Beneath the thalamus lies arguably the most important part of the motor system: the subthalamus. It lies in the midbrain and receives nerve cell nuclei that control certain muscle activities. It represents a pale nucleus; its shape is reminiscent of a lens. This part is one of the regions of the human brain that has hardly been studied so far. For this reason, physicians repeatedly refer to it as an “uncertain zone.”
What is the subthalamus?
The subthalamus, as the name suggests, hides under the thalamus. Better said, it can be found under the thalamus in the embryo; in fact, during human development, the subthalamus is pushed aside by a thick cord filled with a white substance. The subthalamus thus ends up in the cerebrum and is found next to the putamen. Its position is the main reason why it drives numerous anatomists to despair. The subthalamus is composed of the globus pallidus (“the pale nucleus”), the zona incerta (“the uncertain zone”), and the nucleus subthalamicus. Even though the subthalamus was described as early as 1877, many physicians today are still unsure what the function of the subthalamus is. Today, therefore, there is no precise information about its function; predominantly, the descriptions and definitions are pure speculations. This is because the globus pallidus is displaced toward the putamen in the course of ontogeny and, according to experts, also plays an essential role in motor processes.
Anatomy and structure
Beneath the thalamus lies the so-called zona incerta. The zona incerta represents a very small nuclear area surrounded at the bottom and top by a white substance that physicians refer to as Forels field H1 and H2. In the transitional area, which is located below and lies between the midbrain and diencephalon, the nucleus subthalamicus connects. The nucleus, also known as the Luys body, STN, or corpus subthalamicum Luysi, resembles a biconvex lens. Laterally, separated by the capsula interna, lies the globus pallidus, whose shape resembles a cone. Its tip points downward and toward the center. It forms the main nucleus of the subthalamus. Functionally, it belongs to the basal ganglia.
Function and tasks
The subthalamus represents a part of motor control. It receives not only excitatory fiber inputs from the motor cortex but also inhibitory impulses from the globus pallidus. The signals are sent to the inner segment and also to the substantia nigra. According to the medical experts, the control circuits are clearly more important here than individual structures. Namely, the basal ganglia influence the execution of movements. The main loop is responsible for motor activity. This runs from the putamen via the globus pallidus to the thalamus. Since the thalamus is inhibited by the globus pallidus, but itself inhibits the putamen, a double inhibition is subsequently created so that the thalamus can send its excitatory signals to the cortex. In the same process, the secondary loops become the main loops. A main loop also includes the nucleus subthalamicus. Thus, an inner pallidum segment is amplified so that there is internal inhibition acting on the thalamus. Thus, the side loop can prevent unregulated motor activity. However, it is also this secondary loop that – when damaged – can become a problem. Auguste-Henri Forel, a brain researcher from Switzerland, already described “the uncertain zone” almost 130 years ago. In many textbooks, the zona incerta is mentioned, but described only extremely sparsely. In many cases, “the uncertain zone” is not even listed in the register. A reason, why numerous scientists are still uncertain today, which functions actually proceed from the “uncertain zone”. However, there is conjecture and speculation. The zona incerta is believed to not only influence arousal, but also control visceral activity and be responsible for maintaining movement.
Diseases
If damage to the nucleus subthalamicus occurs, for example as a result of an insult (stroke), the clinical picture of ballismus develops. If the physician diagnoses a unilateral disorder in the patient, he or she speaks of hemiballismus. The affected person is no longer “master of his motor function”. The arms or legs are involuntarily “thrown around”; a disorder that, however, is not permanent and mainly affects only one side of the body. This is the opposite side of the damaged brain hemisphere.However, the subthalamus also repeatedly influences the symptoms in Parkinson’s disease. The extent to which the subthalamus is responsible for this cannot be answered, however, and poses a mystery to numerous neuroscientists. However, it is known that a lack of dopamine in the subthalamus causes the symptoms to become more severe. If the lack of dopamine is compensated, there is an improvement in the resting tremor, which causes the patients to tremble. By means of a new method, however, it is possible that the brain stimulation can be influenced. Sufferers receive electrodes that are inserted directly into the brain and constantly emit electrical impulses, thus controlling the overactivity of the subthalamus. Other diseases related to the subthalamus are not known so far. However, because only speculation can be made so far, physicians are unsure whether the subthalamus might not be responsible for other diseases related to motor problems.
