Striatum



The striatum is a subcortical part of the telencephalon. It is the major input station of the basal ganglia system. Anatomically, the striatum is the caudate and the putamen.

History
In the seventeenth and eighteenth centuries, the term "corpus striatum" was used to designate many distinct, deep, infracortical elements of the hemisphere (i.e. Vieussens, 1685). The Vogts (Cécile and Oskar, 1941) simplified the nomenclature by proposing the term striatum for all elements built with striatal elements (see Primate basal ganglia system): the caudate, the putamen and the fundus striati, that ventral part linking the two precedings together ventrally to the inferior part of the internal capsule.

The term neostriatum was forged by comparative anatomists comparing the subcortical structures between vertebrates because it was thought to be a phylogenetically newer section of the corpus striatum.

Structure
The dorsal striatum forms a continuous and large mass, topographically separated by the internal capsule into the caudate nucleus medially, the putamen laterally and the fundus below, linking the two preceding ventrally; but a single entity. The striatum is homogeneous in term of neuronal components. It is built up of 4 neuronal types:
 * spiny neurons relatively close from the pyramidal neurons of the cortex due to the presence of spines with spine apparatus (acanthodendritic neurons), and make up 96% of the striatum.
 * Leptodendritic (Deiter's) neurons(2%) with large, poorly bifurcated, arborisations looking like pallidonigral neurons.
 * Spidery cholinergic interneurons(1%) morphologically entirely different from those observed in rodents (which must lead to very careful interspecific correlations).In primates they are the Tonically Active Neurons (TANs). These briefly stop firing in concomitance to behaviourally salient situations and reward-related events.
 * GABAergic parvalbumin expressing interneurons, which are fast-spiking, and express dopamine receptors
 * GABAergic calretinin expressing interneurons
 * GABAergic somatostatin expressing interneurons, which are low threshold spiking and express dopamine receptors

Organization
The striatum is spatially organized according to several levels.

Anatomical subdivisions and territories
The dorsal striatum is a single entity closed and continuous with a toric topology. The observable anatomical subdivisions of the dorsal striatum (caudate nucleus and putamen) essentially induced by the internal capsule do not completely overlap with now accepted anatomo-functional subdivisions. The selective distribution of the axonal terminal arborisations of cortical sources differentiate the sensorimotor striatum, mainly putaminal but located in its dorsal part and in the lateroinferior part of the caudate. A great part of the remaining of the volume (essentially caudate) receiving from axonal endings from the frontal, parietal, temporal cortex forms the associative striatum. The separation between these two territories is rather clearcut and observable using calbindin immunochemistry. A third entity, the most inferomedial, raises more problems as there is no general agreement about its border with the associative striatum.

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

More information on the anatomical divisions of the striatum can be found in Voorn et al. (2004).

"Compartments"
Immunochemical characteristics particularly acetyl cholinesterase differentiated "compartments" called 'striosomes' and matrix in which 'matrisomes'are sometimes differentiated.

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 the lamina 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. The thalamostriatal afferent essentially comes in primates from the central region (center median parafascicular complex see primate basal ganglia system).This is glutamatergic. The participation of truly intralaminar neurons is much more limited. The striatum receives afferents from other elements of the basal ganglia such as the subthalamic nucleus (glutamatergic) or the external globus pallidus (GABAergic).

Targets
The main efferent target of the striatum is the pallidonigral set.The basal ganglia core is made up of the striatum and its direct targets through the striato-pallidonigral bundle. The striato-pallidonigral bundle is a very dense bundle of fewly myelinated axons giving the whitish aspect to the set. This comprises successively the external globus pallidus (GPe), the internal globus pallidus (GPi), the pars compacta of the substantia nigra (SNc) and the substantia nigra pars reticulata (SNr). This set is made up of the same genus of neurons. Its neurons are inhibited by GABAergic synapses from the dorsal striatum. Among these targets, one does not send axons outside the system (GPe, thus a regulator). Another sends axons to the superior colliculus. Two others are the bases of basal ganglia system output to the thalamus forming two separate channels: one through the internal segment of the globus pallidus to VO and from there to the cortical SMA and another through the substantia nigra to VA and from there to the frontal and the oculomotor cortex.

Function
Metabotropic dopamine receptors are present both on spiny neurons and on cortical axon terminals. Second messenger cascades triggered by activation of these dopamine receptors can modulate pre- and postsynaptic function, both in the short term and in the long term. The striatum is best known for its role in the planning and modulation of movement pathways but is also involved in a variety of other cognitive processes involving executive function. In humans the striatum is activated by stimuli associated with reward, but also by aversive, novel, unexpected or intense stimuli, and cues associated with such events. Recent fMRI evidence suggests that the common property linking these stimuli, to which the striatum is reacting, is saliency under the conditions of presentation. A number of other brain areas and circuits are also related to reward such as frontal areas.

For sources regarding saliency of the reward pathway (thought to be related to dopamine) one can look to the work of Dr. John Salmone (early to late 1990s) and Wolfram Schultz. The ventral tegmental dopaminergic neurons that innervate portions of the striatum have long been accepted to be the site of rewarding feeling. Intracranial stimulation studies from the 1960s show implants in this brain area will elicit bar pressing from rats for many hours at a time. However the collective works of researchers in the 1990s show that blocking dopamine receptors does not remove rewarding sensations, rather it effects how much the animal is willing to work, more motivation to seek reward rather than reward itself.

Pathology
Parkinson's disease results in loss of dopaminergic innervation to the striatum (and other basal ganglia) and to the cascade of subsequent consequences. The lesion of the striatum is also involved in the Huntington's disease, choreas, choreoathetosis and dyskinesias. It is also thought that addiction involves plasticity at striatal synapses.