The thalamus is one of the most important parts of the brain. Not only is it one of the largest encephalic structures, but it is also located in the middle of the brain, as reflected in its name, which comes from the Greek word thalamos (or “inner chamber”).
By occupying so much and being so well communicated with the rest of the brain, the thalamus intervenes in a great number of mental processes that shape our way of perceiving things and acting on the environment that surrounds us… even if we are not aware of it.
What is the thalamus?
The thalamus is basically a collection of grey substance (bodies of neurons) formed by two egg-shaped brain structures below the cerebral cortex. These structures are located next to each other, and in addition to having the same shape and size, they are symmetrically arranged, as are the two brain hemispheres that cover them. They communicate with each other through a kind of bridge that holds them together and is called an interthalamic connection.
The thalamus is part of an area called the diencephalon . The diencephalon is located between the cerebral cortex (and all lobes of the brain) and the top of the brain stem. The diencephalon is made up of the thalamus, hypothalamus (located just below the thalamus), and some other smaller structures.
In addition, the thalamus is symmetrical in shape and, because it is located just below the space separating the two brain hemispheres, it exits on both sides of the brain. To see how it interconnects with these parts, we can take a look at the structures of the thalamus and the types of neurons in it.
The structures of the thalamus
The thalamus is basically a pile of neuron bodies, that is, a structure of grey matter, just like the cerebral cortex. But within this cluster of neurons a number of nuclei of the thalamus can be distinguished :
- Specific connection cores . These send sensory information to specific areas of the cerebral cortex that are specialized in working with that particular type of data from a specific direction.
- Non-specific connection cores . They send information to very large areas of the cerebral cortex, without discriminating by specialization.
- Association cores . They are part of an information circuit that communicates the cerebral cortex with subcortical structures.
Neurons of the thalamus
The thalamus is composed of many other specialized substructures, but all of them are, after all, neurons and glial cells . Like any other part of the brain, the thalamus is only right if it is connected to other areas of the nervous system, and this is reflected in the type of neurons that make it up. In the distribution of these it is noted that they are associated with many other bundles of neurons that arrive from many parts of the central nervous system.
Functionally, the classes of neurons in the thalamus are as follows :
- Local Interneurons . These nerve cells are basically in charge of making the information that comes from other parts of the nervous system be processed in the thalamus, transforming it into a new series of data. Therefore, their main function is to send nerve impulses to other interneurons in the thalamus. They make up approximately 25% of the neurons in the thalamus.
- Projection neurons . These nerve cells are in charge of sending information out of the thalamus, towards the cerebral cortex. They make up 75% of thalamic neurons.
The functions of the thalamus
We have seen that the thalamus is very well communicated, but its role is not simply to be a bridge of communication between relevant parts of the brain. The thalamus itself is a structure that plays an active role in processing information coming to it from other areas. But… what are the functions that this brain structure performs?
Integration of sensory data
The best known and most studied function of the thalamus is that of being one of the first stops in the brain for the information that reaches us through the senses , with the exception of smell.
The thalamus processes this sensory information, discards the parts that are not too important and sends the final result to the brain’s cortex, where this information will continue to be processed.
Thus, it facilitates the integration of sensory information to move from raw data to relatively complex information units and capable of holding a meaning for us. In any case, it must be clear that this process not only takes place in the thalamus, but also involves several networks of neurons distributed throughout virtually the entire brain.
2.The sleep-wake cycle
The thalamus, like its younger brother the hypothalamus, intervenes in regulating the rhythm with which the sensation of sleep comes and goes. This function, apart from being fundamental in regulating all nervous activity in general, is also related to the following.
3.Attention and awareness
Recent research indicates that the thalamus could play a very important role in the emergence of consciousness and everything related to it ; from the ability to think about one’s own thoughts, to the use of language, to the ability to focus attention on concrete information according to the objectives one has at any given time.
However, it is important to note that these processes related to conscious states are not the consciousness itself, although they appear in parallel. We cannot focus our attention on anything when we are not aware that we exist, and we cannot speak or reflect; but when we are conscious, there are aspects of attention and language that are beyond consciousness.
Furthermore, all these complex mental processes related to abstract thought require the participation of many areas of the brain, not only the thalamus ; this part of the diencephalon is a necessary but insufficient component when it comes to making thought, attention and language take place (something that can be said of practically all parts of the brain, because they all work in an interconnected way).
Because the thalamus is so well connected to many areas of the cortex at once, it may be able to intervene in the synchronization of neural activity necessary for the level of consciousness to be maintained. Without it, all other parts of the brain become non-functional, at least in the vast majority of cases. Exceptions can always appear in people born without a thalamus or with a very poorly developed one and who can still live many years; in cases like this, the rest of the brain would have learned to reconfigure itself to carry out the tasks of this absent structure using other networks of neurons.
4.The regulation of emotions
The thalamus is not only connected to circuits that carry sensory information, but also interacts with neural pathways that are directly involved in the emergence of emotional states . It is not in vain that the thalamus is surrounded by the limbic system.
Thus, the thalamus integrates these two pathways and works by bringing together these two types of information, making emotions affect what is perceived and vice versa. In addition, it receives information from the hypothalamus, which in turn intervenes directly in the regulation of emotions and the segregation of different types of hormones in the bloodstream.
The thalamus is one of the largest parts of the brain and, moreover, seems to play a role in a multitude of functions that neither resemble nor have much to do with each other at first glance.
However, this is a reflection of the very functioning of the nervous system, in which all the time, regardless of whether we are asleep or awake, a multitude of processes are taking place in parallel and at the same time in a coordinated manner.
It also has a very relevant role in the appearance and maintenance of the states of brain activation responsible for keeping us aware of our own existence and what is happening around us. This has caused the thalamus to be considered “the switch of consciousness” .
However, the thalamus itself is not the part of the brain where consciousness “resides”. To suppose this would be like thinking that inside our head there is a pixie with a consciousness of its own that is surrounded by unconscious matter just like the pilot of an airplane would do; that is, it would make us fall into the dualism of philosophers like René Descartes.
It is now understood that consciousness is the result of the activity of various parts of the brain (among which the thalamus would be the most important) working together at great speed and in a coordinated manner, and therefore this state of mind cannot be reduced to a single structure.
- Boutros, N. J. (2008). The thalamus. Clinical EEG and Neuroscience, Vol.39(1), p.IV
- Percheron, G. (1982). The arterial supply of the thalamus. In Schaltenbrand; Walker, A. E. (eds.). Stereotaxy of the human brain. Stuttgart: Thieme. pp. 218 – 232.
- Perea-Bartolomé. M. V. and Ladera-Fernández, V. (2004). The thalamus: neurofunctional aspects. Journal of Neurology, 38(7), pp. 697 – 693.
- Sherman, S. Murray; Guillery, R. W. (2000). Exploring the Thalamus. Academic Press.
- Sherman, S. (2006). Thalamus. Scholarpedia1 (9): 1583.
- Shimamura, K; Hartigan, DJ; Martinez, S; Puelles, L; Rubenstein, JL (1995). “Longitudinal organization of the anterior neural plate and neural tube. Development. 121 (12): 3923 – 3933.