Final answer:
The basal ganglia pathways affect action selection by modulating thalamic activity, which then has an impact on movement initiation and suppression. The direct pathway leads to increased motor activity by disinhibiting the thalamus, while the indirect pathway leads to decreased motor activity by reinforcing the inhibition of the thalamus.
Step-by-step explanation:
The basal ganglia pathways engage in action selection by modulating thalamic activity. The basal ganglia consist of two main pathways: the direct and indirect pathways. These pathways influence movement through their interactions with the thalamus and cortical areas.
The direct pathway facilitates movement initiation by causing a net disinhibition of the thalamus - that is, it inhibits a structure (the GPi/SNr) that normally inhibits the thalamus. This allows the thalamus to excite motor areas in the cortex more effectively, leading to increased motor activity.
Conversely, the indirect pathway results in decreased activity in the cerebral cortex and therefore leads to less motor activity, as it causes, or reinforces, the normal inhibition of the thalamus. Through several synaptic connections, including one that inhibits the subthalamic nucleus (which itself provides excitatory input to the GPi/SNr), the indirect pathway ultimately increases inhibition of thalamic neurons, reducing excitation of the motor cortex and suppressing movement.
Both pathways are modulated by signals from the substantia nigra pars compacta, which releases dopamine that modifies activity within these pathways. The balance between these two pathways determines the overall output from the basal ganglia system to the thalamus and is critical for the proper selection and initiation of voluntary movements.