The nucleus subthalamicus is a nucleus (Latin nucleus) located under (Latin sub) the thalamus, the largest part of the diencephalon. In professional circles, the abbreviation STN is mostly used today. Its formerly used epithet, Luysi body, on the other hand, goes back to its discoverer.
What is the nucleus subthalamicus?
The nucleus subthalamicus, along with the globus pallidus and the zona incerta, is part of the subthalamus. This area, which belongs to the brain stem, is located in the diencephalon (medically called the diencephalon) at the junction with the midbrain. However, due to its similar functioning, it is assigned to the basal ganglia, which are usually located under the cerebral cortex. They are diencephalic or endbrain nuclei that, like the subthalamic nucleus, play an important role in motor processes of the body. There is a very close connection between the globus pallidus, a pale nucleus also called the pallidum, and the nucleus subthalamicus. The two are mutually interconnected and form a kind of oscillatory circuit. They receive and send signals to each other on the basis of which certain movements of the human body are then inhibited or permitted. This has an effect primarily on the four limbs, especially the sections close to the torso and those towards the center of the body, respectively.
Anatomy and structure
The bidentate nucleus subthalamicus itself is reminiscent of a biconvex lens with its exterior and is present in both the left and right hemispheres of the brain. However, it is located directly under the thalamus only during the embryonic phase. Thereafter, during development, the entire area of the subthalamus is pushed toward the cerebrum by the adjacent capsula interna, a collection of white matter including nerve fibers. The subthalamus is located in the anterior region of the diencephalon and is also considered part of the extrapyramidal motor system, or EPMS. These are motor pathways that travel into the spinal cord where they activate the muscles of the trunk and limbs. The brain region around the subthalamus is one of the least studied areas of the entire brain. However, while concrete information is now available about the nucleus subthalamicus and globus pallidus, little is still known about the zona incerta.
Function and tasks
The function of the nucleus subthalamicus is mainly to achieve an inhibitory effect on certain movements of the human body. This effect ensures the ability to use all limbs voluntarily and, most importantly, purposefully. Without the movement-inhibiting effect of the nucleus subthalamicus, movements would only be possible in an uncontrolled manner and independent or everyday life would be virtually impossible. The process is controlled by a complex circuitry of the basal ganglia responsible for motor function as well as other areas of the brain stem. This interconnection can be compared to interlocking cogwheels that form a main loop with several subloops. The process is driven by counteracting signals. These have either an inhibitory or excitatory effect and use glutamate as a neurotransmitter. The signals reaching the nucleus subthalamicus originate predominantly from fiber inputs from the cerebral cortex and the adjacent globus pallidus. However, while excitatory impulses arrive from the cortex, the globus pallidus sends inhibitory impulses. The subthalamic nucleus reacts to the latter by sending back excitatory signals and, due to the existing interaction, ensures that the globus pallidus subsequently sends inhibitory impulses to the thalamus again. Indirectly, the nucleus subthalamicus thus counteracts unregulated movements and controls human gross motor activity. The movement-inhibiting function of the nucleus subthalamicus is now also part of Parkinson’s research. Although the exact correlations have not yet been conclusively determined, it has been shown that the characteristic tremor of Parkinson’s patients, the so-called rest tremor, can be noticeably reduced by externally influencing the nucleus subthalamicus. This is accomplished by implanting microelectrodes that calm the overactive nucleus subthalamicus in affected individuals and, as a result, reduce tremors.
Diseases
The only known disease of the nucleus subthalamicus, which is also extremely rare, is ballismus.This manifests itself with uncontrolled and exceptionally violent movements of the arms and legs, in rare cases also of the pelvis and shoulder girdle. There is no control over the affected limbs and even injuries to the own person cannot be excluded. However, the symptoms do not appear during the sleep phases. The ballism usually extends only to one side of the body, so that it is often called hemiballism. In this case, the half of the patient’s body that lies on the opposite side of the nucleus subthalamicus, which is responsible for the disorder, is affected. The cause of ballism is a disturbance or injury of the nucleus. This can be caused, for example, by a brain tumor including metastases, a cerebral infarction or a stroke. Other possible causes include inflammation of the brain, neurosyphilis, and injuries sustained during a previous neurosurgical procedure. Ballismus can be diagnosed with certainty by means of CT or MRI. This is usually followed by treatment with an antiepileptic or a neuroleptic. If this therapy is unsuccessful, various surgical interventions can still be considered. The chances of recovery depend strongly on the cause and are difficult to assess due to the small number of cases so far. They range from spontaneous attenuation of symptoms to paralysis of certain muscle groups.
