The Basal ganglia, otherwise known as the Basal nuclei, are a system of neuron nuclei in the brain most commonly associated with motor function, specifically the integration of intention of the motor action and the emotional state of the mammal with the action itself.
There are four main components to the basal ganglia system: cortical input, a modulatory system, a control processor, an output pathway.
The Basal ganglia receives information primarily from five sources: the parietal cortex (primary and secondary somatosensory information, secondary visual information), temporal cortex (secondary visual and auditory information), cingulate cortex (limbic and emotional status information), the frontal cortex (primary and secondary motor information), and the prefrontal cortex. These structures synapse with medium spiny neurons in the caudate and putamen.
There are two basic pathways for basal ganglia connectivity: direct and indirect. These pathways span from the beginning of the basal ganglia at the Caudate/Striatum (referred to as the "neostriatum" in lower animals) and terminating at the primary motor cortex in the prefrontal gyrus in the frontal lobe.
Caudate/Striatum →- Globus Pallidus Internal (GPi) →- Ventrolateral Thalamus →+Primary Motor Cortex
The stimulation of the Caudate/Striatum in the Direct Pathway inhibits the GPi from inhibiting the Ventrolateral Thalamus. By inhibiting inhibition, the stimulation of the Caudate/Striatum disinhibits the Ventrolateral Thalamus, essentially promoting the Ventrolateral Thalamus to activate the Primary Motor Cortex and cause movement. Therefore, activation of the Direct Pathway increases motor function.
Caudate/Striatum →- Globus Pallidus External (GPe) →- Subthalamic Nucleus (STN) →+ Globus Pallidus Internal (GPi) →- Ventrolateral Thalamus →+ Primary Motor Cortex
The stimulation of the Caudate/Striatum in the Indirect Pathway inhibits the GPe from disinhibiting the Ventrolateral Thalamus, essentially inhibiting the Ventrolateral Thalamus from promoting the Primary Motor Cortex. Therefore, activation of the Indirect Pathway decreases motor function.
The Caudate/Striatum also receives input from the dopaminergic neurons of the Substantia Nigra pars compacta (SNpc), and is embedded with D1 and D2 receptors for the dopamine neurotransmitters being emitted from the SNpc.
When the SNpc is active, D1 receptors are activated, which link to the Direct Pathway portion of the Caudate/Putamen and promote stimulation of the Direct Pathway, further increasing the Direct Pathway's promotion of motor activity.
When the SNpc is active, D2 receptors are also activated, which link to the Indirect Pathway portion of the Caudate/Putamen and promote inhibition of the Indirect Pathway, inhibiting the Indirect Pathway's inhibition of motor activity, essentially promoting motor activity.
In normal circumstances, both the Direct and Indirect Pathways are activated simultaneously, leading to a net zero stimulation of the Primary Motor Cortex - motor function is neither promoted or demoted.
However, when the SNpc is active, the dopaminergic neurons encourage activation of the Direct Pathway and inhibition of the Indirect Pathway, so therefore when both are simultaneously activated, motor activity is promoted.
There are three main neurotransmitters involved in the basal ganglia.
- Dopamine - Neuromodulatory neurotransmitter, used by the Substantia Nigra pars compacta (SNpc) to selectively modulate areas of the caudate/putamen, excites areas of the caudate/putamen with D1 receptors to promote the direct pathway, inhibits areas of the caudate/putamen with D2 receptors to inhibit the indirect pathway
- Glutamate - Excitatory neurotransmitter
- Subthalamic nucleus projects glutamate to stimulate the ventrolateral thalamus
- Ventrolateral thalamus projects glutamate to stimulate the primary localized motor cortex
- GABA - Inhibitory neurotransmitter
- Caudate/Striatum (direct) projects GABA to inhibit the GPi
- GPi projects GABA to inhibit the ventrolateral nucleus
- Caudate/Striatum (indirect) projects GABA to inhibit the GPe
- GPe projects GABA to inhibit the subthalamic nucleus