The suprachiasmatic nucleus is a brain center whose function is to be our internal clock, controlling circadian rhythms. It is responsible for generating the sleep and wake cycles close to 24 hours. It is made up of two small brain structures made up of neurons that regulate biological rhythms. These structures are shaped like wings and are the size of the tip of a pencil; They are located in the anterior part of the hypothalamus.
The suprachiasmatic nucleus triggers a series of neuronal and hormonal events to control different functions of the body in the 24-hour cycle. To do this, it uses approximately 20,000 neurons. This structure interacts with many other brain regions.
Even without external weather signals, these biological rhythms are maintained. However, sunlight and other environmental stimuli influence the maintenance of this 24-hour cycle. In other words, light has to reset the internal clock every morning so that the body remains in sync with the outside world.
The investigations carried out with individual neurons of the suprachiasmatic nucleus show that each one of them is a functional clock. These are synchronized with the activity of their neighboring cells.
It has been found, in many experiments, that the fluctuations of the human circadian cycle are maintained even when we are isolated from daylight.
On the other hand, in experiments with rodents in which the suprachiasmatic nuclei were destroyed, their sleep-wake cycles became totally disorganized.
It seems that this mechanism is not only endogenous, but also has a genetic origin. These rhythms are activated by the cyclical activity of certain genes. Specifically, circadian activity is a reflection of a rhythmic pattern of expression of essential genes. These are known as "clock genes."
Location
The suprachiasmatic nucleus is located at the base of the brain, next to the hypothalamus. Its name comes from the fact that it is located above the optic chiasm, where the optic nerves intersect. They are located bilaterally on each side of the third cerebral ventricle.
This nucleus is in a strategic place to be able to receive signals from the optic nerves, indicating the intensity of the light that is entering the retina.
Features
Living beings have adapted to the existing environment with the aim of maintaining the survival of the species. For this, they have developed two fundamental states of behavior: activity and adaptive behavior, and rest.
In mammals these states are identified as wakefulness and sleep. These happen in precise 24-hour cycles that have evolved as an adaptation to the solar cycle of light and dark.
These circadian rhythms are now known to be found in cells throughout the body. The suprachiasmatic nucleus is the circadian pacemaker that controls times of rest, activity, body temperature, hunger, and hormone secretion. To do this, it coordinates with other brain regions and other body tissues.
With exposure to light, the suprachiasmatic nucleus tells us that it is time to be awake. It raises the body temperature and increases the production of hormones such as cortisol.
In addition, it delays the release of hormones such as melatonin, whose increase is related to the onset of sleep and usually occurs when we perceive that the environment is dark. These levels remain elevated throughout the night so that we can sleep properly.
Neurons emit action potentials in 24-hour rhythms. Specifically, at noon, the firing speed of neurons reaches a maximum level. However, as night falls, the action potentials decrease in frequency.
The dorsomedial part of this nucleus is thought to be responsible for endogenous 24-hour cycles. That is, that we can maintain our circadian rhythms despite staying in the dark.
Functioning
When ambient light hits the retina, it activates photosensitive cells called ganglion cells. These cells convert light particles (photons) into electrical signals. Neurons in the retina send these signals through the optic nerves.
These nerves cross to form the optic chiasm. Later, the visual information reaches the back of the brain, called the occipital lobe. There it is processed in the form of images that we consciously perceive.
However, there is a group of neurons that leave the optic chiasm and reach the suprachiasmatic nucleus to exert the cyclical functions of the organism. Thus, this nucleus decides to activate or inhibit the pineal gland to secrete different hormones. Among them, melatonin.
The circadian influences of neurons in the suprachiasmatic nucleus are spread through the various target organs of the body by different neuronal signals and by the circulation of melatonin.
The suprachiasmatic nucleus regulates the secretion of melatonin from the pineal gland according to the light and darkness of the environment. Melatonin is a substance that controls sleep and other cyclical activities in the body.
Melatonin has a function both as a clock marking each hour of the day, and as a calendar indicating the time of year to all tissues of the body.
Melatonin disturbances have been found to be linked to aging-related sleep disorders, Alzheimer's disease, and other neurodegenerative diseases. In fact, it appears to have antioxidant effects, protecting our neurons.
Suprachiasmatic nucleus alterations
The activity of the suprachiasmatic nucleus can be altered at different stages of life. For example, in adolescents, melatonin levels rise later than in most children and adults. Because of this, they may have a hard time going to sleep early.
On the other hand, in the elderly, there are more awakenings during the night since the release of melatonin is altered as we advance in age.
The functioning of the suprachiasmatic nucleus can be dysregulated by external factors. This is what happens with jet lag or if we do not maintain a daily routine and force our body to stay awake at night.
It is important to note that, in neurodegenerative diseases such as Alzheimer's, circadian rhythms are altered due to the progressive loss of neurons in the suprachiasmatic nucleus.
References
- Benarroch, EE (2008). Suprachiasmatic nucleus and melatonin Reciprocal interactions and clinical correlations. Neurology, 71 (8), 594-598.
- Mirmiran, M., Swaab, DF, Kok, JH, Hofman, MA, Witting, W., & Van Gool, WA (1992). Circadian rhythms and the suprachiasmatic nucleus in perinatal development, aging and Alzheimer's disease. Progress in brain research, 93, 151-163.
- Moore, RY (2007). Suprachiasmatic nucleus in sleep – wake regulation. Sleep medicine, 8, 27-33.
- SLEEP DRIVE AND YOUR BODY CLOCK. (sf). Retrieved on April 20, 2017, from the National Sleep Foundation: sleepfoundation.org.
- Suprachiasmatic nucleus. (sf). Retrieved on April 20, 2017, from Wikipedia: en.wikipedia.org.
- The Human Suprachiasmatic Nucleus. (sf). Retrieved on April 20, 2017, from BioInteractive: hhmi.org.
- THE SUPRACHIASMATIC NUCLEI AND THE PINEAL GLAND. (sf). Retrieved on April 20, 2017, from The brain from top to bottom: thebrain.mcgill.ca.