It is clear that sleep is a very important process for the life of living things, but … What areas of the brain are involved in the act of sleeping? What relationships between them allow the appearance of each phase of sleep?
In this article, we will describe what the sleep process looks like, what phases it is made up of, as well as which regions of the brain are involved, activated or inhibited in each phase, that is, which areas are related to the neural control of the body. sleep.
Definition and phases of sleep
Sleep is a state produced by a decrease in consciousness that allows both body and brain rest.. There is a decrease in brain activity generated by fatigue. Sleep is a fundamental, vital need, and it is essential for the proper functioning and regulation of the different regions involved in this process so that the living being is in an optimal state and can stay alive.
In the waking state, the electrical activity of the brain is not synchronized with the fast frequency waves called beta waves. When the subject is at rest, relaxed, the waves become a little slower giving rise to Alpha waves.
During sleep, there are two main stages: NREM sleep or slow wave, and REM sleep, also called paradoxical sleep because the waves increase their frequency again generating, in the same way as in the morning, beta waves.
In REM sleep, besides the increase in the frequency of brain activity, there is also an increase in eye movements, desynchronizations and a decrease in muscle tone.
NREM sleep is necessary for the brain to rest and recover from the activity produced during wakefulness; on the other hand, REM sleep will be mainly linked to learning processes, consolidating the information obtained during the day.
The parts of the brain related to sleep: organic bases of the act of sleeping
At first, it was believed that sleep acted as a passive process produced by sensory disturbances, a lack of stimulation. But the neuropsychologist Giuseppe Moruzzi has proven that the mesencephalic reticular formation, located in the brainstem, has the function of generating arousal; for its part, the medulla oblongata is responsible for inhibiting the anterior structure allowing rest.
In this way, this scientist was able to assert, given the inhibitory function of the medulla oblongata, that sleep is an active process.
Areas of the brain involved in NREM sleep
As we have already pointed out, in this phase of sleep there is a decrease in brain activity, allowing the brain to rest and recover.
The function of the ventrolateral preoptic zone has been shown to be particularly important for the sleep process, located in the lateral hypothalamus, a region of the brain linked mainly to endocrine function and hormones. It has been verified in different experiments with animals that the injury or destruction of this area causes total insomnia, sleep disturbances; on the contrary, its stimulation causes drowsiness and drowsiness.
The neurotransmitter GABA, a messenger involved in the decrease in brain activity, is projected from the ventrolateral preoptic area to the tuberculo-mammary nucleus, also located in the hypothalamus, to the dorsal protuberance located in the brainstem, to the rafe nucleus located in the brainstem and responsible for the production of serotonin, and in the locus coeruleus, also located in the brainstem and linked to the production of the neurotransmitter norepinephrine. These projections they cause the inhibition of these areas.
As we noted at the beginning, the ventrolateral preoptic area is linked to a decrease in activation and therefore sleep; this fact is reaffirmed by the inhibitory function it produces in the aforementioned areas of the hypothalamus and brainstem. These regions have also been shown to be linked to brain and behavioral activation.
In the same way too the existence of a reciprocal inhibition circuit called a toggle oscillator has been observed, in this circuit takes place the alternating inhibition of the ventrolateral preoptic zone and the projecting regions of the trunk and hypothalamus, this means that the activation of one will result in the deactivation of the other, in this way they will not be able to be found working at the same time, allowing to alternate periods of sleep and wakefulness.
On the other hand, we noticed that this circuit did not always work well and imbalances and decompensations can occur, leading to sleep-wake disorders such as narcolepsy, poultice (loss of muscle tone), sleep paralysis and hypnagogic hallucinations.
Likewise, it has been shown that it is the hypocretinergic neurons of the lateral hypothalamus (i.e. the neurons that secrete hypocretin) that are responsible for regulating and stabilizing the alternative sleep-wake circuit, the ‘oscillator, causing it to stay and thus allow the individual or animal to stay awake or awake.
We also saw that the action of adenosine, a substance that appears after cortical activity, on the basal forebrain (mainly related to cognitive functions such as attention and learning), produces an inhibition or deactivation of it, thus allowing the onset of sleep .
On the other hand, adenosine can also act by inactivating the hypocretinergic neurons of the lateral hypothalamus, linked to the wakeful state, as we have seen.
Areas of the brain involved in REM sleep
As we mentioned in the first sections, the activity or brain waves during REM sleep are similar to those during waking, showing a high electrical frequency observed with the electroencephalogram technique.
A characteristic and distinctive sign of the REM phase is the appearance of PGO (protuberance-geniculo-occipital) waves, thus indicating that the individual is in REM. PGO waves are large, short electrical waves that start at the bulge, travel to the lateral geniculate nuclei, and then to the occipital lobe, specifically the primary visual cortex.
The region most involved in the REM soil is the dorsolateral bulge, which is made up of cholinergic neurons, producers of the neurotransmitter acetylcholine. This region is inhibited by the rafe and locus coeruleus nuclei, mentioned above, by the projection of norepinephrine and serotonin respectively and thus allowing and appearing NREM sleep or wakefulness.
Likewise, it has been observed that the peribrachial zone, located in the protuberance, which is constituted by the pedunculopontin and latero-odorsal tegmental nucleus, as well as the upper intermediate reticular nucleus, are formed by cholinergic neurons producing acetylcholine, which as we said one of its functions is to control REM sleep.
Like that, depending on the region where the cholinergic neurons are projected, they will give rise or allow different functions of the REM phase: Projections to the thalamus (one of the main areas of information passage of the brain) and the basal forebrain allow cortical activation and desynchronization, and connections to the lateral geniculate nucleus allow or relate to PGO waves.
It was also considered that projections to the midbrain tectum, located in the brainstem, control rapid eye movements, and the binding to the lateral preoptic zone could be related to the erection of the penis during sleep.
Finally, the connections with the neurons of the magnocellular nucleus of the medulla oblongata, also located in the trunk of the brain, allow it to inhibit the motoneurons of the spinal cord thus producing muscle atony, a loss of muscle tone, typical of sleep. .
Areas involved in brain activation
It is also interesting to know which areas of the brain are related to activation or arousal, as these will be important and will participate in the neural control of the brain. sleep.
The main region involved in brain activity is the ascending reticular activator system, also known as SARA. These are neurons located in the upper part of the brainstem, hypothalamus, and basal forebrain. These pathways connect the thalamus and the cerebral cortex and allow them to transmit and respond correctly to sensory inputs.
If injury or damage occurs to this system, an alteration and decrease in consciousness will appear. In this way, optimal functioning of the SARA will result in a correct state of wakefulness, not allowing states of relaxation or sleep.
- Taravillo, B., Espejo-Saavedra, JM. and Gómez, M. (2018) Manual for the development of the PIR CEDE. Psychobiology. CEDE: 5ª Edition.
- Velayos, JL., Morales, FJ., Irujo, AM., Yllanes, D. and Paternain, B. (2007) Anatomical bases of sleep. Faculty of Medicine, University of Navarre.