Final answer:
The basal ganglia signal the thalamus using GABA in the direct pathway for movement promotion and in the indirect pathway for movement inhibition. These pathways are modulated by dopamine from the substantia nigra pars compacta, affecting movement initiation.
Step-by-step explanation:
Signaling between the basal ganglia and the thalamus depends on neurotransmitters from various sources. In the direct pathway, the striatum, which receives excitatory input from the cerebral cortex using glutamate, sends inhibitory signals to the internal segment of the globus pallidus (GPi) and the substantia nigra pars reticulata (SNr) using GABA as a neurotransmitter. This inhibits these nuclei, leading to the disinhibition (reduced inhibition) of the thalamus and increased excitation of the cerebral cortex, thereby promoting movement.
Conversely, in the indirect pathway, the striatum projects to the globus pallidus external segment (GPe), then to the subthalamic nucleus (STN), and finally to GPi/SNr. This pathway ultimately reinforces the inhibition of the thalamus, resulting in less excitation of the cerebral cortex and reduced movement. The balance between these pathways is modulated by dopamine from the substantia nigra pars compacta (SNc), which can either stimulate (via D1-type receptors) or inhibit (via D2-type receptors) the direct and indirect pathways, respectively, thus influencing movement initiation.