The Relationships Between Subthalamic Nucleus, Globus Pallidus and Thalamic Parafascicular Nucleus
The current functional relationships between the basal ganglia in normal and pathological conditions, introduced in 1989 (Albin et al., 1989; Alexander and Crutcher, 1990; DeLong, 1990) supports an obvious identification of the input and the outputs of this group of structures. The input is the striatum. The outputs are the pars reticulata of the substantia nigra and the internal segment of the globus pallidus in primates designated the entopeduncular nucleus in rodents. The inputs and ouputs are linked through two circuits in this fonctionnai model. The first one is monosynaptic and is named the direct circuit. The second one is a polysynaptic pathway involving a succession of two structures, the external segment of the globus pallidus, simply labelled globus pallidus in the rodent, and the subthalamic nucleus. This pathway was named the indirect circuit. The main postulate is that the neuronal activity in each of the successive relay structures is plainly related to the changes of activity of the neurons localized in the preceeding structure according to a hierarchical diagram. The discharge rate of the neurons localized in the globus pallidus of rodents or the external segment of the pallidum in the primates, would be determined only by changes in the activity of the striatopallidal afferents. Similarly, the activity of the subthalamic neurons would be related only to the changes in the inhibitory control provided through the pallidosubthalamic afferents. The function of the corticosubthalamic projection was not taken into account though present in this diagrammatic representation. Such an organisation was recently questionned. One of the first attempts was due to Mink and Thach (1993) who proposed that the corticosubthalamic pathway could provide a faster and excitatory input through the subthalamic efferents to the two parts of the globus pallidus and to the substantia nigra.
KeywordsBasal Ganglion Globus Pallidus Subthalamic Nucleus Excitatory Input Ibotenic Acid
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