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What are the parts of the brain associated with sleep?

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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 how the sleep process is, what phases constitute it, as well as what regions of the brain is involved, activated or inhibited in each phase, that is, which areas are linked to the neural control of the 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 to rest.. There is a decrease in brain activity caused by fatigue. Sleeping is a basic, vital need, being essential a good 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 out of synch with fast frequency waves called Beta waves. When the subject is in a resting situation, relaxed, the waves become a little slower, giving rise to Alpha waves.

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

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

NREM sleep is necessary for the brain to rest and recover from waking activity; on the other hand, REM sleep will be linked above all with the learning processes, consolidating the information obtained during the day.

Neurological bases of sleep
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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 disagreements, lack of stimulation. But the neuropsychologist Giuseppe Moruzzi proved that reticular formation midbrain, located in the brainstem, has the function of generating wakefulness; for his part, the medulla oblongata it is in charge of inhibiting the previous 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.

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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 area has been especially important for the sleep process, located in the hypothalamus lateral, brain region mainly related to endocrine function, with hormones. It was found 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 with decreased brain activity, is projected from the ventrolateral preoptic area to the tuberomamillary nucleus, also located in the hypothalamus, to the dorsal protrusion located in the brainstem, to the raphe nucleus located in the brainstem and responsible for the production of serotonin, and to the locus coeruleus, also located in the brainstem and linked to the production of the neurotransmitter norepinephrine. These projections cause inhibition of these areas.

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

Similarly, also the existence of a reciprocal inhibition circuit called a "flip-flop oscillator" has been observed., in this circuit there is alternate 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 the two will not be able to be working, allowing alternating periods of sleep and vigil.

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

Similarly, 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 stay on and thus allowing the individual or animal to remain in a waking or awake state.

Likewise, it has also been seen that the action of adenosine, substance that appears after cortical activity, on the basal forebrain (mainly related to the functions cognitive functions such as attention and learning), produces inhibition or deactivation of this, thus allowing the appearance of sleep.

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

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Areas of the brain involved in REM sleep

As we already mentioned in the first sections, the activity or brain waves during REM sleep are similar to those of 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 (protuberance-geniculate-occipital) waves, thus signaling that the individual is in REM. PGO waves are large and short electrical waves that begin in the pons, going to the lateral geniculate nuclei and later to the occipital lobe, specifically to the primary visual cortex.

The region with the greatest involvement in the REM soil is the dorsolateral protuberance, which is made up of cholinergic neurons, producers of the neurotransmitter acetylcholine. This region is inhibited by the nuclei of the raphe 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 vigil.

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

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

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

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

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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 activator system, also known as SARA. This is made up of neurons in the upper part of the brainstem, 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 injury or damage occurs to this system, there will be an alteration and decreased consciousness. In this way, an optimal functioning of the SARA will give rise to a correct state of wakefulness, not allowing states of relaxation or sleep.

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