Positioned at the base of the forebrain and the top of the midbrain, they have strong connections with the cerebral cortex, thalamus, brainstem and other brain areas.

The substantia nigra is the source of the striatal input of the neurotransmitter dopamine, which plays an important role in basal ganglia function.

The basal ganglia are thought to play a key role in action selection, aiding in the choice of behaviors to execute.

The "behavior switching" that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.

A number of highly addictive drugs, including cocaine, amphetamine, and nicotine, are thought to work by increasing the efficacy of this dopamine signal.

[12] In terms of the development of the nervous system in humans, the central nervous system is often classified based on the original three primitive brain vesicles: These primary vesicles form in the normal development of the neural tube of the embryo and initially include the prosencephalon, mesencephalon, and rhombencephalon, in rostral to caudal (from head to tail) orientation.

During development, the cells that migrate tangentially to form the basal ganglia are directed by the lateral and medial ganglionic eminences.

In contrast to the cortical layer that lines the surface of the forebrain, the basal ganglia are a collection of distinct masses of gray matter lying deep in the brain not far from the junction of the thalamus.

Of note, and not seen in this section, the subthalamic nucleus and substantia nigra lie farther back (posteriorly) in the brain than the striatum and pallidum.

The body and tail show differentiation between the dorsolateral rim and ventral caudate, projecting to the sensorimotor and limbic regions of the striatum respectively.

Substantia nigra pars compacta (SNc) however, produces the neurotransmitter dopamine, which is very significant in maintaining balance in the striatal pathway.

Multiple models of basal ganglia circuits and function have been proposed, however there have been questions raised about the strict divisions of the direct and indirect pathways, their possible overlap and regulation.

[30] The latter interaction has been characterized in more detail by Suzanne Haber and colleagues in their 'spiral model', which postulated how the ventral striatum (limbic circuit) can influence the dorsal striatum (motor circuit) through the midbrain dopamine cells (ventral tegmental area, substantia nigra pars compacta and other regions).

In this model, connections from the ventral tegmental area to the shell region of the nucleus accumbens form a “closed,” reciprocal loop.

However, these projections also extend laterally to influence dopamine neurons that send signals to the rest of the ventral striatum, creating the initial segment of a feed-forward loop, or 'spiral'.

This spiral continues through striato-nigro-striatal pathways, whereby the VS affects cognitive and motor striatal areas via midbrain dopamine neurons.

[31][32] The direct pathway, originating in the dorsal striatum inhibits the GPi and SNr, resulting in a net disinhibition or excitation of the thalamus.

[38] Eye movement is influenced by an extensive network of brain regions that converges on a midbrain area called the superior colliculus (SC).

A "bump" of neural activity in the deep layers of the SC drives an eye movement directed toward the corresponding point in space.

The SC receives a strong inhibitory projection from the basal ganglia, originating in the substantia nigra pars reticulata (SNr).

[38] Neurons in the SNr usually fire continuously at high rates, but at the onset of an eye movement they "pause", thereby releasing the SC from inhibition.

Thus, eye movements begin with activation in the caudate nucleus, which inhibits the SNr via the direct GABAergic projections, which in turn disinhibits the SC.

[28] There is also evidence from non-human primate and human electrophysiology studies that other basal ganglia structures including the globus pallidus internus and subthalamic nucleus are involved in reward processing.

[53] For many years, the term corpus striatum[54] was used to describe a large group of subcortical elements, some of which were later discovered to be functionally unrelated.

The striatum was named on the basis of the striated (striped) appearance created by radiating dense bundles of striato-pallido-nigral axons, described by anatomist Samuel Alexander Kinnier Wilson (1912) as "pencil-like".

The term "locus niger" was introduced by Félix Vicq-d'Azyr as tache noire in (1786), though that structure has since become known as the substantia nigra, due to contributions by Von Sömmering in 1788.

Near the beginning of the 20th century, the basal ganglia system was first associated with motor functions, as lesions of these areas would often result in disordered movement in humans (chorea, athetosis, Parkinson's disease).

Early anatomists, seeing the macroscopic anatomical structure but knowing nothing of the cellular architecture or neurochemistry, grouped together components that are now believed to have distinct functions (such as the internal and external segments of the globus pallidus), and gave distinct names to components that are now thought to be functionally parts of a single structure (such as the caudate nucleus and putamen).

[61] Even in the lamprey (generally considered one of the most primitive of vertebrates), striatal, pallidal, and nigral elements can be identified on the basis of anatomy and histochemistry.

The abrupt rostral re-direction of the pathway from the internal segment of the globus pallidus into the ventral thalamus—via the path of the ansa lenticularis—could be viewed as a footprint of this evolutionary transformation of basal ganglia outflow and targeted influence.