What are the basal ganglia and where are they located
The basal ganglia are massive clusters of neurons forming the deep grey matter of the cerebral hemispheres (along with the thalami and amygdala). Unlike the cortex, which covers the brain externally, these nuclei lie deep within the white matter. Due to their high cell density, they contrast excellently on magnetic resonance and computed tomography scans.
The basal ganglia complex traditionally includes:
- Caudate nucleus (nucleus caudatus);
- Globus pallidus (globus pallidus);
- Putamen (putamen);
- Nucleus accumbens (nucleus accumbens);
- Substantia nigra (substantia nigra).
Corpus striatum and internal connections
A key anatomical feature of the basal ganglia is their spatial relationship with the internal capsule (capsula interna), a major bundle of projection nerve fibers.
The caudate nucleus and globus pallidus do not simply lie in isolation: they are pierced by the internal capsule through multiple bridges of gray matter called pontes grisei. Because of these bridges, this area appears striated on brain sections. This is precisely why the complex of these nuclei was named the corpus striatum (corpus striatum). From below, beneath the fibers of the internal capsule, the nuclei are structurally united by yet another element—the nucleus accumbens.
Age-related changes on CT and MRI
In diagnostic radiology, it is extremely important to distinguish pathological processes from natural tissue aging. The basal ganglia undergo a series of physiological changes that are well visible on scans:
- Iron accumulation. Starting from the second decade of life, iron gradually deposits in the tissues of these deep nuclei. This is a normal physiological process that changes signal intensity on MRI over time.
- Calcification. In patients over 30 years of age, punctate mineralization (calcification) of the basal ganglia is frequently detected on computed tomography scans. In the absence of concomitant neurological symptoms, this is considered a normal variant.
Surrounding structures: thalami and the limbic system
To understand the topography, it is important to mention the thalami (thalami), large paired accumulations of gray matter located posterior and superior to the hypothalamus. The thalami form the majority of the lateral walls of the third ventricle and often touch each other across its cavity (the interthalamic adhesion, or massa intermedia). The posterior border of the thalamus—the pulvinar (pulvinar)—projects into the quadrigeminal cistern above the geniculate bodies.
Nearby, along the medial margins of the hemispheres, lie the structures of the limbic system (amygdala, hippocampus, cingulate gyrus). They are responsible for memory processing, emotions, and fight-or-flight responses.
Tissue architectonics: grey and white matter
Deep nuclei do not function in isolation; they are embedded in the white matter of the brain. All brain tissue (consisting of ~100 billion neurons and consuming about 20% of the body's glucose) is divided into two categories:
- Grey matter. This is the cortex and deep clusters (including the basal ganglia). This is where neuronal cell bodies and synapses are located. The tissue is highly metabolically active and receives the major share of blood flow.
- White matter. The brain center formed by axon bundles with myelin sheaths (synthesized by oligodendrocytes). Due to its high lipid content, white matter has lower density on CT, high signal on T1-weighted images, and low signal on T2-weighted images compared to grey matter.
Nerve pathways running near the ganglia are divided into associative (within a hemisphere), commissural (between hemispheres, e.g., the corpus callosum), and projection (connecting the cortex to lower-lying centers via the internal capsule and corona radiata).
Briefly: answers to topical questions
What clinical syndromes occur in lesions of the basal ganglia?
Lesions of the basal ganglia result in complex motor and cognitive disorders. The main clinical manifestations and syndromes include:
- Parkinsonism (Parkinson's disease) — occurs due to the degeneration of dopaminergic nigrostriatal neurons.
- Wilson's disease — characterized by diffuse nuclear involvement with the development of hypokinesia, hyperkinesia, and tone abnormalities.
- Hypokinesia — poverty of movement.
- Hyperkinesia — excessive involuntary movements (chorea, athetosis, ballism).
- Dystonia — pathological alteration of muscle tone.
Patients may also develop severe extrapyramidal disorders or dementia (for example, as an outcome of carbon monoxide poisoning).
What specific functions do the basal ganglia perform within the extrapyramidal system?
The basal ganglia exert an indirect influence on motor functions, ensuring automation, initiation, and coordination of movements. The main functions include:
- Initiation of movement — formation of primary motor commands (programs for movement onset and termination), initiation and facilitation of voluntary movements.
- Control and correction — adjustment and refinement of voluntary movements via feedback mechanisms, control of complex motor patterns.
- Inhibition — suppression of unwanted involuntary influences to ensure smooth and efficient movements.
- Regulation of tone — participation in the regulation of muscle tone.
To memorize the structure of the corpus striatum, imagine "grey bridges over a white river": the caudate nucleus and globus pallidus are connected by bridges (pontes grisei) laid directly through the white stream of the internal capsule.
Class and exam questions
Why did the corpus striatum get this name?
Due to its specific appearance on a section. The caudate nucleus and globus pallidus are connected by bridges of gray matter through the fibers of the internal capsule, which creates a "striated" texture.
Is basal ganglia calcification considered normal on CT?
Yes, punctate calcification in the basal ganglia region is frequently observed on computed tomography scans in patients over 30 years of age and is a physiological norm.
Which structure unites the basal ganglia nuclei inferiorly?
Inferior to the internal capsule, the caudate nucleus and globus pallidus are connected by the nucleus accumbens.
What else to study on this topic
- Topographical anatomy of the thalamus and its parts
- Limbic system: hippocampus and amygdala
- White matter: classification and radiological semiotics of projection fibers
- Anatomy of the corpus callosum and its clinical significance in neuroradiology
- Glymphatic system and cerebrospinal fluid circulation pathways
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