ventral Tegmental Area (VTA): These midbrain areas provide significant dopaminergic inputs to the striatum, influencing reward prediction and motor control.

The main output nuclei of the basal ganglia are the internal segment of the globus pallidus (GPi) and the substantia nigra pars reticulata (SNr). They give rise to the major inhibitory output pathways from the basal ganglia to the thalamus and superior colliculus^[5].

The striatum communicates with these output nuclei indirectly through the 'indirect pathway' (which involves the external segment of the globus pallidus (GPe) and the subthalamic nucleus (STN)) and directly through the 'direct pathway'.

The striatum itself is traditionally divided into two parts: the dorsal striatum and the ventral striatum.

The dorsal striatum, also known as the neostriatum, is probably what most people refer to as the striatum as its subdivided into the caudate nucleus and the putamen. It's primarily involved in motor and action planning, the execution of movement, and cognitive processes, particularly those associated with habit formation or procedural memory^[7].

The ventral striatum includes the nucleus accumbens and the olfactory tubercle. The nucleus accumbens plays a central role in the reward circuit, with its dopaminergic inputs from the ventral tegmental area in the midbrain and its output to the prefrontal cortex. It's a critical mediator of reward perception and is involved in motivation, pleasure, and reinforcement learning, as well as in the psychological dependence associated with addiction^[8].

Research suggests that the striatum is organized into functional territories across the ventromedial-dorsolateral axis. This conceptual model subdivides the striatum into limbic, cognitive, and motor territories, reflecting its connections with limbic, prefrontal, and sensorimotor cortical areas, respectively^[9].

The striatum (caudate nucleus, putamen, and nucleus accumbens) are all rich in medium spiny neurons (MSNs), which utilise gamma-aminobutyric acid (GABA) as their main neurotransmitter and are also rich in dopamine receptors. These MSNs form the primary output system of the striatum and can be divided into two categories based on their output pathways and the type of dopamine receptor they express: D1-type MSNs contribute to the 'direct' pathway and express D1 dopamine receptors, while D2-type MSNs contribute to the 'indirect' pathway and express D2 dopamine receptors. Other less common neuronal types in the striatum include large spiny cholinergic interneurons and several types of GABAergic interneurons^[11].

The striatum receives a significant amount of dopaminergic inputs from the substantia nigra and the ventral tegmental area, and these inputs are crucial for the proper functioning of the basal ganglia circuitry. Dysfunction or degeneration in these systems has been linked to a number of neurological and psychiatric disorders, such as Parkinson's disease, Huntington's disease, addiction, and schizophrenia^[13].