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The major sections of the brain – the forebrain, midbrain, and hindbrain – can be divided further into subsections. One of these subsections of the forebrain – the section of the brain involved in “higher” cognitive functions such as the forming of thoughts and generally what constitutes sentience – is called the subthalamus.
More specifically, the subthalamus is a subdivision of the diencephalon (cuter names for which are the “interbrain” or “between-brain”). The subthalamus contains nuclei and gray matter like the zona incerta, reticular nucleus, and the perigeniculate nucleus.
General functions the subthalamus are responsible for include sexuality, food and water intake and maintenance of hydration, and cardiovascular activity.
Anatomy of the Subthalamus
As previously noted, the subthalamus is located within the diencephalon, one of the major divisions of the prosencephalon (forebrain). Together, the diencephalon and the telencephalon, which is the cerebrum, comprise the forebrain.
The diencephalon is a small portion of the brain, nestled underneath and between the two cerebral hemispheres, just above the brain stem. The diencephalon is integral to several critical functions in maintaining the brain and body.
This region of the brain is responsible for relaying information between regions of the brain and controlling many autonomic functions in the peripheral nervous system (PNS). It also serves to connect the endocrine system with the nervous and limbic system primarily to regulate emotions and memories.
Although the subthalamus is a structure within the forebrain, it is made up of tissues that arise from the midbrain and is largely interconnected with the basal ganglia.
True its namesake (the thalamus itself), the subthalamus has numerous connections to many different portions of the brain. The zona incerta provides stimulation to the thalamus, the reticular nucleus regulates the thalamocortical pathway, and the perigeniculate nucleus is integral to vision and movement of the eye via its fiber connections to the retina.
The zona incerta is a thin layer of gray matter situated between the thalamic and lenticular fasciculi. (What the heck is a fasciculi, you ask? Well, fasciculi are the plural of fasciculus, which is a nerve tract connecting one structure to another.
The thalamic fasciculus is a bundle of nerve fibers containing crossed and uncrossed fibers connecting the zona incerta to the thalamus. The lenticular fasciculus is a collection of efferent nerve fibers that join the thalamic fasciculus in connecting the zona incerta to the thalamus.)
The zona incerta is highly complex and is home to the convergence of numerous major tracts of the nervous system as one of the final destinations before transmissions arrive in the brain. These tracts are involved in motor control, autonomic movements (reflexes), and more.
There are four major sections of the zona incerta (ZI): the rostral ZI, dorsal ZI, ventral ZI, and caudal ZI.
Each of these sections is responsible for various functions. The rostral ZI is associated with visceral functions (autonomic movements of smooth muscles, cardiac muscles, and glands); the dorsal ZI is associated with arousal; the ventral ZI is associated with the ability to focus attention on tasks and objects, and the caudal ZI is associated with the aforementioned motor functions. (The caudal portion is more special than the others, though, since it receives input from afferent nerve fibers from the basal ganglia, reticular formation, and cortical regions.
The reticular nucleus is a collection of neurons and both afferent and efferent fibers. This structure is integral to many complex functions of the central nervous system (CNS) including the processes that constitute consciousness. The fibers that make up the reticular nucleus give this structure its name because of the “reticulated” pattern of the fibers on the nucleus.
This structure is made up of a thin layer of neurons that cover the lateral portion of the dorsal region of the thalamus and joins with the zona incerta. The reticular nucleus is yet another nervous system structure that can be further divided into smaller, more specific sectors. For the reticular nucleus specifically, these sectors relate to only one modality or functionally-related group of thalamic nuclei each.
The nerve cells in the reticular nucleus transmit neuronal signals to the thalamus in a hierarchical manner, their axons returning to the thalamus with information from “first and higher-order” cells first – in some cases, even sending two branches of a single axon back with information first.
The perigeniculate nucleus (PGN) is actually a component of the reticular nucleus, described above. The PGN plays a major role in the functioning of the human visual sensory system.
