THE BRAIN STRUCTURES THAT WAKE YOU UP AND PUT YOU TO SLEEP

During the Spanish flu pandemic that raged after World War I ended in 1918, a Viennese neurologist, Constantin von Economo, observed that some flu patients fell into a state of lethargy or coma before dying, while others went several days without sleeping, and then died.

Some years later, in 1949, brain researchers Giuseppe Moruzzi and Horace Magoun successfully triggered comas in cats by using coagulation to destroy the central part of the brainstem, known as the reticular formation. From this result they concluded that these comas were due to an absence of wakefulness.

In other experiments, Moruzzi and Magoun found that by stimulating the reticular formation, they could awaken animals from normal sleep. The two researchers also knew that the reticular formation receives many incoming messages, in particular via the sensory pathways. From these two facts, Moruzzi and Magoun developed the concept of the “ascending activating reticular system”. The reticular formation in the brainstem then became the prime contender for the title of “wakefulness centre”.

But some of the conclusions drawn from these earlier, cruder, surgical interventions have since been invalidated by more selective experiments. In these experiments, neurotoxic substances were used to destroy the neurons of the posterior hypothalamus and the reticular formation, while leaving intact the axons that passed through these areas but arose from other structures elsewhere in the nervous system. The result: the wakefulness function was still diminished initially, but quickly returned to normal!

The researchers were thus forced to conclude that these other structures could take over the job of those that had been destroyed and that the wakefulness disorders created in past experiments had probably been attributable to damage to the axons arising from these other wakefulness-maintaining structures and passing through the structures that had been destroyed.

These results cast new doubt on a certain conception of sleep as a passive process, in which being deprived of sensory inputs was what caused people to fall asleep. In subsequent experiments, the application of electrical stimuli to the thalamus of cats while they were awake caused them to fall asleep, thus demonstrating that sleep is not simply a passive process, but rather involves interactions between the thalamus and the cortex.

Later, the finding that blocking the sensory messages reaching the brain in no way disturbed the cycles of sleeping and wakefulness contributed further to the abandonment of this passive model of the process of falling asleep. The discovery of the intense activity in the cortex during REM sleep also dealt a deadly blow to this model.

Today, scientists instead regard sleep as an active phenomenon, and there is no longer any doubt about the important role that structures such as the anterior hypothalamus play in the onset of sleep. Other neurons heavily involved in controlling sleep and wakefulness belong to the various diffuse neuromodulation systems in the brainstem. By diffusing their neuromodulating substances throughout wide areas of the brain, the neurons in these systems act as switches that adjust the cortex’s sensitivity to sensory information.

Brain Imaging

Recent Studies on the Role of Sleep

THE BRAIN DURING REM SLEEP

Electroencephalography is a method of recording the activity of the cortex by means of electrodes placed on the scalp. In the 1950s, electroencephalography revealed that the cortex is just as active when someone is in REM sleep as when he or she is awake. Scientists hence began referring to REM sleep as “paradoxical sleep”, to call attention to this phenomenon.

But with the development of brain imaging technologies in the mid-1990s (follow the Tool module link to the left), researchers discovered other brain structures, many of them located deep below the cortex, whose activity was greatly altered during REM sleep. In some of these areas, activity increased during REM sleep, while in others, it decreased. But what was remarkable was that this increase or decrease in activity was consistent with the particular kind of dreaming that occurs during REM sleep.

Brain imaging studies have found, for example, that the primary visual cortex, the first part of the brain involved in consciously decoding visual signals when people are awake, shows very little activity when they are dreaming during REM sleep. This is no surprise—when people are dreaming, their eyes are closed, and no visual signals are reaching them.

But brain imaging studies have also shown that certain extrastriate visual areasof the cortex, which decode complex visual scenes, are significantly more active during REM sleep. Thus, during REM sleep, these areas are apparently involved in analyzing complex visual scenes. This is completely consistent with the often highly elaborate visual dream scenes that people report when researchers awaken them from REM sleep.

Adapted from Neuroscience, Purves et al., from Hobson et al., 1998

During REM sleep, intense activity is also observed in the limbic system, a set of structures heavily involved in emotions. Two of these structures are especially active: the hippocampal regionand, in particular, the amygdala. Once again, it is interesting to note that this intense limbic activity does not occur during the phases of non-REM sleep , when the dreams that people have are far less emotional.

The frontal cortex is a part of the brain that maintains very close ties with the limbic system. Yet the frontal cortex remains relatively calm during REM sleep. The prefrontal cortex, which is part of the frontal cortex, is heavily involved in thought and judgment when we are awake. Its low activity during REM sleep might thus account for the bizarre, illogical, and often socially inappropriate content of people’s dreams.

The anterior cingulate gyrus, which governs attention and motivation, is also more active during REM sleep, which may be part of the reason that the images we see in dreams are so vivid and so changeable.

Lastly, the pons is also more active during REM sleep, which makes complete sense, because even though the elaborate dreams that occur during REM sleep certainly involve the cerebral cortex, it does not seem to be involved in triggering REM sleep: REM sleep is triggered by certain nuclei (groups of neurons) in the pons.