The basal ganglia or basal nuclei are large masses of grey matter located within the central core of white matter of the cerebral hemispheres. The basal ganglia is composed of the following grey nuclei:
- Caudate nucleus
- Lentiform nucleus
- Amygdaloid nuclear complex (or Amygdala)
- Substantia nigra (within the midbrain)
- Subthalamic nucleus
The lentiform nucleus is composed of two groups of neuronal soma (grey matter) known as the putamen and globus pallidus. The lentiform and caudate nuclei together constitutes a mass of grey matter, the corpus striatum.
Therefore, the basal ganglia can also be defined as a large mass of grey matter composed of the corpus striatum, amygdala and claustrum, which is situated deep within the white matter of the cerebral hemispheres.
Note that the substantia nigra and the subthalamic nucleus are not included in the above definition because they are usually described by several authors as only having a functional relationship with the basal ganglia.
The amygdala lies in the temporal lobe, close to the temporal pole of the cerebral hemisphere. It lies deep to the uncus (of the cerebral cortex) and is related to the anterior end of the inferior horn of the lateral ventricle (1st and 2nd ventricle).
The claustrum is a thin lamina of grey matter that lies lateral to the lentiform nucleus. It is separated from the latter by fibres of the external capsule. Laterally, it is separated by a thin layer of white matter from the cortex of the insula. Connections and functions of the claustrum are unknown.
The subthalamic nucleus (STN) is traditionally described as part of the subthalamic region but functionally grouped with the basal nuclei.
The nucleus is shaped like a biconcave lens. It lies between the thalamus above, and the substantia nigra (of midbrain) below. The upper end of the red nucleus is close to it. The subthalamic nucleus is closely related to the zona incerta, the fascicularis lenticularis intervening between the two.
The fibres connecting the subthalamic nucleus to the globus pallidus (part of the corpus striatum) form a bundle called the subthalamic fasciculus which passes through the internal capsule. The subthalamic nucleus produces the excitatory neurotransmitter glutamate and sends excitatory input to the globus pallidus.
This is a midbrain structure that is closely linked, functionally, to the basal ganglia. It is considered the mesencephalic grey matter portion of the basal ganglia. It is divided into a reticulata and a compacta part based on the arrangements of neurons within each part. Cells (neurons) making up the substantia nigra compacta (SNc) are very close to one another, while those of the reticulata, that is, substantia nigra reticulata (SNr) are further away from one another.
The substantia nigra pars compacta produces the inhibitory neurotransmitter dopamine, which is very significant in maintaining the striatal pathway (the main input pathway of the basal ganglia). There is a fine connection between the substantia nigra pars reticulata and the subthalamic nucleus and the globus pallidus. Through those connections dopaminergic fibres (inhibitory fibres) are sent to the thalamus.
As indicated in the introduction, the corpus striatum is composed of the lentiform and caudate nuclei. Hence, the corpus striatum is made up of the putamen, globus pallidus and caudate nucleus.
The corpus striatum has two parts or units, the striatum and the pallidum. The striatum is composed of caudate nucleus and putamen, while the pallidum is made up of the globus pallidus.
This is composed of the globus pallidus and it forms one unit of the corpus striatum. It sends fibres (efferents) to the substantia nigra pars reticulata and to the thalamus, but receives fibres (afferents) from the striatum.
This is the second unit of the corpus striatum which is composed of the caudate nucleus and putamen. It is the main input unit of the basal ganglia, receiving most of the afferents to the basal nuclei.
Striatum is divided into two parts by a white matter tract known as the internal capsule. Those two parts of the striatum are the caudate nucleus and putamen.
There are two types of cell population within the striatum, the spiny projection neurons (also called medium spiny neurons) which constitutes about 90-95% of its neuronal population. The spiny projection neurons are the principal neurons of the striatum that are largely responsible for its functions, and by extension, the functions of the basal ganglia. The spiny projection neurons are GABAergic, which possess receptors for GABA neurotransmitters. Thus, they are classified as inhibitory neurons.
The second cell group within the striatum consists of interneurons. The interneurons make up about 5-10% of the striatal neurons. They are Cholinergic – releasing and using acetylcholine and are classified as excitatory.
The major efferents (outputs) of the basal ganglia are fibres reaching the pallidum from the striatum. There are also fibres from the striatum to the substantial nigra pars reticularis in the midbrain.
Afferents (inputs) to the basal ganglia, which are also the inputs of the striatum, include the following:
- From the entire cerebral cortex - this is the largest afferent connection of the basal ganglia. The fibres are glutamatergic – releasing the neurotransmitter glutamate to excite the striatum or striatal neurons.
- From the substantia nigra - fibres arising in the pars compacta of the substantia nigra reach the striatum, forming the nigrostriatal connections. This very important connection of the basal ganglia ensures a continuous supply of dopamine to the striatum. The nigrostriatal fibres are dopaminergic – releasing the neurotransmitter dopamine.
- From the thalamus - fibres from the thalamus to the basal ganglia form the thalamostriatal connections or the thalamostriatal afferents. Those connections or pathway carry glutamate and thus are glutamatergic.
- From the reticular formation of the brainstem (specifically from the midbrain) - afferents from the reticular formation are noradrenergic, supplying noradrenaline neurotransmitter to the basal ganglia.
- From the locus coeruleus - the striatum also receives fibres from the locus coeruleus. Those fibres are serotonergic, supplying serotonin.
There are a growing number of studies focused on the functions of the basal ganglia, as its functions are yet to be fully understood. However, the following are established functions involving the basal nuclei:
- Planning and modulation of movement pathways
- The basal nuclei is involved in the control of movement and learning
- Coordinating motivation with body movement. Specifically, the basal ganglia inhibits individual behavior in a complex social interaction and also inhibits small voluntary movement