Where are the basal ganglia?
It was as early as 1664 when the first clear identification of distinctsubcortical structures was published by the English anatomist ThomasWillis (Parent, 1986). What is now functionally known as the basalganglia was then referred to as the corpus striatum. It held such a central position,striped with a wide range of cortical and brainstem fibers, that at the timeit was believed to be the "sensorium commune" as defined by Aristotle. Itwas a structure thought to both receive all sensory modalities and initiate all motor acts. This idea appeared to be anatomically reinforced byits central position and clearly visible ascending and descending fibersystems.
Two subsequent events relegated the corpus striatum to anobscure and less defined position. The attractiveness of the histologicalorganization of the cortex, and the possibility of localizing higher mentalfunctions drew many neurologists of both the 18th and 19th centuries tocortical research. Amongst those that continued studying the corpus striatum,there was a sudden realization that many of the functions originallyassigned to it were in fact properties of neighbouring corticospinal paths(Wilson, 1914). As Wilson (page 428) describes, the corpus striatum"seemed to fall from its high estate and depreciate in physiologicalsignificance".
At the beginning of the 20th century there were serious attempts to providedetailed comparative descriptions of the corpus striatum(Wilson 1914, Cajal 1911). It began to gain importance once again withthe discoveries that lesions of these areas would often result in disorders of motor functions in humans(Wilson 1914, Vogt 1911). The corpus striatum came to be viewed as the major componentsof the "extrapyramidal motor system" (Parent, 1986). This term loosely groupedthe corpus striatum with an array of brain stem nuclei and reflected theassumption that this grouping constituted a complete and independent motorunit (Carpenter 1981). The term "basal ganglia" has been generally used torefer to these major anatomical telencephalic subcortical nuclei at the baseof the forebrain. More formally this definition groups the corpus striatum(striatum and globus pallidus) with the substantia nigra and subthalamicnucleus.