What Are the Parts of the Brain Associated with Sleep?

It is clear that sleep is a very important process for the life of living beings, 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 is like, what phases it consists of, as well as what regions of the brain are involved, activated or inhibited in each phase, that is, what areas are linked to the neural control of sleep.

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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 basic, vital need, with proper functioning and regulation of the different regions involved in this process being essential so that the living being is in an optimal state and can remain alive.

In the waking state, the electrical activity of the brain is desynchronized with fast frequency waves called Beta waves. When the subject is at rest, relaxed, the waves become a little slower, giving rise to Alpha waves.

There are two main stages during sleep: NREM or slow wave sleep, and REM sleep, also called paradoxical sleep because the waves increase their frequency again, generating, in the same way as when awake, Beta waves.

In REM sleep, apart from the increase in the frequency of brain activity, an increase in eye movements, desynchronizations and a decrease in muscle tone is also observed.

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 linked above all to learning processes, consolidating the information obtained during the day.

The parts of the brain linked to sleep: organic bases of the act of sleeping

At first it was believed that sleep acted as a passive process produced by sensory deafferentation, lack of stimulation. But the neuropsychologist Giuseppe Moruzzi proved that the mesencephalic reticular formation, located in the brain stem, has the function of generating wakefulness; For its part, the medulla oblongata is responsible for inhibiting the anterior structure, allowing rest.

In this way, this scientist was able to affirm, given the inhibitory function of the medulla oblongata, that sleep is an active process

Brain areas involved in NREM sleep

As we have already noted, 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 area has been seen to be especially important for the sleep process, located in the lateral hypothalamus, a region of the brain mainly related to endocrine function, with hormones. It was proven in different experiments with animals that the injury or destruction of this area causes total insomnia, difficulty sleeping, on the contrary, its stimulation generates drowsiness and drowsiness.

The neurotransmitter GABA, a messenger involved in the decrease in brain activity, is projected from the ventrolateral preoptic area to the tuberomammillary nucleus, also located in the hypothalamus, to the dorsal pons located in the brain stem, to the raphe nucleus located in the brain stem. brain and responsible for the production of serotonin, and the locus coeruleus, also located in the brain stem and linked to the production of the neurotransmitter norepinephrine. These projections cause inhibition of these areas

As we noted at the beginning, the ventrolateral preoptic area is related to decreased activation and therefore sleep; This fact is reaffirmed by the inhibitory function it produces in the aforementioned areas of the hypothalamus and brain stem. It has also been proven that these regions would be related to brain and behavioral activation.

Likewise, also The existence of a reciprocal inhibition circuit called “flip-flop oscillator” has been observed in this circuit there is alternating inhibition of the ventrolateral preoptic area and the regions of the trunk and hypothalamus that project, this means that the activation of one will cause the deactivation of the other, in this way both cannot be found functioning, allowing them to alternate periods of sleep and wakefulness.

On the other hand, it has been noted that this circuit does not always work well and Imbalances and decompensations can occur, leading to sleep-wake disorders, such as narcolepsy cataplexy (a loss of muscle tone occurs), sleep paralysis and hypnagogic hallucinations.

Likewise, it has been proven that the hypocretinergic neurons of the lateral hypothalamus (that is, the neurons that secrete hypocretin) are responsible for regulating and stabilizing the alternating sleep-wake circuit, the flip-flop oscillator, causing it to be keep it on and thus allowing the individual or animal to remain in a state of vigil or awake.

Likewise, it has also been seen 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 inhibition or deactivation of it, thus allowing the onset of sleep.

On the other hand, adenosine can also act by deactivating the hypocretinergic neurons of the lateral hypothalamus, linked to the state of wakefulness, as we have seen.

Brain areas involved in REM sleep

As we already mentioned in the first sections, the activity or brain waves during REM sleep are similar to those during wakefulness, showing a high electrical frequency observed with the electroencephalogram technique

A characteristic and distinctive sign of the REM phase is the appearance of PGO (pontine-geniculate-occipital) waves, thus signaling that the individual is in REM. PGO waves are large, brief electrical waves that begin in the pons, heading to the lateral geniculate nuclei and subsequently to the occipital lobe, specifically to the primary visual cortex.

The region with the greatest involvement in the REM floor is the dorsolateral pons, which is made up of cholinergic neurons, producers of the neurotransmitter acetylcholine. This region is inhibited by the raphe nuclei and the locus coeruleus, mentioned above, through the projection of norepinephrine and serotonin respectively and thus allowing and appearing NREM sleep or the state of wakefulness.

Likewise, it has been observed that the peribrachial area, located in the pons, which is made up of the pedunculopontine and laterodorsal tegmental nuclei, together with the superior intermediate reticular nucleus, are formed by cholinergic neurons that produce acetylcholine, which, as we have said, one of its functions is to control REM sleep.

So, Depending on the region where the cholinergic neurons project, they will give rise to or allow different functions of the REM phase: Projections to the thalamus (one of the main information passing regions of the brain) and the basal forebrain allow cortical activation and desynchronization, and connections with the lateral geniculate nucleus allow or are related to PGO waves.

It has also been considered that projections to the midbrain tectum, located in the brain stem, control rapid eye movements and the link with the lateral preoptic area could be related to penile erection during sleep.

Finally, the connections with the neurons of the magnocellular nucleus of the medulla oblongata, also located in the brain stem, allow it to inhibit the motoneurons of the spinal cord, thus producing muscle atony, loss of muscle tone, typical of REM sleep.

Areas involved in brain activation

It is also interesting to know which areas of the brain are related to activation or arousal, since they will be important and will participate in the neural control of sleep.

The main region involved with brain activity is the ascending reticular activating system, also known by the name SARA. This is made up of neurons from the upper part of the brain stem, the hypothalamus and the basal forebrain. These pathways connect the thalamus and the cerebral cortex, allowing them to transmit and respond correctly to sensory inputs.

If an injury or damage occurs to this system, an alteration and decrease in consciousness will appear. In this way, optimal functioning of the SARA will lead to a correct state of wakefulness, not allowing states of relaxation or sleep.

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