Direct motor pathway mechanism (facilitates movement)
The direct motor pathway goes from the striatum to the globus pallidus internus (GPi) via fibres that inhibit GPi activity.1,2 When this occurs, the GPi no longer inhibits the thalamus, which is now able to send an excitatory message to the cortex to initiate movement.2 Thus, activity of the direct motor pathway facilitates body movement through the motor neurons.2
Indirect motor pathway mechanism (inhibits movement)
Activity routed to the GPi via the indirect pathway involves several extra steps.1,2 First, impulses go from the striatum to the external segment of the globus pallidus (GPe), then onward to the subthalamic nucleus and the GPi.2
Inhibitory output from the striatum to the GPe shuts down inhibition of the subthalamic nucleus, which then sends excitatory messages to the GPi.2 This increases the GPi’s inhibitory activity on the thalamus, which reduces the excitatory output from the thalamus to the cortex.2 This results in a lower amount of excitatory activity by the cortex on motor neurons in the body.2 Thus, the indirect motor pathway inhibits body movement.2
References:
1.Augustine GJ, Groh JM, Huettel SA, et al. Neuroscience, 7th edition. New York, USA: Oxford University Press, 2023.