Basal ganglia pathways

Basics

The “basal ganglia” or basal nuclei, refers to a group of nuclei situated deep within the cerebral hemispheres and form a major portion of the “extrapyramidal system”. This system receives inputs from wide areas of the cerebral cortex and returns it, via the thalamus, to the cortex and brainstem.

The major structures that compose the basal ganglia are:

• the striatum (Str), which includes the caudate nucleus and the putamen, 
• the globus pallidus (GP) which is divided into two segments, the internal (GPi) and external parts (GPe), 
• the subthalamic nucleus (STN)
• the substantia nigra (SN) which is also divided into two parts, the reticular part (SNr) and the compact part (SNc). 

All these structures lay in the core of the cerebral hemispheres, wrapped around by the ventricular system and separated between them with an abundant network of descending and ascending pathways that connect the cerebral cortex and the brainstem.

The basal ganglia or nuclei are heavily interconnected and play an important role in motor planning and modulation. By choosing the intended movement, the basal ganglia use different pathways to initiate and terminate the motor program, by controlling the muscle tone, muscle length, speed, and strength of the movement by using the pyramidal system as the executor.

To make this possible, the basal ganglia use three pathways: the direct, indirect and hyperdirect pathway.

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Direct pathway

The direct pathway starts from the cortex and projects to the striatum (caudate nucleus and putamen) with excitatory glutamatergic (glu) neurons. The neurons from the striatum, which are inhibitory GABAergic, send their axons to the medial (internal) globus pallidus and substantia nigra, pars reticulata (SNr). 

The neurons from the internal globus pallidus and SNr send their axons to the thalamus, and they are also inhibitory. The fibers that travel from the pallidum to the thalamus, form two white matter fascicles called ansa lenticularis and lenticular fasciculus, that fuse into one pathway called thalamic fasciculus just before they enter the thalamus.

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From the thalamus, excitatory pathways go to the cortex (prefrontal, premotor and supplementary cortex) where they affect the planning of the movement by synapsing with the neurons of the corticospinal and corticobulbar tracts in the brainstem and spinal cord.

In summary, we have the following connections:

• Cortex - Striatum (glu)
• Striatum - GPi/SNr (GABA)
• GPi/SNr - Thalamus (GABA)

This entire system functions on the principle of positive feedback. Since the two of the inhibitory synapses are serially connected, that means that the first inhibitory neuron (striatum) suppresses the activity of the second inhibitory neuron (globus pallidus). The result of this is a reduction of the inhibitory influence that the globus pallidus has over the thalamus, so-called disinhibition of the thalamus, which is equivalent to the excitation of the motor cortex. So the final function of the direct pathway of the basal ganglia is to excite the motor cortex or to increase the motor activity.

Indirect pathway

This pathway begins (like the direct pathway) from the cortex, projecting to the striatum. Instead of sending axons directly to the GPi and SNr, they project to the external globus pallidus. 

The neurons from the GPe send inhibitory fibers to the subthalamic nucleus instead of sending directly to the thalamus (hence its name “indirect”). From the subthalamic nucleus, neurons send their axons to the GPi/SNr and then continue as the direct pathway with GABAergic inhibitory neurons to the thalamus and glutamate excitatory efferents to the cortex. 

So, functionally, the striatum inhibits the external globus pallidus, and that causes disinhibition of the subthalamus. For that reason, the neurons of the subthalamus become more active, and they excite the internal segment of the globus pallidus which in the end, inhibits the thalamic nuclei. The final result of this pathway is a decreased activity of the cortical motor neurons and consequential suppression of the extemporaneous movement.

Hyperdirect pathway

The hyperdirect pathway consists of neurons projecting from the cortex directly to the subthalamic nucleus (STN). Those fibers convey strong excitatory signals to the GPi/SNr with shorter conduction time than the direct and indirect pathway, bypassing the striatum. 

When receiving glutamatergic inputs from the cerebral cortex directly to the subthalamic nucleus, it then excites the GPi/SNr thus suppressing thalamic activity on the cerebral cortex and increasing inhibitory influences on the upper motor neurons. As a result, together with the indirect pathway, only the selected motor program is executed and other competing motor programs are canceled.

In summary, when a given motor pattern is computed by the cerebral cortex, it is first conveyed to the basal ganglia via glutamatergic projections to the striatum, with the purpose of releasing the intended movement and suppressing the unintended ones. The direct pathway funnels the information from the striatum to GPi/SNr via GABAergic inhibitory projections thus selectively reducing its activity and releasing firing from the thalamocortical neurons to initiate the movement.

Along with the initial signal to the striatum, the cerebral cortex suppresses competing motor programs by the indirect and hyperdirect pathways. When excited by the glutamatergic inputs of the cerebral cortex, striatum sends inhibitory signals to the GPe which normally exerts GABAergic inhibition on the STN. Therefore, the glutamatergic excitatory neurons of the STN can then excite the GPi/SNr thus suppressing thalamic activity on the cerebral cortex and increasing inhibitory influences on the upper motor neurons.

Modulation of the basal ganglia

The neuronal circuits that modulate the function of the basal ganglia are:

• The nigrostriatal pathway
• The thalamostriatal pathway