Scientists have discovered rare neurons in the cerebral cortex that may promote the onset of sleep. The study showed that the cortex does not simply enter a state of rest under the influence of deeper brain structures, but may itself initiate and synchronize activity characteristic of sleep. The work, led by Renata Batista-Brito of the Icahn School of Medicine at Mount Sinai, was published in the journal Nature.
The neurons in question are Sst-Chodl neurons, which make up only about 0.2% of all cortical neurons. Unlike most inhibitory neurons, which primarily interact with neighboring cells, these neurons have long-range connections and can therefore influence the activity of large areas of the cortex.
The researchers studied the activity of these neurons in mice. While the animals were awake, Sst-Chodl cells remained mostly inactive. As the animals began to fall asleep and entered deep slow-wave sleep, the activity of these neurons increased along with the emergence of slow, synchronized rhythms in the cortex that are characteristic of sleep.
However, the scientists found that the neurons do not simply respond to the onset of sleep. When the researchers artificially activated Sst-Chodl cells, electrical activity in the neocortex became slower and more synchronized, taking on characteristics of sleep. The stimulation also made the mice fall asleep more easily and increased the duration of their sleep.
According to Batista-Brito, this suggests that these neurons may be directly involved in the transition from wakefulness to sleep, rather than becoming active only after sleep has already begun.
The fact that Sst-Chodl neurons have been preserved throughout evolution for hundreds of millions of years was of particular interest to the researchers. They have been found in amphibians, reptiles and mammals, including humans. However, their role in the body has remained poorly understood. Experiments in mice showed that they are involved in synchronizing cortical activity and regulating sleep, but it is not yet known whether they perform the same function in humans.
The researchers also suggest that these neurons may somehow respond to the brain's increasing need for sleep. During wakefulness, so-called sleep pressure gradually builds up, but there is currently no evidence showing whether Sst-Chodl cells can detect this signal or trigger the transition to sleep in response to it.
The discovery could help researchers investigate the mechanisms underlying sleep disturbances associated with various neurological, neurodevelopmental and psychiatric conditions, including Alzheimer's disease and autism spectrum disorder. Scientists still need to determine whether the activity of these neurons changes in sleep disorders and whether dysfunction of these cells could contribute to the development of such conditions.
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