The basal ganglia select, adjust, censor and coordinate the outflow of behavioral signals from the central nervous system and intervene in the underlying processes of the cerebral cortex.
In other words, a nuclear complex of exceptional importance.
cf. Grillner, S. et al. (2005) Mechanisms for selection of basic motor programs - roles for the striatum and pallidum. Trends Neurosci 28:364−370

The basal ganglia consist of a group of larger and smaller nuclei located in the end, middle and midbrain (in the tel, di- and mes-encephalon); such a group on either side of the middle line.

Basal ganglia have long included:
1/ Caudate nucleus
2/ shell core (putamen)
3/ Subthalamic nucleus (nucleus subthalamicus)
4/ pale nucleus (globus pallidus, pallidum) consisting of an outer part GPe and an inner part GPi
5/ black nucleus (substantia nigra) consisting of a dense/compact part/SNc and a loose/reticular part/SNr..

The concepts 1/ to 5/ constitute, what one might call the, the traditional basal ganglia. One concern is the nucleus accumbens (see list below) which is sometimes counted here and sometimes not. The same can be said about the amygdala, which was previously counted as the basal ganglia (see Frank Netter's classic illustrations ref 34) but which subsequently fell away and is now on its way back again due to its return traffic with the ventral striatum.

In connection with the basal ganglia, the following names usually also appear:
The nucleus lentiformis = the accumulation of grey matter by the shell nucleus (putamen) and the pale nucleus (globus pallidus) together.
corpus striatum = 1/ + 2/ + 4/.

The list below summarizes the names of the core areas of the basal ganglia (cf. 1 - 5 above and note the newly added components) as presented in a larger modern textbook.
(The Human Central Nervous System: A Synopsis and Atlas. Nieuwenhuys R., Voogd J., and Van Huijzen, C. Springer-Verlag Berlin-Heidelberg New York . 4.th edition. 2008, 427-489.)


Striatum
    Dorsal striatum
        Nucleus caudate
        Putamen
    Ventral striatum
        Accumbens nucleus
        Tuberculum olfactorium
Nucleus subthalamicus
Globus pallidus
    Pallidum dorsale
        Gpe
        Gpi
    Pallidum ventrale
        Substantia innominata voting party
        Parts of the nucleus basalis Meynert 
Substantia nigra
    Pars compacta, SNc
    Pars reticulata, SNr
Area tegmentalis ventralis, VTA
Nucleus tegmentalis pedunculopontinus, TPP.

Heimer L, Switzer RD, Van Hosen GV (1982) Ventral striatum and ventral pallidum. Components of the motor system? Trends Neurosci 5:83-87
Mena-Segovia J., Bolam J. and Magill PJ.( 2004 Pedunculopontine nucleus and basal ganglia: distant relatives or part of the same family? Trends Neurosci 27, 585-588.
Obeso, YES. (2011) Past, present and future of the pathophysiological model of the basal ganglia. Front. Neuroanate. 12 July

The basal ganglia can receive signals from, broadly speaking, the entire cerebral cortex, especially the frontal and parietal lobes, as well as from several thalamus nuclei.

The incoming signals are processed in the basal ganglia and then transmitted from GPi-SNr to certain thalamus nuclei, to the TPP nucleus, to the upper quadriculus (colliculus superior) and to formatio reticularis (including its pattern generators).

In the thalamus nuclei (CM, PF, MD, VA and VL nuclei), the information from the basal ganglia is weighed together with other incoming information and the result is returned, a few hundredths of a second later, to the very cortex areas that began the activation of the basal ganglia, whereupon the signals immediately spread to and activate the primary motor cortex .

The contact signals are switched around in a circuit, in a loop, which starts from a limited area of the cerebral cortex. The signals first reach the subthalamic nucleus and striatum, then proceed to the loose part of the pale nucleus and black nucleus, and then arrive at the thalamus, which in turn delivers, the now processed, signal flow to the same bark area from which the original flow originated.

Depending on which part of the cerebral cortex directs the signal flow to the basal ganglia, one speaks of (at least) three different feedback circuits or loops.
1/ The sensori-motor feedback circuit/loop.
2/ The associative/cognitive feedback circuit/loop.
3/ The limbic/emotional feedback circuit/loop.

Common to the three circuits are:
that the starting signals from the cerebral cortex are excitatory and are mediated by glutamate,
that the starting signals from the cerebral cortex seek their way to the subthalamic nucleus and to the striatum in order to be reconnected,
that the starting signals from the cerebral cortex, in the subthalamic nucleus, are switched to driving (excitatory) glutamate-ergic neurons which contact GPi-SNr that the signal spread from the cortex via the subthalamic nucleus to the GPi-SNr occurs along the so-called "hyperdirect pathway through the basal ganglia"
that the starting signals from the cerebral cortex, in the striatum, are switched to inhibitory GABA-ergic neurons of two types; one with type 1 dopamine receptors (D1) and the other with type 2 dopaminereceptors (D2)
that the D1 neurons are also substance P and dynorphin positive and redirect the signals directly to GPi-SNr that the signal propagation from the cortex via the D1 neurons to the GPi-SNr takes place along the so-called "Direct path through the basal ganglia" and
that the D2 neurons are also enkephalin positive and are included as a step in the so-called "Indirect pathway through the basal ganglia" which, also ends, in GPi-SNr.

Talking about different types of feedback circuits, each of which uses a hyperdirect, a direct and an indirect pathway through the basal ganglia, are assumptions, which are based on animal experimental studies (mice, rats and monkeys; rarely humans), and which facilitate the understanding of the various disease signs/diseases associated with disorders of the basal ganglia. The feedback circuits with their "pathways" appear to be well integrated with each other, forming a complicated neuronal network.

Despite the great importance of the basal ganglia for the normal implementation of learned and coordinated movements, it is the primary motor cortex that, via the motor neurons of the brainstem and spinal cord, initiates the necessary muscle contractions. The various feedback circuits of the basal ganglia open up (the striatum and the subthalamic nucleus are activated) "first" when the committed muscle cells have begun to contract, i.e. about 0.1 sec after the activation signal left the primary motor cortex Ref.18 pages 725-749.

See: Basal ganglia dysfunction.

Reading about and trying to understand the basal ganglia with the help of modern textbooks is not so easy.
To take part in and to try to understand the scientific results of recent decades regarding the basal ganglia is almost impossible in terms of time and leads to a difficult balancing act between elation and uncertainty.
Are the basal ganglia in fact the "Gray Eminence" of the central nervous system?
/CHB