The basal ganglia are not a single structure. They are a family of nuclei working together, and the components matter because each plays a distinct role in the gating circuit. The major basal ganglia nuclei are the caudate, the putamen, the globus pallidus, the subthalamic nucleus, and the substantia nigra (which we already met in the brainstem).
The caudate and putamen together are called the striatum, because they look striated under the microscope — the gray matter is broken up by white-matter bands. The striatum is the input nucleus of the basal ganglia: it receives signals from the entire cortex. Every cortical area sends projections here, and the topography is preserved — motor cortex projects to the dorsolateral putamen, prefrontal cortex projects to the caudate head, and so on.
The globus pallidus is the output nucleus. It has two segments — internal (GPi) and external (GPe) — which play different roles in the direct and indirect pathways we will discuss next. GPi projects to the thalamus, which projects back to cortex, closing the basal ganglia loop.
The subthalamic nucleus (STN) sits just below the thalamus. It is a small but critical node in the indirect pathway. The STN is also the target of choice for deep brain stimulation in Parkinson's disease, where high-frequency electrical stimulation reduces tremor and rigidity remarkably effectively.
The substantia nigra we met in Stage 2 sits in the midbrain but functions as part of the basal ganglia system. Its dopaminergic neurons project to the striatum (the nigrostriatal pathway), where dopamine modulates the activity of both the direct and indirect pathways.
The functional logic is straightforward: cortex sends a proposal to the striatum; the striatum sends commands through direct and indirect pathways to the globus pallidus; the globus pallidus, via the thalamus, sends a refined signal back to cortex; the loop closes. Dopamine from the substantia nigra tunes the balance between direct and indirect pathways, biasing the system toward action or inaction.
Hold the components. In the next concepts we will trace the direct and indirect pathways through this circuitry and see why dopamine loss produces Parkinson's, why D2 blockade produces antipsychotic side effects, and why the nucleus accumbens — a different part of the striatum — handles reward rather than movement.