The striatum (pl.: striata) or corpus striatum is a cluster of interconnected nuclei that make up the largest structure of the subcortical basal ganglia. The striatum is a critical component of the motor and reward systems; receives glutamatergic and dopaminergic inputs from different sources; and serves as the primary input to the rest of the basal ganglia. Functionally, the striatum coordinates multiple aspects of cognition, including both motor and action planning, decision-making, motivation, reinforcement, and reward perception. The striatum is made up of the caudate nucleus, the putamen, and the ventral striatum. The lentiform nucleus is made up of the larger putamen, and the smaller globus pallidus. Strictly speaking the globus pallidus is part of the striatum. It is common practice, however, to implicitly exclude the globus pallidus when referring to striatal structures. In primates, the striatum is divided into the ventral striatum and the dorsal striatum, subdivisions that are based upon function and connections. The ventral striatum consists of the nucleus accumbens and the olfactory tubercle. The dorsal striatum consists of the caudate nucleus and the putamen. A white matter nerve tract (the internal capsule) in the dorsal striatum separates the caudate nucleus and the putamen. Anatomically, the term striatum describes its striped (striated) appearance of grey-and-white matter.
Structure
The striatum is the largest structure of the basal ganglia. The striatum is divided into two subdivisions, a ventral striatum and a dorsal striatum, based upon function and connections. It is also divisible into a matrix and embedded striosomes.
Ventral striatum The ventral striatum is composed of the nucleus accumbens and the olfactory tubercle. The nucleus accumbens is made up of the nucleus accumbens core and the nucleus accumbens shell, which differ by neural populations. The olfactory tubercle receives input from the olfactory bulb but has not been shown to play a role in processing smell. In non-primate species, the islands of Calleja are included. The ventral striatum is associated with the limbic system and has been implicated as a vital part of the circuitry for decision making and reward-related behavior.
Dorsal striatum The dorsal striatum is composed of the caudate nucleus and the putamen. Primarily it mediates cognition and involves motor and executive function. The dorsal striatum can be further subdivided into the dorsomedial striatum, and the dorsolateral striatum. Both of these areas have different roles in the acquisition of learnt behaviour and skill formation. The dorsomedial region receives projections from the frontal and the parietal cortices. The dorsolateral region receives projections from the sensorimotor cortex.
Matrix and striosomes Neurochemistry studies have used staining techniques on the striatum that have identified two distinct striatal compartments, the matrix, and the striosome (or patch). The matrix is seen to be rich in acetylcholinesterase, while the embedded striosomes are acetylcholinesterase-poor. The matrix forms the bulk of the striatum, and receives input from most areas of the cerebral cortex. Clusters of neurons in the matrix, called matrisomes receive a similar input. Their output goes to both regions of the globus pallidus and to the substantia nigra pars reticulata. The striosomes receive input from the prefrontal cortex and give outputs to the substantia nigra pars compacta. There are more striosomes present in the dorsal striatum making up 10-15% of the striatal volume, than in the ventral striatum.
Cell types
Types of cells in the striatum include:
Medium spiny neurons (MSNs), which are the principal neurons of the striatum. They are GABAergic and, thus, are classified as inhibitory neurons. Medium spiny projection neurons comprise 95% of the total neuronal population of the human striatum. Medium spiny neurons have two characteristic types: D1-type MSNs and D2-type MSNs. A subpopulation of MSNs contain both D1-type and D2-type receptors, with approximately 40% of striatal MSNs expressing both DRD1 and DRD2 mRNA. Cholinergic interneurons release acetylcholine, which has a variety of important effects in the striatum. In humans, other primates, and rodents, these interneurons respond to salient environmental stimuli with stereotyped responses that are temporally aligned with the responses of dopaminergic neurons of the substantia nigra. The large aspiny cholinergic interneurons themselves are affected by dopamine through D5 dopamine receptors. Dopamine also directly controls communication between cholinergic interneurons. There are many types of GABAergic interneurons. The best known are parvalbumin expressing interneurons, also known as fast-spiking interneurons, which participate in powerful feedforward inhibition of principal neurons. Also, there are GABAergic interneurons that express tyrosine hydroxylase, somatostatin, nitric oxide synthase and neuropeptide-y. Recently, two types of neuropeptide-y expressing GABAergic interneurons have been described in detail, one of which translates synchronous activity of cholinergic interneurons into inhibition of principal neurons. These neurons of the striatum are not distributed evenly. There are two regions of neurogenesis in the brain – the subventricular zone (SVZ) in the lateral ventricles, and the dentate gyrus in the hippocampal formation. Neuroblasts that form in the lateral ventricle adjacent to the striatum, integrate in the striatum. This has been noted in the human striatum following an ischemic stroke. Injury caused to the striatum stimulates the migration of neuroblasts from the SVZ, to the striatum, where they differentiate into adult neurons. The normal passage of SVZ neuroblasts is to the olfactory bulb but this traffic is diverted to the striatum after an ischemic stroke. However, few of the new developed neurons survive.
Inputs
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