Basal ganglia
The basal ganglia consist of 4 main formations, formed by:
- striated
- globus pallidus
- substantia nigra (consisting in turn of a pars compacta and a pars reticulata)
- subthalamic nucleus
Coronal section representation of the basal ganglia (source h ttp: //mindblog.dericbownds.net/)
As for the striatum, it in turn is constituted by the caudate nucleus, putamen and ventral striatum which includes the nucleus accumbens.
The striatum is important because it receives the main afferents of the nuclei of the base, from the cerebral cortex, from the thalamus and from the trunk of the encephalon (figure 1). Its neurons project to the globus pallidus and to the substantia nigra.
From these two nuclei, whose neurons have morphologically similar somes or bodies, the main projections of the nuclei of the base take origin.
Streak functions
The striatum is composed of two distinct parts, the compartments called matrix and striosomes. These compartments have different histological features and have different receptors. The striosome compartment receives afferents mainly from the limbic cortex and projects mainly to the pars compacta of the substantia nigra. To better understand the operation of the striatum, it is appropriate to mention how the circuitry or the communication between the different brain areas works.
All areas of the cerebral cortex send excitatory glutamatergic projections to specific areas of the striatum. The striatum also receives excitatory signals from the intralaminar nuclei of the thalamus, dopaminergic projections from the midbrain and serotoninergic from the raphe nuclei.
In particular, the striatum is made up of different cell types, but 90-95% of the cells that make it up are made up of GABAergic projection neurons. They represent the main target of projections coming from the cerebral cortex and are also the only source of efferent projections. Normally they are silent neurons, except during movement or after the application of peripheral stimuli. The striatum is also made up of local inhibitory interneurons which, thanks to their developed axonal collaterals, reduce the activity of the efferent striatal neurons. Although these neurons are present in small quantities, they are responsible for most of the striatum tonic activity.
As for the circuitry, the striatum projects to the nuclei from which the efferent pathways originate through two ways, a direct pathway which is excitatory and an indirect pathway of an inhibitory type.
Thus through interactions with the cerebral cortex, the basal ganglia contribute to voluntary movement, but also to other forms of behavior such as skeletal-motor, oculomotor, cognitive and emotional functions. For example, in some individuals with Huntington's disease it has been observed that some lesions at the level of the nuclei of the base produce negative emotional and cognitive effects.
Neuropathology of Huntington's disease
Pathologically, Huntington's disease is manifested by atrophy of the striatum which, as described in the previous section, consists of caudate and putamen. Atrophy causes neuronal loss, also associated with a state of gliosis (a process of astrocyte proliferation in damaged areas of the brain). In 1985 the scholar Vonsattel classified this disease from grade 0 (where no changes occur) to grade 4, in relation to the extent of striatal atrophy. It has also been shown that the degree of atrophy that occurs at the level of the striatum also correlates with the degeneration of other non-striatal brain structures.
In the striatum the most affected neurons are the spiny medium neurons which represent the most numerous population present in the striatum and which use GABA, an inhibitory type amino acid, as neurotransmitter.
It has also been shown that Huntington's disease is characterized by the presence of intraneuronal inclusions and protein aggregates in dystrophic axons and in striatal and cortical neurons; it is a phenomenon also present in other polyglutamine disorders (ie triplet expansions, as in the case of Huntington's disease). It has also been seen that intranuclear inclusions manifest themselves before brain weight loss, but also before body weight loss and before the onset of neurological symptoms.
