Neurons Making Up 0.2% of the Cortex Helped Put Mice to Sleep

A population of neurons making up roughly 0.2% of the mouse cortex did more than become active during sleep. When researchers selectively activated the cells, activity across distant cortical regions slowed and synchronized, and the animals entered sleep more readily. The result, published in Nature on September 9, gives the cerebral cortex an active role in the transition from wakefulness rather than casting it only as a recipient of commands from deeper brain centres.

A long-range brake inside the cortex

Most inhibitory interneurons act locally, dampening nearby cells. Sst-Chodl neurons are unusual because their axons reach across long distances. Jacob Ratliff, Geoffrey Terral, Renata Batista-Brito and colleagues combined anatomical tracing, recordings of neural activity and targeted activation to examine what this sparse network does across the sleep-wake cycle.

The cells were largely silent while mice were awake and alert. Their activity increased as the animals became drowsy and moved into slow-wave sleep, when large populations of cortical neurons alternate together between active and quiet phases. Selective stimulation pushed electrical activity toward that slower, more coordinated pattern across multiple cortical regions.

Sst-Chodl cells account for about 0.2% of cortical neurons, yet activating them was sufficient to promote sleep in mice.

That causal manipulation is the important step. Correlation alone would show only that the neurons accompany sleep. By changing their activity and observing both cortical synchronization and behaviour, the researchers found evidence that the cells participate in producing the transition. A separate experiment using 14 mice, reported in the paper's analyses, found that chemogenetic activation increased slow-wave and rapid-eye-movement sleep while reducing wakefulness.

Why the finding changes the map of sleep

Classic models place major sleep-control switches in the brainstem and hypothalamus. The new work does not displace those systems. It adds a cortical circuit that may cooperate with them, suggesting that sleep emerges from communication between deep structures and the cortex.

The cells appear conserved across amphibians, reptiles and mammals, including humans. That makes the circuit a useful target for further research, but conservation is not proof of equivalent human function. The team also has not shown whether Sst-Chodl neurons sense rising sleep pressure, whether disrupting them causes a sleep disorder, or whether manipulating them would be safe.

For context, NewTqnia has previously covered an earbud chip that detects deep sleep and an observational study linking very long sleep with an Alzheimer-related blood marker. This study addresses a different question: which cortical cells may help the brain enter sleep in the first place.