The Subthalamus

As its name suggests, the subthalamus is hidden directly beneath the thalamus. Or rather, it is found there in the embryo – during development, a thick strand of white matter, the internal capsule, pushes most of the subthalamus to the side. Finally, it ends up in the cerebrum, right next to the putamen. This poses structural problems for anatomists.

The subthalamus consists of three areas of gray matter: the globus pallidus, Latin for “pale nucleus;” the nucleus subthalamicus; and the zona incerta, the “uncertain zone.” Although first described in 1877, the function of this nucleus is indeed uncertain: to this day, no one knows its function; there is only speculation.

But that's just where problems begin.Not only is the globus pallidus displaced toward the putamen during ontogenesis, it also plays a key role in motor processes – the classic task of the basal ganglia, which also include the putamen. The globus pallidus is closely connected to the putamen and is therefore also considered part of the basal ganglia in terms of function. The subthalamic nucleus is also part of this motor regulation and is assigned to either the basal ganglia or the subthalamus, depending on the author's preference.
 

Just below the lowest point of the thalamus lies the zona incerta, a small nucleus surrounded above and below by white matter, known as Forel's field H1 and Forel's field H2. Below this, in the transition zone between the midbrain and diencephalon, is the even smaller subthalamic nucleus. This nucleus, also called Luys' body after its discoverer, has the shape of a biconvex lens. To the side of it, separated from both nuclei by the internal capsule, is the much larger globus pallidus in the shape of a lying cone, whose tip points downwards and towards the center. It is the main nucleus of the subthalamus. However, as mentioned above, it is functionally part of the basal ganglia and is therefore discussed there in a separate text, including its motor control circuits.

The subthalamic nucleus is also part of the complex motor control system: it receives excitatory fiber inputs directly from the motor cortex, as well as inhibitory impulses from the outer segment of the globus pallidus while sending signals primarily to the inner segment of the globus pallidus and to the substantia nigra.

Since control loops are obviously more important than individual structures at this point, here is a brief excursion into the interconnection of the basal ganglia: These influence the execution of movements via several interconnection pathways. The main loop promotes motor function. It runs from the putamen via the inner segment of the globus pallidus to the thalamus. Since the globus pallidus inhibits the thalamus but is itself inhibited by the putamen, the inhibition of the thalamus is reduced according to the principle of double inhibition, allowing it to send its excitatory signals to the motor cortex. At the same time, several secondary loops modulate the main loop. One of these includes the subthalamic nucleus: it excites the internal pallidum segment, thereby increasing inhibition of the thalamus. This secondary loop thus counteracts unregulated motor function.

This is particularly evident in cases of damage to the subthalamic nucleus, for example as a result of a stroke. This can lead to a condition known as hemiballismus, in which those affected suddenly and involuntarily flail their arms or legs. Fortunately, this disorder is not permanent and only affects one side of the body: the opposite side of the damaged hemisphere.

The subthalamic nucleus also influences the symptoms of Parkinson's patients. Exactly how it does this remains a mystery to neuroscientists. However, they do know that when there is a dopamine deficiency in the striatum, the subthalamic nucleus fires signals more intensely. When it is “switched off,” the resting tremor, the typical slow tremor experienced by patients, subsides. This switch-off is achieved using the method of deep brain stimulation: electrodes are implanted in the brain of the patient, which constantly emit electrical impulses and counteract the overactivity of the subthalamic nucleus.

When Swiss brain researcher Auguste-Henri Forel first described the zona incerta, he called it “a region about which nothing certain can be said.” Not much has changed in the nearly 130 years since then: most textbooks only briefly mention the zona incerta as existing and do not even list it in their index. Its function is still unknown today. However, there are many assumptions: it is thought to influence arousal, control the activity of the intestines, maintain movement, or generally generate responses to sensory stimuli.

First published on August 28, 2011
Last updated on October 1, 2025