It works alongside the pulvinar (the pulvinar nuclei are a cluster of neuron cell bodies that are strongly associated with the visual cortex), referred to collectively as the PGN/pulvinar couple. (Fun fact: the pulvinar couple is a major part of the reason why you, a human, are capable of learning to read!)
More specifically, the PGN is associated with the Precision Optical System (POS). You can think of the POS as the analog to the optional settings, or manual controls, of your camera (no, not your camera phone, a real camera). The POS is a group of neurological structures that control the “pointing, accommodation, and aperture control functions” that constitute vision.
The structures that make up the POS are the photoreceptors of the foveola, PGN, pulvinar, oculomotor nuclei, oculomotor muscles, and the thalamic reticular nucleus.
Within the POS, the primary functions of the PGN are
- to control the direction in which both eyes are pointing,
- the physiological process by which the visual sensory information gathered from both eyes converges at a certain distance (where the two distinct images of the eyes join into one so you’re no longer seeing double but one whole picture),
- the distance at which the eyes can focus (your prescription, basically),
- and the process that allows the analysis of fine details and the ability to read.
Another portion of the subthalamus – the largest, as a matter of fact – is the subthalamic nucleus. The subthalamic nucleus is heavily involved in the integration of somatic motor functions (this is why, in a lot of the scientific literature you may come across, the subthalamus is often discussed largely in terms of the subthalamic nuclei).
An additional reason as to why the subthalamic nucleus is so special is that it is the only “excitatory structure” within the basal ganglia. This is perhaps what contributes to its role as the major neuronal transmission processing unit of the basal ganglia.
The subthalamic nucleus can be divided into several more subsections – of course, just like many other structures of the nervous system. There are three of these subsections in total: a dorsolateral motor territory, ventromedial associative territory, and medial limbic territory, each of them having their own unique connections to the cerebral cortex and outsourcing information to various target nuclei in the globus pallidus, substantia nigra, pars reticulata, and ventral pallidum.
The dorsolateral motor territory is associated with motor activities, supporting connections to the primary motor cortex, and the external and internal segments of the globus pallidus.
The ventromedial associative territory is connected to the dorsolateral prefrontal and lateral orbitofrontal cortex, controlling the oculomotor and cognitive functions of the nervous system.
Lastly, the medial limbic territory, as its name suggests, controls behavioral responses to emotional stimuli and is connected to the limbic and paralimbic cortex, the hippocampus, and the amygdala.
Significance of the Subthalamus
Perhaps a reason why the subthalamus isn’t typically discussed in-depth as a whole structure but referred to instead by its components is because of the diversity of functions of all of the aforementioned anatomical structures.
Its significance as one organ can be overlooked, given its location – it is the most ventral portion of the diencephalon in between the thalamus and midbrain. I mean, come on! It’s surrounded by some of the most important organs in the nervous system and body as a whole.
Still, the subthalamus has what these other structures do not – and that is a core part in the functionality of the basal ganglia, the ultimate processing center of movement, emotion, and cognitive nerve signals – almost everything that the human body needs to live. It also serves as the primary connection between the midbrain and diencephalon.
Its role in motor functions implicates the subthalamus in movement disorders, usually the result of some kind of trauma to the head that can affect the basal ganglia, thalamus, and/or subthalamus. Some of these known disorders are tremors, dystonia, parkinsonism, myoclonus, and choreiform movements.
Interestingly enough, subthalamic cells have been found to be inactive a majority of the time due to inhibition by the cells of the external pallidal segment (a portion of the external globus pallidus).
The cessation of this inhibition results in the hyperactivity of not only the subthalamic cells but the globus pallidus as well – a key trait of Parkinson’s disease. (This is why one of the treatments of Parkinson’s disease is the delivery of electrical impulses to the subthalamic nucleus, called deep-brain stimulation, to interfere with the functionality of the subthalamic nucleus, reducing its neuronal output.)
The subthalamus is truly an underdog of the nervous system, but don’t be mistaken – it is not merely an extension of the thalamus, but an essential organ to the maintenance of the human body.
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