What is the striatum?

The striatum forms part of the basal ganglia, the portion of the brain involved in the control of voluntary movement, and participates in emotional, motivational, associative, and cognitive aspects of movement as well (Herrero et al., 2002). The basal ganglia belong to the extrapyramidal system as there is no direct connection to the spinal cord. The basal ganglia also control facial muscles and voluntary saccadic eye movements (Hikosaka, et al., 2000). The basal ganglia are important also because they are affected by Parkinson’s disease, which is now recognised to occur worldwide (Zhang and Roman, 1993, Zhang et al., 2005).
Anatomically, the basal ganglia comprise a set of synaptically connected structures, whose main input from the cortex is the corpus striatum, which corresponds to the nuclei caudatus and putamen in humans. The striatum, located inside the capsula interna, anterior to the thalamus, projects to the medial and lateral portions of the globus palidus, located medially with respect to the striatum. The nucleus subthalamicus, located ventrally with respect to the thalamus underneath the area tegmentalis, receives inputs from the lateral portion of the globus palidus, and projects to the substantia nigra pars reticulata, forming the indirect pathway. The projection from the striatum to the medial portion of the globus palidus constitutes the direct pathway. An important portion of the basal ganglia is the substantia nigra pars compacta that projects to the striatum.
The main input to the basal ganglia is the cerebral cortex, which projects to the striatum. The main output is to the thalamus. It is now accepted that the maps in the basal ganglia are topographycally organised. The striatum, for example, contains two partially overlapping body maps arising from the primary motor cortex and supplementary motor cortex respectively (Romanelli et al., 2005). This separation is not only anatomical, but also functional. Among the two outputs of the basal ganglia to the thalamus, the GPi conveys sensory-motor information, and the SNr conveys associative and cognitive information.