1. Basal Ganglia
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2. Basal Ganglia
The basal ganglia are a part of the motor system. The principal nuclei of the basal ganglia are the caudate nucleus, the putamen, and the globus pallidus, all of which lie in the subcortical white matter of the telencephalon. These nuclei are connected to each other, and to the motor cortex, in complex regulatory circuits. They exert both excitatory and inhibitory effects on the motor cortex. They play an important role in the initiation and modulation of movement and in the control of muscle tone.

3. Lesions of the basal ganglia, and of other, functionally related nuclei, such as the substantia nigra and the subthalamic nucleus, can produce either an excess or a deficiency of movement-related impulses, and/or pathological alterations of muscle tone. The most common disease of the basal ganglia is Parkinson disease, which is characterized by the clinical triad of rigidity, akinesia, and tremor.

4. Nuclei
The basal ganglia include all of the functionally interrelated nuclei within the deep white matter of the telencephalon that are embryologically derived from the ganglionic eminence (anterior portion of the telencephalic vesicle). The major nuclei of the basal ganglia are the caudate nucleus, the putamen, and part of the globus pallidus; other nuclei that are considered part of the basal ganglia on embryological grounds are the claustrum and the amygdala.

7. The caudate nucleus forms part of the wall of the lateral ventricle and, like it, has an arched shape, due to the rotation of the telencephalon during embryonic development. The head of the caudate nucleus forms the lateral wall of the lateral ventricle; its tail forms the roof of the inferior horn of the lateral ventricle in the temporal lobe, extending as far forward as the amygdala, which lies at the anterior end of the inferior horn. The caudate nucleus can therefore be seen in two separate locations on some coronal sections, in the lateral wall of the body of the lateral ventricle as well as in the roof of the inferior horn. The rostral portion (head) of the caudate nucleus is continuous with the putamen.

11. The putamen lies lateral to the globus pallidus (or pallidum, so called because of its relatively pale coloration), covering it like a shell and extending somewhat beyond it both rostrally and caudally. The putamen and globus pallidus are separated by a thin layer of white matter called the medial medullary lamina.

12. The caudate nucleus and putamen are connected by numerous small bridges of gray matter, which are seen as stripes in anatomical sections. These two nuclei together have, therefore, been given the alternative name corpus striatum (striped body), or striatum for short. The striation arises during development, when the Fibers of the internal capsule grow through the originally uniform basal ganglion.

13. Globus pallidus. The third major nucleus of the basal ganglia is made up of an internal and an external segment (pars interna and pars externa). Because the globus pallidus is phylogenetically older than the other nuclei, it is also called the paleostriatum. Part of it is, embryologically speaking, a component of the diencephalon. The putamen and globus pallidus are collectively termed the lentiform or lenticular nucleus (lens-shaped nucleus).

14. Associated nuclei. Further nuclei that are closely functionally related to the basal ganglia include two midbrain nuclei—the substantia nigra (reciprocally connected to the striatum) and the red nucleus—and one diencephalic nucleus, the subthalamic nucleus (reciprocally connected to the globus pallidus). The globus pallidus caudally borders the rostral portion (red zone) of the substantia nigra. The pallidum, substantia nigra, and red nucleus contain large amounts of iron. The dark pigmentation of the substantia nigra (“black substance”) is due to its high melanin content.

15. Connections of the Basal Ganglia
The neural connections of the basal ganglia with one another and with other regions of the brain are not yet completely understood. The major afferent and efferent pathways will be described in this section.

16. Afferent Pathways
Afferent pathways to the corpus striatum. The corpus striatum receives afferent input from extensive areas of the cerebral cortex, particularly the motor areas of the frontal lobe, i.e., Brodmann areas 4, 6aα, and 6a". These cortical afferents are derived from projection neurons of the cerebral cortex (pyramidal cells of the fifth layer of the cortex), are glutamatergic, run ipsilaterally, and are topically organized. There are probably no reciprocal fibers running from the corpus striatum back to the cortex.

17. A further point-to-point afferent input to the corpus striatum is derived from the centromedian nucleus of the thalamus, and is probably excitatory. This afferent pathway transmits impulses from the cerebellum and the midbrain reticular formation to the striatum. The substantia nigra sends dopaminergic afferent fibers to the striatum, whose loss is the cause of Parkinson disease (see below). Finally, the striatum also receives a serotonergic input from the raphe nuclei.

18. Other afferent pathways
Other afferent pathways. The globus pallidus derives its major afferent input from the corpus striatum and receives no direct afferent fibers from the cerebral cortex. Cortically derived afferent fibers do, however, travel to the substantia nigra, red nucleus, and subthalamic nucleus.

19. Efferent Pathways
Efferent pathways of the corpus striatum: The major efferent projections of the corpus striatum go to the external and internal segments of the globus pallidus. Further efferent fibers travel to the pars compacta and pars reticulata of the substantia nigra. The cells of origin of the striatal efferent fibers are GABAergic spiny neurons, the most common cell type in the striatum.

20. Efferent pathways of the globus pallidus
Efferent pathways of the globus pallidus. The major contingent of efferent fibers runs to the thalamus, which, in turn, projects to the cerebral cortex, completing a feedback loop. The functional interpretation of the afferent and efferent projections of the basal ganglia requires consideration of the particular neurotransmitter substances and receptors involved, and of the types of neurological deficit that are produced when certain pathways cease to function normally. Thus, Parkinson disease is characterized by degeneration of the dopaminergic neurons of the substantia nigra that project to the corpus striatum. The clinical deficits observed in Parkinson disease provide a clue to the probable functions of the nigrostriatal system in normal individuals.

21. Participation of the Basal Ganglia in Regulatory Circuits
The basal ganglia and their afferent and efferent connections are integral parts of complex regulatory circuits that excite and inhibit the neurons of the motor cortex. Neural transmission within these circuits can be characterized in terms of the anatomical course along which the impulses travel, as well as the particular neurotransmitters and receptors that are involved at each synapse. One of the more important circuits conveys impulses along two separate paths from the cortex, via the corpus striatum, to the globus pallidus, and then to the thalamus and back to the cortex. In addition to this major regulatory circuit, there are other feedback loops that will not be explicitly described in this book.

23. Cortico-striato-pallido-thalamo-cortical pathway
Cortico-striato-pallido-thalamo-cortical pathway. The motor and sensory cortex sends a topographically organized projection to the striatum that uses the excitatory neurotransmitter, glutamate. Beyond the striatum, the basal ganglionic circuit splits into two parts, which are known as the direct and indirect pathways.

24. Direct pathway: The direct pathway is GABAergic and runs from the striatum to the internal pallidal segment. Substance P is used as a co-transmitter. From the pallidum, the pathway proceeds to the glutamatergic projection neurons of the thalamus, which complete the loop back to the cerebral cortex

25. Indirect pathway: The indirect pathway, which uses the neurotransmitters GABA and enkephalin, runs from the striatum to the external pallidal segment. From this point, a further GABAergic projection proceeds to the subthalamic nucleus, which, in turn, sends a glutamatergic projection to the internal pallidal segment. The further course of the indirect pathway is identical to that of the direct pathway, i.e., from the thalamus back to the cerebral cortex

26. It follows from the combination of inhibitory and excitatory neurotransmitters used by these two pathways that the overall effect of stimulation of the direct pathway on the cerebral cortex is excitatory, while that of stimulation of the indirect pathway is inhibitory. The dopaminergic projection from the substantia nigra (pars compacta) plays a modulating role in this system.