the cerebral cortex, via the striatum. The putamen is interconnected with the following structures: This description is rudimentary and does not nearly exhaust even the basic established circuitry of the putamen. The cortico-subcortico-cortical circuits with putaminal involvement are dense and complicated, consisting of a wide range of axonal, dendritic, chemical, afferent, and efferent substrates. The putamen's outputs are highly arborized across output structures, and cortical efferents arise from layers III-VI of the cortex, dependent on gyri and location within the putamen. Topographical organization of the putamen combines the following elements: anterior-to-posterior functional and somatotopic gradients, lateral-to-medial functional and somatotopic gradients, diffuse terminal output, patchy localized terminal output, segregated terminals from adjacent regions, finely interdigitated terminals from distal cortical regions in a seemingly overlapping fashion. Caudate nucleus The caudate works with the putamen to receive the input from cerebral cortex. Collectively, they can be considered the "entrance" to the basal ganglia. Projections from the putamen reach the caudate directly via the caudolenticular grey bridges. The putamen and caudate are jointly connected with the substantia nigra, however the caudate outputs more densely to the substantia nigra pars reticulata while the putamen sends more afferents to the internal globus pallidus. Substantia nigra The substantia nigra contains two parts: the substantia nigra pars compacta (SNpc) and the substantia nigra pars reticulata (SNpr). The SNpc obtains input from the putamen and caudate, and sends information back. The SNpr also obtains input from the putamen and caudate. However, it sends the input outside the basal ganglia to control head and eye movements. The SNpc produces dopamine, which is crucial for movements. The SNpc is the part that degenerates during Parkinson's disease. Globus pallidus The globus pallidus contains two parts: the globus pallidus pars externa (GPe) and the globus pallidus pars interna (GPi). Both regions acquire input from the putamen and caudate and communicate with the subthalamic nucleus. However, mostly the GPi sends GABAergic inhibitory output to the thalamus. The GPi also sends projections to parts of the midbrain, which have been assumed to affect posture control. Physiology Types of pathways The putamen (and striatum in general) has numerous, parallel circuits that allow for cortico-subcortico-cortico communication loops. These have been described, broadly, as the direct, indirect, and hyper direct pathways. GABAergic projections of the putamen have an inhibitory effect on the thalamus. Thalamic projections from the centromedian and parafascicular nuclei have an excitatory effect on the putamen.