Network models of the basal ganglia.
The basal ganglia: A vertebrate solution to the selection problem?
There are two distinct pathways that process signals through the basal ganglia: the direct pathway and the indirect pathway. These two pathways have opposite net effects on thalamic target structures. Excitation of the direct pathway has the net effect of exciting thalamic neurons (which in turn make excitatory connections onto cortical neurons). Excitation of the indirect pathway has the net effect of inhibiting thalamic neurons (rendering them unable to excite motor cortex neurons). The normal functioning of the basal ganglia apparently involves a proper balance between the activity of these two pathways. One hypothesis is that the direct pathway selectively facilitates certain motor (or cognitive) programs in the cerebral cortex that are adaptive for the present task, whereas the indirect pathway simultaneously inhibits the execution of competing motor programs. An upset of the balance between the direct and indirect pathways results in the motor dysfunctions that characterize the extrapyramidal syndrome (see below).
Functional and pathophysiological modelsof the basal ganglia.
The indirect pathway starts with a different set of cells in the striatum. These neurons make inhibitory connections to the external segment of the globus pallidus (GPext). The GPext neurons make inhibitory connections to cells in the subthalamic nucleus, which in turn make excitatory connections to cells in the GPint. (Remember that the subthalamic-GPint pathway is the only purely excitatory pathway within the intrinsic basal ganglia circuitry.) As we saw before, the GPint neurons make inhibitory connections on the thalamic neurons. To see the net effects of activation of the indirect pathway, let us work backwards from the GPint. When the GPint cells are active, they inhibit thalamic neurons, thus making cortex less active. When the subthalamic neurons are firing, they increase the firing rate of GPint neurons, thus increasing the net inhibition on cortex. Firing of the GPext neurons inhibits the subthalamic neurons, thus making the GPint neurons less active and disinhibiting the thalamus. However, when the indirect pathway striatal neurons are active, they inhibit the GPext neurons, thus disinhibiting the subthalamic neurons. With the subthalamic neurons free to fire, the GPint neurons inhibit the thalamus, thereby producing a net inhibition on the motor cortex.