Anatomical composition and topography
The basal ganglia comprise functionally interconnected nuclei of the telencephalon. The main motor formations are:
- Caudate nucleus. During embryogenesis, the telencephalon undergoes a twisting motion, causing the nucleus to acquire a curved C-shape that follows the contour of the lateral ventricle. Its massive head (rostral part) forms the lateral wall of the lateral ventricle. The putamen covers this head externally. The tail of the nucleus extends into the temporal lobe, forming the overhanging roof of the inferior horn, and reaches anteriorly all the way to the amygdala. Due to this curvature, on a coronal section, the caudate nucleus is visualized simultaneously in two areas: in the wall of the body and in the roof of the inferior horn of the ventricle.
- Putamen (outer part of the lentiform nucleus).
- Globus pallidus (inner part of the lentiform nucleus).
Topographical diagrams show that the nodes lie in close proximity to the diencephalon: the thalamus is located most medially, the globus pallidus and putamen more laterally, and the caudate nucleus curves around them. Based on embryonic origin, the claustrum (whose function is unclear) and the amygdala (part of the limbic system) are also formally classified as subcortical nodes, but they do not participate in motor regulation.
Functions and connections
The cerebral cortex acts as the highest center for movement regulation. It forms the pyramidal system—the fastest, most direct descending pathways to the motor nuclei of the brainstem and spinal cord.
The basal ganglia function as "supplementary" centers. They form complex feedback regulatory loops with the motor areas of the cortex and are capable of both exciting and inhibiting them. Their main tasks are:
- Initiation of movements.
- Coordination of complex motor acts.
- Regulation of normal muscle tone.
Previously, medicine used the term "extrapyramidal system", which united all non-pyramidal motor structures. Today, this concept is considered outdated. It has been proven that the pyramidal system and basal ganglia do not function in isolation. For instance, extensive bidirectional connections between the cortex and the striatum (striatum) have been identified. They constitute a unified apparatus for movement regulation.
Phylogeny of the motor system
To deeply understand the anatomy of the subcortical nuclei, it is important to know their evolutionary development:
- Ancient centers: The spinal cord and the primitive reticular formation apparatus.
- Paleostriatum (globus pallidus): Became a new motor node at the next stage of evolution.
- Neostriatum (caudate nucleus and putamen): Phylogenetically the youngest structures. They expanded in parallel with the development of the cortex and reached their maximum in higher mammals and humans.
The principle of subordination operates here: as the cortex developed, it formed the pyramidal system, which increasingly subjugated the older centers (neo- and paleostriatum). While in lower animals the ancient structures maintain automated movements (a cat can run after the removal of part of the motor cortex), in humans the motor function depends entirely on the pyramidal system. The old centers cannot compensate for its damage—the person loses the ability to walk. However, it is precisely these ancient nuclei that generate synkinesias (involuntary associated movements) during the development of spastic paralyses.
Briefly: answers to topical questions
What anatomical structures are bordered by the caudate nucleus along its extent?
Along its extent, the caudate nucleus is bordered by the ventricular system, the thalamus, and other basal nuclei.
- Head — forms the lateral wall of the lateral ventricle (lies adjacent to its anterior horn), covered externally by the putamen, with which it connects via cellular bridges.
- Body — is located in the lateral wall of the body of the lateral ventricle, separated from the thalamus by the terminal sulcus (sulcus terminalis).
- Tail — curves around the thalamus posteriorly, forms the roof of the inferior horn of the lateral ventricle in the temporal lobe, and continues to the amygdala (corpus amygdaloideum).
The nucleus also curves around the lentiform nucleus.
Where is the claustrum located topographically, and what separates it from neighboring structures?
The claustrum is topographically located in the deep parts of the white matter of the telencephalon, lateral to the lentiform nucleus. On a coronal section, this structure is visualized in direct proximity to the insular cortex (insula). The claustrum is separated from the adjacent putamen (putamen) by a layer of white matter called the external capsule (capsula externa). The function of this structure is not yet fully understood.
What specific neural pathways connect the basal ganglia with the cortex, thalamus, and brainstem?
The basal ganglia are connected to other structures through complex regulatory loops and extrapyramidal tracts.
- With the cortex: corticostriatal, corticorubral, corticonigral, and corticoreticular pathways.
- With the thalamus: thalamic fasciculus (fasciculus thalamicus), lenticular fasciculus (fasciculus lenticularis), ansa lenticularis (ansa lenticularis).
- With the brainstem: nigrostriatal pathway (from the substantia nigra to the striatum), as well as pathways from the globus pallidus to the reticular formation, red nucleus, and inferior olive.
Direct and indirect pathways, as well as the subthalamic fasciculus (fasciculus subthalamicus), are distinguished within the system.
Which neurotransmitters ensure signal transmission within the basal structures of the ganglia?
Signal transmission in the basal ganglia is provided by a complex of excitatory, inhibitory, and modulatory neurotransmitters.
- Glutamate — an excitatory neurotransmitter used by the cortex and subthalamic nucleus (ensures cortico-striatal connections).
- GABA (gamma-aminobutyric acid) — an inhibitory neurotransmitter used by the striatum and globus pallidus.
- Dopamine — a modulatory neurotransmitter secreted by the substantia nigra (acts on D1 and D2 receptors in the putamen).
- Serotonin — transmitted via serotonergic fibers from the raphe nuclei.
Substance P (in the direct pathway) and enkephalin (in the indirect pathway) also act as co-transmitters.
The caudate nucleus resembles the letter "C". Because of this C-shaped curve, it wraps around the thalamus, and on a single coronal section we see it twice: above (head/body) and below (tail).
Class and exam questions
What happens when the basal ganglia are damaged?
A pathological alteration of muscle tone occurs, along with an excess or deficiency of motor impulses. The most frequent example is Parkinson's disease, manifested by rigidity, akinesia, and tremor.
Can subcortical nuclei compensate for cortical damage in humans?
No. Unlike animals, in humans the old subcortical nerve centers are unable to compensate for impaired function of the new cortical centers. Walking depends entirely on the pyramidal system.
What else to study on this topic
- Functional connections of the basal ganglia with the substantia nigra
- The role of the subthalamic nucleus in the motor system
- The mechanism of synkinesia onset in spastic paralyses
- Topographical anatomy of diencephalic structures
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