How do the basal ganglia (basal nuclei) facilitate motor cortex excitation and increase motor activity?

From the Guidelines

The basal ganglia facilitates motor cortex excitation and increases motor activity through a complex disinhibitory pathway, where the striatum receives excitatory input from the cerebral cortex and inhibits the internal segment of the globus pallidus and substantia nigra pars reticulata, ultimately releasing the thalamus from inhibition and allowing it to excite the motor cortex. This process is crucial for voluntary movements, such as reaching and walking, and is mediated by the direct pathway through the basal ganglia, which disinhibits specific thalamic neurons that project to motor cortical areas controlling limb movement 1. The basal ganglia's role in motor control is also evident in conditions like Parkinson's disease, where reduced dopamine leads to excessive inhibition of the thalamus and subsequent reduced motor cortex activation, resulting in movement difficulties 1.

Key aspects of this pathway include:

• The striatum's receipt of excitatory input from the cerebral cortex
• The inhibition of the internal segment of the globus pallidus and substantia nigra pars reticulata by the striatum
• The release of the thalamus from inhibition, allowing it to excite the motor cortex
• The involvement of the direct pathway through the basal ganglia in facilitating voluntary movements
• The impact of dopamine reduction on motor control in conditions like Parkinson's disease

The most recent and highest-quality study on this topic, published in 2024, highlights the importance of the prefrontal–basal ganglia–thalamocortical network in reactive response inhibition, and the role of the subthalamic nucleus and globus pallidus in this process 1. This study provides valuable insights into the neural correlates of motor control and the basal ganglia's role in facilitating motor cortex excitation.

From the Research

Basal Ganglia Structure and Function

• The basal ganglia consist of five interconnected nuclei in the basal forebrain that influence cortical control of voluntary movement 2.
• The basal ganglia have dense fiber connections with the cerebral cortex and thalamus, forming 4-5 distinct loops or circuits to allow parallel processing of information 3.
• The motor loop of the basal ganglia comprises two distinct direct and indirect pathways, which have opposing effects on motor cortex excitation 3, 4.

Mechanism of Motor Cortex Excitation

• The direct pathway of the basal ganglia disinhibits the powerful inhibition of the internal segment of the globus pallidus/substantia nigra pars reticulata upon thalamic ventrolateral nuclei, resulting in a facilitatory influence upon the motor cortex 3.
• The basal ganglia output nuclei tonically inhibit the motor nuclei of the thalamus, and the basal ganglia facilitate motor activity by disinhibiting the thalamus 2.
• The striatal projection neurons in the basal ganglia are subdivided into two types: one that projects directly to the output stage (direct pathway) and promotes action, and the other that targets the output nuclei via intercalated basal ganglia nuclei (indirect pathway) and has an opposite effect 4.

Role of Basal Ganglia in Motor Control

• The basal ganglia play a crucial role in selecting cortical dynamics for motor behavior, and the selection is thought to determine the selection of motor repertoires for instrumental action 5.
• The basal ganglia may modulate cortical dynamics by preserving coarse-grained spatiotemporal information throughout cortico-cortical pathways, allowing for the production of different motor trajectories by controlling the same set of joint actuators 5.
• The basal ganglia also interact with the cerebellum to influence motor coordination, reward processing, and movement vigor, with cerebellar projections to the basal ganglia contributing to motor deficits, reward-based learning, and addictive behavior 6.