Thalamus: anatomy, structures and functions

The thalamus is one of the parts of the brain more important. Not only is it one of the largest brain structures, but it is also located in the heart of the brain, as reflected by its name, which comes from the Greek word we thall (or "inner chamber").

By occupying so much and being so well communicated with the rest of the parts of the brain,  the thalamus intervenes in a large number of mental processes that shape our way of perceiving things and acting on the environment that surrounds us... even if we don't realize it.

What is the thalamus?

The thalamus is basically  a set of gray matter (bodies of neurons) formed by two egg-shaped brain structures that lie 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 cerebral hemispheres that cover them. They communicate with each other through a kind of bridge that holds them together and is called the inter-thalamic connection.

The thalamus is part of an area called diencephalon. The diencephalon is situated between the cerebral cortex (and all lobes of the brain) and the upper part of the brain stem. In turn, the diencephalon is composed of the thalamus, the hypothalamus (located just below the first one) and some other smaller structures.

In addition, the thalamus has a symmetrical shape and being located just below the space that separates the two cerebral hemispheres, it has an outlet 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 what's in this.

Structures of the thalamus

The thalamus is basically a collection of neuron bodies, that is, a gray matter structure, just like the cerebral cortex. But within this set of neuronal groups a series 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 specific type of data from a specific sense.
• Nonspecific connection nuclei. They send information to very wide areas of the cerebral cortex, without discriminating by specialization.
• Association nuclei. They are part of an information circuit that communicates the cerebral cortex with subcortical structures.

Thalamus neurons

Thalamus It is made up of many other specialized substructures, but they are all, after all, neurons and glial cells. Like any other part of the brain, the thalamus only has a reason for being if it is connected to other areas of the nervous system, and this is reflected in the type of neurons that compose it. In the distribution of these it is noted that they are associated with many other bundles of neurons that come from many parts of the central nervous system.

From a functional point of view, the classes of neurons in the thalamus are as follows:

• Local interneurons. These nerve cells are basically responsible for making the information that arrives from other parts of the nervous system processed in the thalamus, transforming it into a new series of data. Therefore, its main function is to send nerve impulses to other interneurons in the thalamus. They make up about 25% of the neurons in the thalamus.
• Projection neurons. These nerve cells are responsible for sending information out of the thalamus, towards the cerebral cortex. They are 75% of thalamic neurons.

The functions of the thalamus

We have seen that the thalamus is very well communicated, but its role is not that of being a simple communication bridge between relevant parts of the brain. The thalamus itself is a structure that plays an active role in processing information that comes to it from other areas. But... What are the functions of this brain structure?

1. Sensory data integration

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 comes to 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 end result to the cortex of the brain, where this information will remain 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 does not only take place in the thalamus, but also involves several networks of neurons distributed throughout practically 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, in addition to being essential to regulate all nervous activity in general, is also related to the following.

3. Attention and awareness

Recent research indicates that the thalamus could have a very important role in the appearance of consciousness and everything related to it; from the ability to think about one's own thoughts, to the use of language, through ability to focus attention on specific information according to the objectives of each moment.

However, it is important to note that those processes related to conscious states are not 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.

In addition, all these complex mental processes related to abstract thinking require the participation of many areas of the brain, not just the thalamus; This part of the diencephalon is a necessary but insufficient component in making thought, attention, and language take place (something that can be said for practically all parts of the brain, because they all work in interconnected).

Since the thalamus is so well connected to many areas of the cortex at once, it might be able to intervene in the synchronization of neuronal activity necessary to maintain the level of consciousness. Without it, the other parts of the brain become non-functional, at least in the vast majority of cases. Exceptions can always appear of people born without a thalamus or with a very underdeveloped one and that despite this they can live for many years; in such cases, 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 with circuits that carry sensory information, but  it also interacts with neural pathways that are directly involved in the appearance of emotional states. Not for nothing is the thalamus surrounded by the limbic system.

Thus, the thalamus integrates these two pathways and works by bringing these two types of information together, causing emotions to affect what is perceived and vice versa. In addition, it receives information from the hypothalamus, which in turn is directly involved in the regulation of emotions and the secretion of different types of hormones in the bloodstream.

Conclution

The thalamus is one of the largest parts of the brain and, in addition, it seems to have a role in a multitude of functions that neither look very similar 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 whether we are asleep or awake, a multitude of processes are taking place in parallel and simultaneously coordinated.

It also has a very relevant role in the appearance and maintenance of brain activation states. responsible for keeping ourselves aware of our own existence and what happens to our around. This has caused the thalamus has come 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 self-awareness that is surrounded by non-conscious matter just as the pilot of an airplane would; that is, it would make us fall into the dualism of philosophers like René Descartes.

Currently it is understood that consciousness is the result of the activity of various parts of the brain (among which the thalamus would stand out) working with each other at high speed and in a coordinated manner, and consequently this state of mind cannot be reduced to a single structure.

Bibliographic references:

• 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. AND. (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.