Although we often take it for granted that the human brain is basically the organ that allows us to think and be aware of things, the truth is that it also performs all kinds of automatic and unconscious functions. It is not simply the biological basis of the human intellect; it also carries out a multitude of processes essential to our survival.
The suprachiasmatic nucleus is an example of this . While certain regions of the brain stem are responsible for making the heart beat or regulating the body’s temperature so that our cells do not die, this brain structure acts as our internal clock. We will now see exactly what this means and what anatomical characteristics the supraquiasmatic nucleus has.
What is the suprachiasmatic nucleus?
We understand a suprachiasmatic nucleus as a small structure formed by some 20,000 neurons located in the area of the hypothalamus closest to the face, that is, in the lower part of the diencephalon. It is composed of gray matter.
It should be noted that there is a supraquiasmatic nucleus in each cerebral hemisphere , that is, two per individual on each side of the head.
As its name suggests, the suprachiasmatic nucleus is located above the optic chiasm , which is an area at the base of the brain where the optic nerves cross, passing into the opposite hemicorium. It is also possible to locate it by taking the hypothalamus as a reference, since it is located in the anterior part of this brain structure, bordering on both sides of the third brain ventricle.
The fact that the optic chiasm is located just above the optic nerves is not accidental; in fact, its functioning has to do with the light signals that are captured by the retina, as we will see.
Functions of the Suprachiasmatic Nucleus
The main task of the supraquiasmatic nucleus is to regulate the circadian rhythms that govern the levels of activity of the body depending on the time we are in. Circadian rhythms are the cycles that determine when there is the greatest need for rest and when there is a great amount of energy available and therefore we will move more, think better, etc.
That is, the supraquiasmatic nucleus intervenes in the sleep-wake cycles, and makes us more likely to sleep at certain times and wake up at others, for example, and not have the same energy at 12 noon as after dinner.
The cycles regulated by the supraquiasmatic nucleus last 24 hours, since evolution has made them adapt to what lasts a natural day from the luminosity captured through our eyes.
Thus, when we are exposed to light, this is interpreted by this brain structure as evidence that it is time to stay awake longer, and the massive secretion of melatonin , a hormone that is much more numerous just before we start sleeping and while we remain in the sleep phase, is delayed.
When we look somewhere, the light that reflects what we direct our eyes to is projected onto the retina, a layer of cells located inside the eye and which some scientists consider part of the diencephalon.
This membrane collects the electrical signals into which the light patterns of what we see are translated , and sends this information to the brain through the optic nerves. The usual route for most of this information is through the thalamus and the occipital lobe, where the visual information begins to be integrated into larger, more complete units.
However, some of this information deviates from this route at the optic chiasm, located at the “entrance” to the brain, to reach the supra-khiasmic nucleus. This structure does not recognize details of light patterns, shapes or movement, but is sensitive to the overall amount of light being collected by the retinas. This causes commands to be sent to other areas of the body related to circadian rhythms, such as the pituitary gland, which is located nearby.
In this way, our body adapts to what are interpreted as the demands of the environment. In the end, if we are designed in a way that generates more efficiency during the light hours, it is better to take advantage of those moments and leave the dark hours to rest, according to the logic of natural selection.
However, the use of artificial light sources can turn this against us and, for example, exposing us to the light of a computer screen shortly before going to sleep can cause us to feel sleepless despite being tired from a long day’s work. This makes our body try to respond to a strange situation for which it has not been prepared: days with many more hours of light.