basal ganglia circuits
The basal ganglia are a group of nuclei located deep within the cerebral hemispheres of the brain. They play a crucial role in various motor and non-motor functions, including voluntary movement control, procedural learning, cognition, and emotion regulation. Basal ganglia circuits involve complex interconnected pathways between different nuclei within the basal ganglia and other regions of the brain, including the cerebral cortex and thalamus.
The main components of the basal ganglia include the following nuclei:
- Striatum: The striatum is the largest component of the basal ganglia and consists of the caudate nucleus and putamen. It receives inputs from the cerebral cortex and thalamus and serves as the main input nucleus of the basal ganglia.
- Globus pallidus: The globus pallidus consists of two segments: the external segment (GPe) and the internal segment (GPi). The GPe receives inputs from the striatum and projects to the subthalamic nucleus, while the GPi serves as the primary output nucleus of the basal ganglia.
- Subthalamic nucleus (STN): The STN receives excitatory inputs from the cerebral cortex and inhibitory inputs from the GPi. It projects back to the GPi and substantia nigra pars reticulata (SNr).
- Substantia nigra: The substantia nigra consists of two parts: the substantia nigra pars compacta (SNc) and substantia nigra pars reticulata (SNr). The SNc is involved in the modulation of motor function through the release of dopamine, while the SNr serves as an output nucleus of the basal ganglia.
The basal ganglia circuits involve several interconnected pathways that facilitate the initiation, modulation, and termination of voluntary movements. These pathways include the direct pathway, indirect pathway, and hyperdirect pathway:
- Direct pathway: This pathway facilitates movement initiation. It involves excitatory projections from the striatum to the GPi/SNr, resulting in disinhibition of the thalamus and facilitation of movement.
- Indirect pathway: This pathway facilitates movement inhibition. It involves inhibitory projections from the striatum to the GPe, which then inhibits the STN. Inhibition of the STN leads to decreased excitation of the GPi/SNr, resulting in increased inhibition of the thalamus and suppression of movement.
- Hyperdirect pathway: This pathway provides rapid and strong inhibition of movement. It involves excitatory projections from the cortex directly to the STN, bypassing the striatum. Activation of this pathway results in rapid inhibition of movement via increased excitation of the GPi/SNr.
These pathways work together to modulate the activity of thalamocortical circuits, ultimately influencing voluntary movement and other motor and non-motor functions. Dysfunction within basal ganglia circuits can lead to various movement disorders, such as Parkinson’s disease, Huntington’s disease, and dystonia.