User:Keepstherainoff/striatum

Potentially useful link (or possibly later reviews/papers that cite it): http://www.ncbi.nlm.nih.gov/pubmed/7754303 Neurophysiology and cognitive function of the striatum

Organization

Dorsal and ventral division

The striatum is typically divided into dorsal and ventral parts. The dorsal striatum is the collective term for three contiguous structures. The structures are:

• The caudate nucleus
• The putamen
• The fundus, linking the caudate nucleus and the putamen.

In primates, the caudate nucleus resembles a C-shape structure with a wider "head" (caput in Latin) at the front, tapering to a "body" (corpus) and a "tail" (cauda). The putamen, along with the globus pallidus, forms the lense-shaped lentiform nucleus. The front of the caudate nucleus is continuous with the front part of the putamen. The lentiform nucleus and the body of the caudate nucleus are seperated by the internal capsule.

The ventral striatum comprises:

• The nucleus accumbens
• The olfactory tubercle

In primates, the ventral striatum is clearly delineated by tracing the subicular (subiculum) territory. This corresponds to the olfactory tubercle and the nucleus accumbens, which is not a nucleus, but is a striatal part made up of striatal elements.

Gross Anatomy

Although the dorsal and ventral striatum are largely physically seperate in the primate, they are less so in other species, being contiguous in the rodent. Despite the widespread use of the terms dorsal and ventral striatum, the striatum can be divided according to several criteria with each producing a different boundary.

Whilst the division of dorsal and ventral striatum is commonly defined as the border between the nucleus accumbens and caudate–putamen complex this border has no clear histological or immunohistochemical foundation.

A straight line from the inferior tip of the lateral ventricle medially to the most medial extension of the external capsule laterally has also been adopted as an imaginary boundary between dorsal and ventral striatum.

Cytoarchitecture

The caudate nucleus and putamen may be considered cytologically identical (DeLong and Georgopoulos 1981). The densities and relative proportions of the various cell types do not differ strongly between striatal regions, although cytoarchitectonic and chemoarchitectonic complexity is greater in ventral striatum [21,22]. However, no clear boundary between the dorsal and ventral striatum can be established on the basis of cytoarchitecture, myeloarchitecture or chemoarchitecture, although there are suggestions of a gradiated difference, although on a dorsallateral/ventromedial axis.

Functionality

However, despite being defined by their connection to functionally distinct regions of the brain, ventral areas of dorsal striatum can also mediate the motivational functions ascribed to the ventral striatum.

Connectivity

Others have used connectivity as the defining criterion, where ventral striatum is equated with limbic-innervated striatum (i.e. that receiving input the hippocampus, amygdala, prefrontal cortex and major serotonin-releasing pathways) whilst the dorsal striatum is equated with areas of striatum receiving input from sensorimotor areas.

A division between dorsalateral and ventromedial striatum has been proposed, based on the external connectivity (Voorn et al. 2004)

Subdivisions within the dorsal and ventral striatum also differ, depending on the criterion used. All regions can be subdivided into patches, or striosomes, based on neurochemistry and connectivity (Gerfen and Wilson 1996).

Dorsal striatum subdivision

The dorsal striatum can be subdivided anatomically into the caudate nucleus and putamen based on their physcial seperation by the internal capsule. However, this subdivision does not completely overlap with now accepted subdivisions based on function or connections with other brain regions. Sensorimotor cortical projections which deliniate the sensorimotor striatum terminate mostly in the dorsal putamen but also in the lateroinferior part of the caudate. The rest of the dorsal striatum (essentially caudate nucleus) receives projections from the frontal, parietal, temporal cortex, and forms the associative striatum. However, a third division of viscerolimbic striatum exists, which has no general agreement about its border with the associative striatum.

The striatum can also be differentiated based on immunochemical characteristics—particularly with regard to acetylcholinesterase—into "compartments", consisting of 'striosomes' and 'matrisomes'. The separation between caudate and putamen is rather clearcut and observable using calbindin immunochemistry.

Overview of the main circuits of the basal ganglia. Striatum is shown in blue. Picture shows 2 coronal slices that have been superimposed to include the involved basal ganglia structures. + and - signs at the point of the arrows indicate respectively whether the pathway is excitatory or inhibitory in effect. Green arrows refer to excitatory glutamatergic pathways, red arrows refer to inhibitory GABAergic pathways and turquoise arrows refer to dopaminergic pathways that are excitatory on the direct pathway and inhibitory on the indirect pathway.

Inputs (afferent connections)

The most important afferent in terms of quantity of axons is the corticostriatal connection. Many parts of the neocortex innervate the dorsal striatum. The cortical pyramidal neurons projecting to the striatum are located in layers II-VI, but the most dense projections come from layer V. They end mainly on the spines of the spiny neurons. They are glutamatergic, exciting striatal neurons. Another well known afferent is the nigrostriatal connection arising from the neurons of the substantia nigra pars compacta. While cortical axons synapse mainly on spine heads of spiny neurons, nigral axons synapse mainly on spine shafts. In primates, the thalamostriatal afferent essentially comes from the center median-parafascicular complex of the thalamus (see primate basal ganglia system). This afferent is glutamatergic. The participation of truly intralaminar neurons is much more limited. The striatum also receives afferents from other elements of the basal ganglia such as the subthalamic nucleus (glutamatergic) or the external globus pallidus (GABAergic).

Targets (efferent connections)

The basal ganglia core is made up of the striatum along with the regions to which it projects directly, via the striato-pallidonigral bundle. The striato-pallidonigral bundle is a very dense bundle of sparsely myelinated axons, giving a whitish appearance. This projection comprises successively the external globus pallidus (GPe), the internal globus pallidus (GPi), the pars compacta of the substantia nigra (SNc) and the pars reticulata of substantia nigra (SNr). The neurons of this projection are inhibited by GABAergic synapses from the dorsal striatum. Among these targets, the GPe does not send axons outside the system. Others send axons to the superior colliculus. Two others comprise the output to the thalamus, forming two separate channels: one through the internal segment of the globus pallidus to the ventral oralis nuclei of the thalamus and from there to the cortical supplementary motor area (SMA) and another through the substantia nigra to the ventral anterior nuclei of the thalamus and from there to the frontal cortex and the occulomotor cortex.