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basal ganglia influence behavior by these output nuclei projecting to the ventral thalamus and
then back to the cortex or by projecting to subcortical `premotor' regions (figure 5) (see Albin
et al., 1989; DeLong, 1990; Bolam and Bennett, 1995; Gerfen and Wilson, 1996; Smith et al.,
1998 for review).
Figure 5. Basal ganglia motor circuit. Areas of the motor cortex project in a somatotopic pattern to the
striatum, where they synapse through excitatory glutamatergic neurons onto the medium spiny striatal
neurons. These striatal neurons use GABA as their primary neurotransmitter and substance P (SP) or
enkephalin (Enk) as co-transmitters, and are organized into two pathways: the 'direct' (D) and the
'indirect' (I) pathway. The direct pathway connects the striatum to the internal segment of the globus
pallidus (GPi) and the substantia nigra pars reticulata (SNr). The GPi and SNr are the output nuclei of
the basal ganglia (GPi/SNr) and project to the brainstem and the thalamus and from the latter to the
cortex. The influence of the GPi and SNr on the thalamus is inhibitory, whereas the thalamic projection
to the cortex is excitatory. The indirect pathway also connects the striatum to the output nuclei of the
basal ganglia but these fibres first pass through synaptic connections in the external segment of the
globus pallidus (GPe) and then the subthalamic nucleus (STN). Output from the STN to the GPi/SNr is
excitatory. Dopaminergic neurons (DA) of the SNc provide a massive feedback projection to the
striatum and modulate the flow of cortical information. Excitatory projections are shown in black;
inhibitory connections are shown in mottled gray lines.
The transmission of cortical information through the basal ganglia occurs through two
routes, the `direct' and `indirect' pathways (Albin et al., 1989; DeLong, 1990). In the direct
pathway corticostriatal information is transmitted directly from the striatum to the output
nuclei (SNr/GPi). In the indirect pathway corticostriatal information is transmitted indirectly
to the output nuclei via the complex network interconnecting the GPe and STN.
The great complexity of the circuits and the manner in which the basal ganglia
accomplish their diverse functions is better explained through the analysis of a single
neuronal cell type in the striatum: the medium sized spiny neuron (MSN) (figure 6). The
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